Abstract
Inspired by the recent theory of EntropyTransport problems and by the \({\mathbf {D}}\)distance of Sturm on normalised metric measure spaces, we define a new class of complete and separable distances between metric measure spaces of possibly different total mass. We provide several explicit examples of such distances, where a prominent role is played by a geodesic metric based on the HellingerKantorovich distance. Moreover, we discuss some limiting cases of the theory, recovering the “pure transport” \({\mathbf {D}}\)distance and introducing a new class of “pure entropic” distances. We also study in detail the topology induced by such EntropyTransport metrics, showing some compactness and stability results for metric measure spaces satisfying Ricci curvature lower bounds in a synthetic sense.
Introduction
With motivations from pure Mathematics as well as from applied sciences, over the last decades a growing attention has been paid to the problem of “comparing objects”, which come naturally endowed with a distance/metric and a weight/volume form/measure. From the mathematical point of view, such objects are formalised as metric measure spaces (m.m.s. for short) \((X,{\mathsf {d}},\mu )\), where the metric structure \((X,{\mathsf {d}})\) describes the geometry and the mutual distance of points, and the measure \(\mu \) “weights” the relative importance of different parts of the object.
The flexibility of such a framework allows to unify the treatment of a series of problems stemming from various fields of science and technology, e.g. chemistry [24], data science [33], multiomics data alignment [13], computer vision [40], language processing [1], graph [46] and shape [42, 49] matching, barycenters & shape analysis [34], generative networks [6, 11], machine learning [47]. The theory of metric measure spaces has been flourishing in pure Mathematics as well, providing a unified setting to investigate concentration of measure phenomena [26, 41], the theory of Ricci limit spaces [8, 19] and, more generally, synthetic notions of Ricci curvature lower bounds [4, 30, 43, 44].
In order to “quantify the similarities and differences between two such objects”, it is thus natural to investigate appropriate notions of distance between metric measure spaces. This idea has its roots in the work of Gromov [23, Chapter 3\(\frac{1}{2}\)]), who first recognized the importance of studying the “space of spaces” \(\varvec{\mathrm {X}}\) as a metric space in its own right. Formally, \(\varvec{\mathrm {X}}\) denotes the set of equivalence classes of metric measure spaces \((X,{\mathsf {d}},\mu )\), where \((X,{\mathsf {d}})\) is a complete and separable metric space, and \(\mu \) is a finite, nonnegative, Borel measure; we are naturally identifying two m.m.s. \((X_1,{\mathsf {d}}_1,\mu _1)\), \((X_2,{\mathsf {d}}_2,\mu _2)\) if there exists an isometry \(\psi :{\mathsf {supp}}(\mu _1)\rightarrow {\mathsf {supp}}(\mu _2)\) such that \(\psi _{\sharp }\,\mu _1=\mu _2\). Here by \({\mathsf {supp}}(\mu )\) we denote the support of the measure \(\mu \) (see the preliminary section for more details).
In the recent years, the theory has been pushed forward by the works of Sturm [43, 45] and Memoli [32] who realized that ideas from mass transportation can be used to produce new relevant distances between metric measure spaces. Such distances have been successfully applied in different fields, but suffer from a major restriction which is intrinsic of the Wasserstein distances coming from optimal transport: they can be used to compare only spaces with the same total mass.
The goal of the present paper is to overcome this limitation by taking advantage of the theory of optimal EntropyTransport problems [29]. In contrast with the classical transport setting, these problems allow the description of phenomena where the conservation of mass may not hold; for this reason they are also known in the literature as “unbalanced optimal transport problems”. The corresponding theory is fairly recent and is becoming increasingly popular in applications, e.g. gradient flows to train neural networks [9, 37], supervised learning [18], medical imaging [17] and video [27] registration. Indeed, the EntropyTransport relaxation seems to outperform classical optimal transport in all the problems where the input data is noisy or a normalization procedure is not appropriate. We refer to [38] and references therein for more applications of unbalanced optimal transport.
As we are going to explain in detail below, inspired by the construction of the \({\mathbf {D}}\)distance of Sturm [43], we are able to produce a new class of complete and separable distances between metric measure spaces by replacing the Wasserstein distance with an EntropyTransport distance. Such metric structures on \(\varvec{\mathrm {X}}\) also turn out to be geodesic (resp. length) when the underlying EntropyTransport distance is geodesic (resp. length).
Optimal transport and Sturm distances. Let \((X,{\mathsf {d}})\) be a metric space and \(\varvec{\mathrm c}:X\times X\rightarrow [0,+\infty ]\) be a lower semicontinuous cost function. The optimal transport problem between two probability measures \(\mu _1,\mu _2\) consists in the minimization problem:
Here \(\Pi (\mu _1,\mu _2)\) denotes the set of measures \(\varvec{\gamma }\) in the product space \(X\times X\) whose marginals satisfy the constraint \(\pi ^i_{\sharp }\varvec{\gamma }=\mu _i\), where \(\pi ^i\) denotes the projection map \(\pi ^i(x_1,x_2)=x_i\).
A typical choice for the cost function is \(\varvec{\mathrm c}(x_1,x_2)={\mathsf {d}}^p(x_1,x_2)\), \(p\ge 1\). In this situation, the transport cost \(\mathrm {T}\) is the ppower of the celebrated pWasserstein distance \({\mathcal {W}}_p\), a metric on the set \({\mathscr {P}}_p(X)\) of probability measures over X with finite pmoment. Starting from the seminal work of Kantorovich, the metric space \(({\mathscr {P}}_p(X), {\mathcal {W}}_p)\) has been thoroughly studied: it inherits many geometric properties of the underlying space \((X,{\mathsf {d}})\) (such as completeness, separability, geodesic property) and induces the weak topology (with pmoments) of probability measures. We refer to the monograph [48] for a detailed overview of the topic.
As observed by Sturm [43], one can lift the metric \({\mathcal {W}}_p\) to a distance between metric measure spaces by defining:
where the infimum is taken over all complete and separable metric spaces \(({\hat{X}},\hat{{\mathsf {d}}})\), and all isometric embeddings \(\psi ^i:{\mathsf {supp}}(\mu _i)\rightarrow {\hat{X}}\). It is proved in [43, Theorem 3.6] that \({\mathbf {D}}_p\) is a complete, separable and geodesic distance on the set
EntropyTransport problems and SturmEntropyTransport distances. The idea at the core of EntropyTransport problems is to relax the marginal constraints typical of the classical Kantorovich formulation (1) by adding some suitable penalizing functionals which keep track of the deviation of the marginals \(\gamma _i:=\pi ^i_{\sharp }\varvec{\gamma }\) from the data \(\mu _i\), \(i=1,2\).
Following the approach of Liero, Mielke and Savaré [29], given a superlinear, convex function \(F:[0,+\infty )\rightarrow [0,+\infty ]\) such that \(F(1)=0\) (for simplicity here we assume F to be superlinear, see definition (21) for the general case), one considers the entropy functional (also called Csiszár Fdivergence [12])
Here \({\mathscr {M}}(X)\) denotes the set of finite, nonnegative, Borel measures over X. A classical example is given by the choice \(F=U_1(s):=s\ln (s)s+1\), that corresponds to the celebrated KullbackLeibler divergence (note that when \({\varvec{\gamma }}\) and \(\mu \) are probability measures, \(D_{U_1}\) coincides with the celebrated BoltzmannShannon entropy \(\mathrm{Ent}(\rho \mu  \mu )=\int \rho \log \rho \, {\mathrm d}\mu \)).
Given \(\mu _1,\mu _2\in {\mathscr {M}}(X)\), the EntropyTransport problem induced by the entropy function F and the cost function \(\varvec{\mathrm c}\) is then defined as
We emphasize that the problem (4) makes perfect sense even when \(\mu _1(X)\ne \mu _2(X)\).
As in the case of optimal transport problems, it is natural to consider cost functions of the form \(\varvec{\mathrm c}(x_1,x_2)=\ell ({\mathsf {d}}(x_1,x_2))\), where \({\mathsf {d}}\) is a distance on X and \(\ell :=[0,\infty )\rightarrow [0,\infty ]\) is a general function. With a careful choice of the functions F and \(\ell \) (see [14] for a discussion on the metric properties of EntropyTransport problems), one is able to produce a distance on the space \({\mathscr {M}}(X)\) by taking a suitable power of the EntropyTransport cost , namely for a certain \(a\in (0,1]\).
In the paper we introduce the class of regular EntropyTransport distances. The formal definition of this class of distances is given in Definition 2, here we just mention than any regular EntropyTransport distance is a complete and separable metric on \({\mathscr {M}}(X)\) of the form , for an EntropyTransport cost induced by sufficiently regular functions F and \(\ell \).
For any regular EntropyTransport distance, the SturmEntropyTransport distance between the (equivalence classes of) m.m.s. \((X_1,{\mathsf {d}}_1,\mu _1)\), \((X_2,{\mathsf {d}}_2,\mu _2)\) is then defined as
where the infimum is taken over all complete and separable metric spaces \(({\hat{X}},\hat{{\mathsf {d}}})\), and all isometric embeddings \(\psi ^1:{\mathsf {supp}}(\mu _1)\rightarrow {\hat{X}}\) and \(\psi ^2:{\mathsf {supp}}(\mu _2)\rightarrow {\hat{X}}\).
The main result of the paper (Theorem 2) is that every SturmEntropyTransport distance defines a complete and separable metric structure on \({\mathbf {X}}\). Moreover it satisfies the geodesic (resp. length) property if the distance satisfies the geodesic (resp. length) property on the space of measures.
We also study in detail the notion of convergence induced by such distances, showing that it corresponds to the weak measuredGromov convergence introduced in [21]. As a consequence, we obtain a compactness result for the class of m.m.s. \((X,{\mathsf {d}},\mu )\) satisfying the \({\mathsf {CD}}(K,N)\) condition, having bounded diameter and satisfying \(0<v\le \mu (X)\le V\). We refer to Theorem 4 for the precise statement and to the preliminaries for the definition of the curvaturedimension condition \({\mathsf {CD}}(K,N)\).
At a technical level, the proofs of our results are inspired by the corresponding ones given by Sturm in [43], but they require new ideas in order to deal with general cost functions and with the entropic part of the problem. Two key results of independent interest are contained in Proposition 2 and Lemma 6, where we show that the infimum in the right hand side of (5) is actually a minimum, and we give an explicit formulation of the SturmEntropyTransport distance, namely
for some optimal measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) and optimal pseudometric coupling \(\hat{{\mathsf {d}}}\) between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) (see the preliminaries for the definition of pseudometric coupling). Also the proof of one of the main results, Theorem 2, despite being inspired by [43], departs from it and needs some new ideas:

in order to show that \({\mathbf {D}}_p\) defines a nondegenerate distance function (i.e.
$$\begin{aligned} {\mathbf {D}}_p \left( (X_1,{\mathsf {d}}_1,\mu _1), (X_2,{\mathsf {d}}_2,\mu _2) \right) =0 \end{aligned}$$implies that \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\) are isomorphic as metric measure spaces), Sturm [43] establishes a comparison result with Gromov’s \({\underline{\Box }}_{1}\) distance, of independent interest; this permits to inherit the nondegeneracy of \({\mathbf {D}}_p\) by the one of \({\underline{\Box }}_{1}\).
Instead, we argue directly: thanks to the aforementioned Proposition 2 and Lemma 6, we can exploit the existence of an optimal coupling both at the level of space and measure and infer the nondegeneracy of directly;

in order to show that the \({\mathbf {D}}_p\) distance is length, Sturm [43] argues by approximation via finite metric spaces, taking advantage of the “pure transport” behaviour of \({\mathbf {D}}_p\).
Due to the entropy contribution in the distance, we argue differently: the main point is to embed everything in a complete, separable and geodesic ambient space, obtained by a slight modification of Kuratowski embedding.
The class of regular EntropyTransport distances includes some of the main examples of EntropyTransport distances known in the literature, including:

The HellingerKantorovich geodesic distance [10, 25, 28, 29] induced by the choices
$$\begin{aligned} a=1/2\, , \qquad F(s)=U_1(s)\, , \qquad \ell (d)={\left\{ \begin{array}{ll} \log \left( {\cos ^2(d)}\right) \ \ &{}\text {if} \ d<\frac{\pi }{2}, \\ +\infty \ \ &{}\text {otherwise}.\end{array}\right. } \end{aligned}$$ 
The socalled Gaussian HellingerKantorovich distance [29] that corresponds to the choices
$$\begin{aligned} a=1/2\, , \qquad F(s)=U_1(s)\, , \qquad \ell (d)=d^2. \end{aligned}$$ 
The quadratic powerlike distances studied in [14] corresponding to
$$\begin{aligned} a=1/2\, , \quad F(s)=U_p(s):=\frac{s^pp(s1)1}{p(p1)}\, , \quad \ell (d)=d^2, \quad 1<p\le 3. \end{aligned}$$
Moreover, our analysis is not restricted to regular EntropyTransport distances. By a limit procedure we also discuss some singular cases covering:

The “pure entropy” setting that corresponds to the choice
$$\begin{aligned} \varvec{\mathrm c}(x_1,x_2)={\left\{ \begin{array}{ll}0 \ &{}\text {if} \ x_1=x_2, \\ +\infty &{}\text {otherwise.} \end{array}\right. } \end{aligned}$$In this situation we construct a family of distances between metric measure spaces inducing a notion of strong convergence (see Theorems 5 and 6 for the details).

The “pure transport” setting, corresponding to
$$\begin{aligned} a=1/p\, , \qquad F(s)={\left\{ \begin{array}{ll} 0 \ &{}\mathrm {if} \ s=1, \\ +\infty &{}\mathrm {otherwise}, \end{array}\right. } \qquad \ell (d)=d^p, \end{aligned}$$where we recover the \({\mathbf {D}}_p\)distances introduced by Sturm.

The PiccoliRossi distance \(\mathsf {BL}\) [35, 36] (also known as boundedLipschitz distance), induced by the choices
$$\begin{aligned} a=1\, , \qquad F(s)=s1\, , \qquad \ell (d)=d. \end{aligned}$$By an analogous procedure to the one described in (5), in Theorem 8 we show that the distance \(\mathsf {BL}\) can be lifted to a complete distance \({\mathbf {BL}}\) on the set \({\mathbf {X}}\).
Note on the preparation. Some of the results of the paper (often under additional assumptions) have been presented at different seminars and included in the Phd thesis of the first named author [15, Chapter 5], where the construction of the SturmEntropyTransport distances induced by the HellingerKantorovich and the quadratic powerlike distances is developed.
Only during the final stage of preparation of the present manuscript (September 2020), we became aware of the independent work [39], which defines a class of distances (denoted by \(\mathrm {CGW}\), for “conic GromowWasserstein”) between unbalanced metric measure spaces starting from the construction of the Gromov–Wasserstein distance introduced in [32] and the conical formulation of the EntropyTransport problems (see [10, 14, 29] and Remark 1 for a discussion on the “cone geometry” of EntropyTransport problems). The paper [39] also provides some interesting numerical discussions on the topic, while it is not present a study on the analytic and geometric properties of this class of distances (such as completeness, separability, the length and geodesic property, compactness). An expert reader will notice that our distance is an unbalanced counterpart of Sturm’s \({\mathbf {D}}_p\) distance [43] for probability metric measure spaces, while SéjournéVialardPeyré \(\mathrm {CGW}\) distance [39] is an unbalanced counterpart of Memoli’s [32] Gromov–Wasserstein distance. A major difference between the two approaches is that while our distance is complete (see Theorem 2), the Gromov–Wasserstein distance of [32] is not complete, and the same is expected for the \(\mathrm {CGW}\) distance of [39]. The relation between and \(\mathrm {CGW}\) is analysed in Sect. 5, where we prove an upper bound of the latter in terms of the former.
Preliminaries and notation
Metric and measure setting
A function \({\mathsf {d}}:X\times X\rightarrow [0,\infty ]\) is a pseudometric on the set X if \({\mathsf {d}}\) is symmetric, satisfies the triangle inequality and \({\mathsf {d}}(x,x)=0\) for every \(x\in X\). We say that \({\mathsf {d}}\) is a metric possibly attaining the value \(+\infty \) if it is a pseudometric such that \({\mathsf {d}}(x,y)=0\) implies \(x=y\). When \({\mathsf {d}}\) is also finitevalued, we simply say that \({\mathsf {d}}\) is a metric. A pseudometric space (resp. metric space) will be a couple \((X,{\mathsf {d}})\), where \({\mathsf {d}}\) is a pseudometric (resp. metric) on the set X.
On a pseudometric space we will always consider the topology induced by the open balls \(B_r(x):=\{y\in X: {\mathsf {d}}(x,y)<r\}.\) A Polish space is a separable completely metrizable topological space. We will denote by \(\mathsf {diam}(X)\) the diameter of a metric space X.
An isometry between two metric spaces \((X_1,{\mathsf {d}}_1)\), \((X_2,{\mathsf {d}}_2)\) is a map \(\psi :X_1\rightarrow X_2\) such that for every \(x,y\in X_1\) we have
Let \(\{(X_{\alpha },{\mathsf {d}}_{\alpha }){\alpha }\in A\}\) be an indexed family of metric spaces, we define its disjoint union as
endowed with a pseudometric \(\hat{{\mathsf {d}}}\), called pseudometric coupling between \(\{{\mathsf {d}}_{\alpha }\}\), such that \(\hat{{\mathsf {d}}}((x,\alpha ),(y,\alpha ))={\mathsf {d}}_{\alpha }(x,y)\) for every \(x,y\in X_{\alpha }\). The inclusion map
is thus an isometry with image \(X_{\alpha }\times \{\alpha \}\). We will often identify, with a slight abuse of notation, the space \(X_{\alpha }\) with \(X_{\alpha }\times \{\alpha \}\).
Lemma 1
Let \((X_1,{\mathsf {d}}_1)\), \((X_2,{\mathsf {d}}_2)\) be two complete and separable metric spaces. Let \({\hat{{\mathsf {d}}}}\) be a finite valued pseudometric coupling between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\). Then the space
endowed with the distance
is a complete and separable metric space. Here \([x]\in {\tilde{X}}\) denotes the equivalence class of the point \(x\in X_1\sqcup X_2\).
Proof
We firstly notice that \({\tilde{{\mathsf {d}}}}\) is well defined on \({\tilde{X}}\). Indeed, if \(x_1 \sim {\tilde{x}}_1\) and \(x_2 \sim {\tilde{x}}_2\) we have
which implies \({\tilde{{\mathsf {d}}}}(x_1,x_2)={\tilde{{\mathsf {d}}}}({\tilde{x}}_1,{\tilde{x}}_2)\).
It is clear that \({\tilde{{\mathsf {d}}}}\) is a metric on \((X_1\sqcup X_2)/\sim \).
The separability is a consequence of the fact that \((X_1\sqcup X_2,{\hat{{\mathsf {d}}}})\) is separable, being the union of two separable space (recall that \({\hat{{\mathsf {d}}}}={\mathsf {d}}_i\) on \(X_i\), \(i=1,2\)).
To prove the completeness, let us consider a Cauchy sequence \(\{y_j\}\in {\tilde{X}}\). It is sufficient to show that a subsequence is converging with respect to \({\tilde{{\mathsf {d}}}}\). Let \(p:X_1\sqcup X_2\rightarrow {\tilde{X}}\) be the quotient map and, recalling that \(X_1\sqcup X_2=X_1\times \{0\}\cup X_2\times \{1\}\), we can suppose without loss of generality that there exists a subsequence \(\{p^{1}(y_{j_k})\}\in X_1\times \{0\}\) (the case \(\{p^{1}(y_{j_k})\}\in X_2\times \{1\}\) being analogous). Up to identifying \((X_1\times \{0\},{\hat{{\mathsf {d}}}})\) with \((X_1,{\mathsf {d}}_1)\), we can infer that \(\{p^{1}(y_{j_k})\}\) is a Cauchy sequence in the complete space \((X_1,{\mathsf {d}}_1)\) and thus it converges. It is immediate to check that \(\{y_{j_k}\}\) is converging in \({\tilde{X}}\) with respect to \({\tilde{{\mathsf {d}}}}\) and the proof is complete. \(\square \)
Starting from a metric space \((X,{\mathsf {d}})\), we define the cone over X as the space
If \((X,{\mathsf {d}})\) is a pseudometric space, we denote by \({\mathscr {M}}(X)\) the space of finite, nonnegative measures on the Borel \(\sigma \)algebra \({\mathscr {B}}(X)\), and by \({\mathscr {P}}(X)\subset {\mathscr {M}}(X)\) the space of probability measures. We endow \({\mathscr {M}}(X)\) with the weak topology, inducing the following notion of convergence:
where \(C_b(X)\) denotes the set of real, continuous and bounded functions defined on X.
A subset \({\mathscr {K}}\subset {\mathscr {M}}(X)\) is bounded if \(\sup _{\mu \in {\mathscr {K}}}\mu (X)<\infty \) and it is equally tight if
Compactness properties with respect to the weak topology on \({\mathscr {M}}(X)\) are guaranteed by the following version of Prokhorov’s Theorem:
Theorem 1
Let X be a Polish space. A subset \({\mathscr {K}}\subset {\mathscr {M}}(X)\) is bounded and equally tight if and only if it is relatively compact with respect to the weak topology.
We recall that the set of measures of the form
where \(M\in {\mathbb {R}}_+\), \(N\in {\mathbb {N}}\) and \(x_n\in X\), is dense in \({\mathscr {M}}(X)\). Moreover, if X is separable, the measures of the form (11), with \(M\in {\mathbb {Q}}_+\) and \(x_n\) in a countable dense subset of X, form a countable dense subset of \({\mathscr {M}}(X)\), proving that also the latter is a separable space.
A metric measure space will be a triple \((X,{\mathsf {d}},\mu )\) where \((X,{\mathsf {d}})\) is a complete, separable metric space and \(\mu \in {\mathscr {M}}(X)\). If there exists a point \(x_0\in X\) such that
we will say that the measure \(\mu \in {\mathscr {P}}(X)\) has finite pmoment. We denote by \({\mathscr {P}}_p(X)\) the space of measures \(\nu \in {\mathscr {P}}(X)\) with finite pmoment.
The support of the measure \(\mu \) is the smallest closed set \(X_0:={\mathsf {supp}}(\mu )\) such that \(\mu (X{\setminus } X_0)=0\). We notice that the set \({\mathsf {supp}}(\mu )\) has a natural structure of metric measure space with the induced distance, \(\sigma \)algebra and measure (which will be denoted in the same way).
We say that \(\varphi \) is a curve connecting \(x,y\in X\), if \(\varphi :[a,b]\rightarrow X\) is a continuous map such that \(\varphi (a)=x\) and \(\varphi (b)=y\). The length of a curve is defined as
where the supremum is taken over all the partitions \(a=t_0<t_1<\ldots <t_n=b\).
We will always assume that a curve of finite length is parametrized by constant speed, i.e.
A metric space \((X,{\mathsf {d}})\) is called length space if for all \(x,y\in X\)
A geodesic is a curve \(\varphi :[a,b]\rightarrow X\) such that
Notice in particular that if \(\varphi \) is a geodesic then
A metric space \((X,{\mathsf {d}})\) is geodesic if any pair of points \(x,y\in X\) is connected by a geodesic.
For a metric space \((X,{\mathsf {d}})\), the KantorovichWasserstein distance \({\mathcal {W}}_p\) of order p, \(p\ge 1\), is defined as follows: for \(\mu _0,\mu _1 \in {\mathscr {M}}(X)\) we set
where the infimum is taken over all \(\varvec{\gamma } \in {\mathscr {M}}(X \times X)\) with \(\mu _0\) and \(\mu _1\) as the first and the second marginal, i.e. \((\pi ^i)_{\sharp }\varvec{\gamma }=\mu _i\) where \(\pi ^i:X\times X\rightarrow X\) denotes the projection map \(\pi ^i(x_1,x_2)=x_i\), \(i=1,2\). A measure \(\varvec{\gamma } \in {\mathscr {M}}(X \times X)\) achieving the minimum in (16) with given marginals is said a \({\mathcal {W}}_p\)optimal coupling for \((\mu _0,\mu _1)\). It is clear that \({\mathcal {W}}_p(\mu _1,\mu _2)=+\infty \) when \(\mu _1(X)\ne \mu _2(X)\).
If \((X,{\mathsf {d}})\) is complete and separable, \(({\mathscr {P}}_p(X),{\mathcal {W}}_p)\) is a complete and separable metric space. It is geodesic when \((X,{\mathsf {d}})\) is geodesic. Moreover, for any sequence \(\mu _n\in {\mathscr {P}}_p(X)\) we have
where the latter means that for some (thus any) \(x_0\)
For a proof of these last facts, see [48, Theorem 6.18].
CurvatureDimension condition
It is out of the scopes of this brief section to give a full account of the curvaturedimension condition and its properties; we will limit to schematically recalling the basic definitions involved. The interested reader is referred to the original papers [3,4,5, 7, 16, 20, 21, 30, 43, 44], the survey [2] and the monograph [48].

For any \(K\in {\mathbb {R}}, \, N\in (1,\infty ), \,\theta >0\) and \(t\in [0,1]\), define the distortion coefficients by
$$\begin{aligned} \tau ^{(t)}_{K,N}(\theta ) := t^{\frac{1}{N}} \sigma ^{(t)}_{K, N1}(\theta )^{\frac{N1}{N}}, \end{aligned}$$where
$$\begin{aligned} \sigma ^{(t)}_{K,N}(\theta ):= {\left\{ \begin{array}{ll} \infty &{}\text {if }K\theta ^2\ge N\pi ^2\\ \frac{\sin (t\theta \sqrt{K/N})}{\sin (\theta \sqrt{K/N})} &{}\text {if }0<K\theta ^2< N\pi ^2\\ t &{}\text {if }K\theta ^2=0\\ \frac{\sinh (t\theta \sqrt{K/N})}{\sinh (\theta \sqrt{K/N})} &{}\text {if } K\theta ^2<0 \end{array}\right. }. \end{aligned}$$ 
For every \(N\in (1,\infty )\), define the NRényi entropy functional relative to \(\mu \), \({{\mathcal {U}}}_N(\cdot \, \mu ):{{\mathscr {P}}}(X) \rightarrow [\infty ,0]\) as
$$\begin{aligned} {{\mathcal {U}}}_N(\nu  \mu ):= \int _{X} \rho ^{1\frac{1}{N}} {\mathrm d}\mu , \quad \text {where }\nu = \rho \mu +\nu ^s\text { and }\nu ^s\perp \mu . \end{aligned}$$ 
Define also the BoltzmannShannon entropy functional relative to \(\mu \), \(\mathrm{Ent}(\cdot \, \mu ):{{\mathscr {P}}}(X) \rightarrow (\infty ,+ \infty ]\) as
$$\begin{aligned} \mathrm{Ent}(\nu  \mu ):= \int _{X} \rho \,\log (\rho )\, {\mathrm d}\mu , \quad \text {if }\nu = \rho \mu \ll \mu \text { and }\rho \log \rho \in L^1(X,\mu ), \end{aligned}$$and \(+\infty \) otherwise.

\({\mathsf {CD}}(K,\infty )\) condition: given \(K\in {\mathbb {R}}\), we say that \((X,{\mathsf {d}},\mu )\) verifies the \({\mathsf {CD}}(K,\infty )\) condition if for any pair of probability measures \(\nu _0,\nu _1\in {{\mathscr {P}}}_2 (X) \) with
$$\begin{aligned} \mathrm{Ent}(\nu _0 \mu ), \mathrm{Ent}(\nu _1 \mu )<+\infty , \end{aligned}$$there exists a \({\mathcal {W}}_2\)geodesic \((\nu _t)_{t\in [0,1]}\) from \(\nu _0\) to \(\nu _1\) such that
$$\begin{aligned} \mathrm{Ent}(\nu _t \mu )\le (1t)\, \mathrm{Ent}(\nu _0 \mu )+ t\, \mathrm{Ent}(\nu _1 \mu ) \frac{K}{2} t (1t) {\mathcal {W}}^2_2(\mu _0, \mu _1), \end{aligned}$$for any \(t\in [0,1]\).

\({\mathsf {CD}}(K,N)\) condition: given \(K\in {\mathbb {R}}\), \(N\in (1,\infty )\) we say that \((X,{\mathsf {d}},\mu )\) verifies the \({\mathsf {CD}}(K,N)\) condition if for any pair of probability measures \(\nu _0,\nu _1\in {{\mathscr {P}}}{_2}(X) \) with bounded support and with \(\nu _0,\nu _1\ll \mu \), there exists a \({\mathcal {W}}_2\)geodesic \((\nu _t)_{t\in [0,1]}\) from \(\nu _0\) to \(\nu _1\) with \(\nu _t \ll \mu \), and a \({\mathcal {W}}_2\)optimal coupling \(\varvec{\gamma } \in {{\mathscr {P}}}(X \times X)\) such that
$$\begin{aligned} {{\mathcal {U}}}_{N'}(\nu _t \mu )\le  \int \left[ \tau ^{(1t)}_{K,N'}({\mathsf {d}}(x,y))\rho _0^{\frac{1}{N'}}+ \tau ^{(t)}_{K,N'}({\mathsf {d}}(x,y)) \rho _1^{\frac{1}{N'}} \right] {\mathrm d}\varvec{\gamma } (x,y), \end{aligned}$$for any \(N'\ge N\), \(t\in [0,1]\).

Consistency property: A smooth Riemannian manifold (resp. weighted Riemannian manifold) M satisfies the \({\mathsf {CD}}(K,N)\) condition for some \(K \in {\mathbb {R}}, N\in (1,\infty )\) if and only if \(\mathrm{dim}(M)\le N\) and the Ricci curvature is bounded below by K (resp. if and only if the NBakryÉmeryRicci tensor is bounded below by K).

Define the slope of a real valued function \(u:X\rightarrow {\mathbb {R}}\) at the point \(x\in X\) as
$$\begin{aligned} \nabla u(x):= {\left\{ \begin{array}{ll} \limsup _{y\rightarrow x} \frac{u(x)u(y)}{{\mathsf {d}}(x,y)} &{}\text {if { x} is not isolated} \\ 0 &{}\text {otherwise}. \end{array}\right. } \end{aligned}$$We denote with \(\mathrm{LIP} (X)\) the space of Lipschitz functions on \((X,{\mathsf {d}})\).

Let \(f\in L^2(X, \mu )\). The Cheeger energy of f is defined as
$$\begin{aligned} \mathsf {Ch}(f):= \inf \left\{ \liminf _{n\rightarrow \infty }\frac{1}{2}\int \nabla f_n^2 {\mathrm d}\mu \, \, f_n\in \mathrm{LIP}(X)\cap L^2(X,\mu ), \Vert f_nf\Vert _{L^2}\rightarrow 0 \right\} . \end{aligned}$$One can check that the Cheeger energy \(\mathsf {Ch}:L^2(X,\mu )\rightarrow [0,\infty ]\) is convex and lower semicontinuous. Thus it admits an \(L^2\)gradient flow, called heat flow.

The metric measure space \((X,{\mathsf {d}},\mu )\) is said infinitesimally Hilbertian if \(\mathsf {Ch}\) is a quadratic form, i.e. it satisfies the parallelogram identity.
One can check that \((X,{\mathsf {d}},\mu )\) is infinitesimally Hilbertian if and only if the heat flow for every positive time is a linear map from \(L^2(X,\mu )\) to \(L^2(X,\mu )\).
If \((X,{\mathsf {d}},\mu )\) is the metric measure space associated to a smooth Finsler manifold, one can check that \((X,{\mathsf {d}},\mu )\) is infinitesimally Hilbertian if and only if the manifold is actually Riemannian.

Given \(K\in {\mathbb {R}}\) and \(N\in (1,\infty ]\), we say that \((X,{\mathsf {d}},\mu )\) verifies the \({\mathsf {RCD}}(K,N)\) condition if it satisfies the \({\mathsf {CD}}(K,N)\) condition and it is infinitesimally Hilbertian.

Pointed measured GromovHausdorff convergence: Let \((X_n,{\mathsf {d}}_n,\mu _n)\), \(n\in {\mathbb {N}}\cup \{\infty \}\), be a sequence of metric measure spaces and let \({\bar{x}}_n\in X_n\) for every \(n\in {\mathbb {N}}\cup \{\infty \}\) be a sequence of reference points. We say that \((X_n,{\mathsf {d}}_n,\mu _n, {{\bar{x}}}_n)\rightarrow (X_\infty ,{\mathsf {d}}_\infty ,\mu _\infty , {{\bar{x}}}_\infty )\) in the pointed measured Gromov Hausdorff (pmGH) sense, provided for any \(\varepsilon ,R>0\) there exists \(N({\varepsilon ,R})\in {\mathbb {N}}\) such that for all \(n\ge N({\varepsilon ,R})\) there exists a Borel map \(f^{R,\varepsilon }_n:B_R({{\bar{x}}}_n)\rightarrow X_\infty \) such that

\(f^{R,\varepsilon }_n({{\bar{x}}}_n)={{\bar{x}}}_\infty \),

\(\sup _{x,y\in B_R({{\bar{x}}}_n)}{\mathsf {d}}_n(x,y){\mathsf {d}}_\infty (f^{R,\varepsilon }_n(x),f^{R,\varepsilon }_n(y))\le \varepsilon \),

the \(\varepsilon \)neighbourhood of \(f^{R,\varepsilon }_n(B_R({{\bar{x}}}_n))\) contains \(B_{R\varepsilon }({{\bar{x}}}_\infty )\),

\((f^{R,\varepsilon }_n)_\sharp (\mu _n\llcorner {B_R({{\bar{x}}}_n)})\) weakly converges to \(\mu _\infty \llcorner {B_R(x_\infty )}\) as \(n\rightarrow \infty \), for a.e. \(R>0\).
If in addition there exists \({\bar{R}}>0\) such that \(\mathrm{diam}(X_n)\le {\bar{R}}\) for every \(n\in {\mathbb {N}}\cup \{\infty \}\), then we say that \((X_n,{\mathsf {d}}_n,\mu _n)\rightarrow (X_\infty ,{\mathsf {d}}_\infty ,\mu _\infty )\) in the measured Gromov Hausdorff (mGH for short) sense. In this case it is enough to consider only \(R={\bar{R}}\) in the above requirements.


Stability: Let \(K\in {\mathbb {R}}\) and \(N\in (1,\infty ]\) be given. Assume that \((X_n, {\mathsf {d}}_n, \mu _n)\) satisfies \({\mathsf {CD}}(K,N)\) (resp. \({\mathsf {RCD}}(K,N)\)), for every \(n\in {\mathbb {N}}\), and that \((X_n, {\mathsf {d}}_n, \mu _n, {\bar{x}}_n )\rightarrow (X_\infty ,{\mathsf {d}}_\infty ,\mu _\infty , {{\bar{x}}}_\infty )\) in the pmGH sense. Then \((X_\infty , {\mathsf {d}}_\infty , \mu _\infty )\) satisfies \({\mathsf {CD}}(K,N)\) (resp. \({\mathsf {RCD}}(K,N)\)) as well.
Entropy functionals
In this section we assume that X is a Polish space.
A function \(F:[0,+\infty )\rightarrow [0,+\infty ]\) belongs to the class \(\Gamma _0({{\mathbb {R}}_{+}})\) of the admissible entropy functions if F is convex, lower semicontinuous and \(F(1)=0\). We define the recession constant as
and we say that F is superlinear if \(F'_{\infty }=+\infty \).
We also define the perspective function induced by \(F\in \Gamma _0({\mathbb {R}}_+)\) as the function \({\hat{F}}:[0,+\infty )\times [0,+\infty )\rightarrow [0,+\infty ]\), given by
The function
is called reverse entropy.
Let \(F\in \Gamma _{0}({\mathbb {R}}_{+})\) be an admissible entropy function. The Fdivergence (also called Csiszár’s divergence or relative entropy) is the functional \(D_F:{\mathcal {M}}(X)\times {\mathcal {M}}(X)\rightarrow [0,+\infty ]\) defined by
where \(\gamma =\sigma \mu +\gamma ^{\perp }\) is the Lebesgue’s decomposition of the measure \(\gamma \) with respect to \(\mu \). When F is superlinear \(D_F(\gamma \mu )=+\infty \) if \(\gamma \) has a singular part with respect to \(\mu \). Moreover, it is clear that \(D_F(\mu \mu )=0\).
We now collect some useful properties of the relative entropies. For the proof see [29, Sect. 2.4].
Lemma 2
The functional \(D_F\) is jointly convex and lower semicontinuous in \({\mathscr {M}}(X)\times {\mathscr {M}}(X)\). More generally, if \(F\in \Gamma _0({\mathbb {R}}_+)\) is the pointwise limit of an increasing sequence \((F_n)\subset \Gamma _0({\mathbb {R}}_+)\) and \(\gamma ,\mu \in {\mathscr {M}}(X)\) are the weak limit of a sequence \((\gamma _n,\mu _n)\subset {\mathscr {M}}(X)\times {\mathscr {M}}(X)\) then we have
Lemma 3
If \({\mathcal {K}}\subset {\mathscr {M}}(X)\) is bounded and \(F'_{\infty }>0\) then the set
is bounded for every \(C\ge 0\). Moreover, if \({\mathcal {K}}\) is also equally tight and F is superlinear, then \({\mathbf {K}}_C\) is equally tight for every \(C\ge 0\).
The last lemma of this section shows an invariance result for the Fdivergences.
Lemma 4
Let \(F\in \Gamma _0({\mathbb {R}}_{+})\) be an admissible entropy function, X, Y be two Polish spaces and \(f:X\rightarrow Y\) be a Borel injective map. Then, for any \(\gamma ,\mu \in {\mathscr {M}}(X)\) it holds
Proof
Let us consider the Lebesgue’s decompositions
Since \(f_{\sharp }\gamma \) and \(f_{\sharp }\mu \) have support contained in f(X), we can suppose without loss of generality that f is bijective.
For any Borel set \(A\subset X\) we have
By the uniqueness of the Lebesgue’s decomposition (see [29, Lemma 2.3]) it follows that \(\sigma ={\tilde{\sigma }}\circ f\) up to \((\mu + \gamma )\)negligible sets and \(\gamma ^{\perp }(X)={\tilde{\gamma }}^{\perp }(f(X))={\tilde{\gamma }}^{\perp }(Y)\). In particular
\(\square \)
EntropyTransport problem and distances
Let \(\varvec{\gamma }\in {\mathscr {M}}(X\times X)\). In the sequel we denote by \(\gamma _i:=(\pi ^i)_{\sharp }\varvec{\gamma }\) the marginals of \(\varvec{\gamma }\).
We are now ready to define the EntropyTransport problem.
Definition 1
Let \(F\in \Gamma _{0}({\mathbb {R}}_{+})\) and let \(\varvec{\mathrm c}:X\times X\rightarrow [0,+\infty ]\) be a lower semicontinuous function. The EntropyTransport functional between the measures \(\mu _1,\mu _2\in {\mathscr {M}}(X)\) is the functional
We define the EntropyTransport problem between \(\mu _1\) and \(\mu _2\) as the minimization problem
To highlight the role of the entropy function F and the cost function \(\varvec{\mathrm c}\), we also say that is the cost of the EntropyTransport problem induced by \((F,\varvec{\mathrm c})\).
We are particularly interested in cost functions of the form \(\varvec{\mathrm c}(x_1,x_2)=\ell ({\mathsf {d}}(x_1,x_2))\) for a certain function \(\ell :[0,\infty )\rightarrow [0,\infty ]\).
In the next Proposition we recall some properties of EntropyTransport problems (for a proof see [29]).
Proposition 1
Let us suppose that the EntropyTransport problem between the measures \(\mu _1,\mu _2\in {\mathscr {M}}(X)\) is feasible, i.e. there exists \(\varvec{\gamma }\in {\mathscr {M}}(X\times X)\) such that \({\mathcal{ET}\mathcal{}}(\varvec{\gamma }\mu _1,\mu _2)<\infty \), and that F is superlinear. Then the infimum in (26) can be replaced by a minimum and the set of minimizers is a compact convex subset of \({\mathscr {M}}(X\times X)\). Moreover, the functional is convex and positively 1homogeneous (thus subadditive).
Remark 1
An important role in the theory of EntropyTransport problems is played by the marginal perspective cost H, that we are going to define.
Given a number \(c\in [0,+\infty )\) and an admissible entropy function F, we first introduce the marginal perspective function \(H_c:[0,+\infty )\times [0,+\infty )\rightarrow [0,+\infty ]\) as the lower semicontinuous envelope of the function
where R is the reverse entropy defined in (20). If \(c=+\infty \), we set
When \(\varvec{\mathrm c}:X_1\times X_2\rightarrow [0,+\infty ]\) is a lower semicontinuous cost function on two metric spaces \(X_1,X_2\), the induced marginal perspective cost
is defined as
One can give some equivalent formulations of the problem (26) in terms of the marginal perspective cost (see for instance [29, Theorem 5.8]). Moreover, the metric properties of the entropytransport cost defined in (26) can be read in terms of the properties of H, studied as a function on the space \({\mathfrak {C}}(X)\times {\mathfrak {C}}(X)\). This point of view, which links the EntropyTransport structure with the conical geometry of the problem, has been deeply investigated by Liero, Mielke and Savaré for the HellingerKantorovich distance [29, Sect. 7] (see also [10, 14] and [15, Chapters 3,4] for general marginal perspective functions).
For brevity, we do not enter into the details of all these formulations (but see Sect. 5 for some details on the conical construction performed in [39]). Here we only remark that for any complete and separable metric space \((X,{\mathsf {d}})\) the cost induces a distance on the space of measures \({\mathscr {M}}(X)\) if and only if \(H^a\) is a distance on the cone \({\mathfrak {C}}(X)\), \(a\in (0,1]\). In general, it is not difficult to identify conditions on F and \(\varvec{\mathrm c}\) for which the induced function H is nonnegative, symmetric and \(H(x_1,r;x_2,t)=0\) if and only if \((x_1,r)=(x_2,t)\) as points on the cone (see [14, Proposition 4]); on the contrary, proving the triangle inequality for (a power of) H is a much more challenging problem.
Regular EntropyTransport distances
In the next definition we introduced the class of regular EntropyTransport distances.
Definition 2
We say that is a regular EntropyTransport distance if

There exist \(a\in (0,1]\), \(F\in \Gamma _0({\mathbb {R}}_+)\) and a function \(\ell :[0,\infty )\rightarrow [0,\infty ]\) such that for every complete and separable metric space \((X,{\mathsf {d}})\), setting \(\varvec{\mathrm c}(x_1,x_2):=\ell ({\mathsf {d}}(x_1,x_2))\), the function coincides with the power a of the EntropyTransport cost induced by \((F,\varvec{\mathrm c})\), namely
(28) 
The function \(\ell \) is continuous, convex and \(\ell (s)=0\) if and only if \(s=0\).

F is superlinear and finite valued.

For every complete and separable metric space \((X,{\mathsf {d}})\), the related EntropyTransport distance is a complete and separable metric on \({\mathscr {M}}(X)\) inducing the weak topology.
We also write that the distance is induced by \((a,F,\ell )\) with obvious meaning.
We notice that if is a regular EntropyTransport distance induced by \((a,F,\ell )\) then \(\ell \) is an increasing function and \(\lim _{d\rightarrow +\infty }\ell (d)=+\infty \).
We conclude the section with a list of examples of regular EntropyTransport distances.
Examples 1

(1)
HellingerKantorovich: Let \(F(s)=U_1(s):=s\log {s}s+1\) and
It is proved in [29, Sect. 7] that induces a regular EntropyTransport distance, called HellingerKantorovich distance. We refer also to [28] for a discussion on “weighted versions” of the HellingerKantorovich distance.

(2)
Gaussian HellingerKantorovich: Let \(F(s)=U_1(s)=s\log {s}s+1\) and \(\ell _2(d):=d^2\).
The triple \((1/2,U_1,\ell _2)\) induces a regular EntropyTransport distance, as discussed in [29, Sect. 7.8]. It is called Gaussian HellingerKantorovich distance.

(3)
Quadratic powerlike distances: Let
$$\begin{aligned} F(s)=U_p(s):=\frac{s^pp(s1)1}{p(p1)}, \qquad p>1 \end{aligned}$$and \(\ell _2(d)=d^2\).
Then, for every \(1<p\le 3\) the triple \((1/2,U_p,\ell _2)\) induces a regular EntropyTransport distance, as proved in [14, Theorem 6 and Corollary 1].
We notice that the class of entropy functions \(\{U_p\}\) satisfies \(\lim _{p\rightarrow 1} U_p(s)=U_1(s)\), justifying the notation we have used (see also [29, Example 2.5]).

(4)
Linear powerlike distances: Let
$$\begin{aligned} F(s)=U_p(s):=\frac{s^pp(s1)1}{p(p1)}, \qquad p>1 \end{aligned}$$and \(\ell _1(d):=d\).
For every \(p>1\), \((1/2,U_p,\ell _1)\) induces a regular EntropyTransport distance (see again [14, Theorem 6 and Corollary 1]).
SturmEntropyTransport distance
We say that two metric measure spaces \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\) are isomorphic if there exists an isometry \(\psi :{\mathsf {supp}}(\mu _1)\rightarrow {\mathsf {supp}}(\mu _2)\) such that \(\psi _{\sharp }\,\mu _1=\mu _2\), where \(\psi _{\sharp }\) denotes the pushforward through the map \(\psi \). A necessary condition in order to be isomorphic is that \(\mu _1(X_1)=\mu _2(X_2).\)
The family of all isomorphism classes of metric measure spaces will be denoted by \(\varvec{\mathrm {X}}\). From now on, we will identify a metric measure space with its class.
We recall now the definition of the \({\mathbf {D}}_p\)distance due to Sturm.
Definition 3
([43]) Fix \(p\ge 1\). Let \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\) be two metric measure spaces, the Sturm \({\mathbf {D}}_p\)distance is defined as
where the infimum is taken over all complete and separable metric spaces \(({\hat{X}},\hat{{\mathsf {d}}})\) with isometric embeddings \(\psi ^1:{\mathsf {supp}}(\mu _1)\rightarrow {\hat{X}}\) and \(\psi ^2:{\mathsf {supp}}(\mu _2)\rightarrow {\hat{X}}\).
It is proved in [43, Theorem 3.6] that \({\mathbf {D}}_p\) is a complete, separable and geodesic metric on the set
We are now going to define the SturmEntropyTransport distance in a similar way.
Definition 4
Let \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\) be two metric measure spaces, we define the SturmEntropyTransport distance induced by the regular EntropyTransport distance as
where the infimum is taken over all complete and separable metric spaces \(({\hat{X}},\hat{{\mathsf {d}}})\) with isometric embeddings \(\psi ^1:{\mathsf {supp}}(\mu _1)\rightarrow {\hat{X}}\) and \(\psi ^2:{\mathsf {supp}}(\mu _2)\rightarrow {\hat{X}}\).
It is not difficult to prove that the definition is wellposed. Indeed, let us suppose \((X'_i,{\mathsf {d}}'_i,\mu '_i)\) is isomorphic to \((X_i,{\mathsf {d}}_i,\mu _i)\) through the map \(\varphi ^i\), \(i=1,2\). Then, for every metric space \({\hat{X}}\) and every isometric embedding \(\psi ^i:{\mathsf {supp}}(\mu _i)\rightarrow {\hat{X}}\), \(i=1,2\), we have that
It is often convenient to work with explicit realisations of the ambient space \(({\hat{X}},\hat{{\mathsf {d}}})\), a particularly useful one is given by the disjoint union that we now discuss.
Given two metric spaces \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\), let \(X_1\sqcup X_2\) be their disjoint union. We say that a (resp. pseudo)metric \(\hat{{\mathsf {d}}}\) on \(X_1\sqcup X_2\) is a (resp. pseudo)metric coupling between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) if \(\hat{{\mathsf {d}}}(x,y)={\mathsf {d}}_1(x,y)\) when \(x,y\in X_1\) and \(\hat{{\mathsf {d}}}(x,y)={\mathsf {d}}_{2}(x,y)\) when \(x,y\in X_2\).
A finite valued metric coupling \(\hat{{\mathsf {d}}}\) between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) always exists: to construct it, fix two points \({\bar{x}}_1\in X_1, {\bar{x}}_2\in X_2\), a number \(c\in {\mathbb {R}}_+\), and define \(\hat{{\mathsf {d}}}\) as:
Moreover, from any finite valued pseudometric coupling \(\hat{{\mathsf {d}}}\) of \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) and any \(\delta >0\) we can obtain a complete, separable metric \(\hat{{\mathsf {d}}}_{\delta }\) which is again a coupling of \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) in the following way:
We say that a measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) is a measure coupling between \(\mu _1\) and \(\mu _2\) if
for all Borel sets \(A\subset X_1\) and \(B\subset X_2\). We keep the notation \(\gamma _i\) for the marginals of the measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\), \(i=1,2.\)
A more explicit formulation of the function is given in the following Proposition.
Proposition 2
Let \((X_1,{\textsf {d} }_1,\mu _1)\) and \((X_2,{\textsf {d} }_2,\mu _2)\) be two metric measure spaces and a regular EntropyTransport distance induced by \((a,F,\ell )\).

(i)
In Definition 4 we can suppose without loss of generality that \({\hat{X}}=X_1\sqcup X_2\), \(\psi ^1=\iota _1\), \(\psi ^2=\iota _2\) be respectively the inclusion of \(X_1\) and \(X_2\) in \(X_1\sqcup X_2\) and the infimum is taken over all the pseudometric couplings \({\hat{\textsf {d} }}\) between \({\textsf {d} }_1\) and \({\textsf {d} }_2\).

(ii)
In the situation of (i) we will identify \(\mu _k\) with \((\iota _k)_\sharp \,\mu _k\), \(k=1,2\), and it holds
(34)where
$$\begin{aligned} C:= & {} \{(\varvec{\gamma },{\hat{{\mathsf {d}}}}): \varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2), \nonumber \\&\ {\hat{{\mathsf {d}}}} \ \text {finite valued pseudometric coupling for} \ {\mathsf {d}}_1,{\mathsf {d}}_2\} \end{aligned}$$(35)
Proof
\(\mathrm{(i)}\) We first show that the infimum as in \(\mathrm{(i)}\) is less or equal to the infimum as in Definition 4. Let \(({\hat{X}},\hat{{\mathsf {d}}})\) be a complete and separable metric space with isometric embeddings \(\psi ^1:{\mathsf {supp}}(\mu _1)\rightarrow {\hat{X}}\), \(\psi ^2:{\mathsf {supp}}(\mu _2)\rightarrow {\hat{X}}\), and let \(\hat{\varvec{\gamma }}\in {\mathscr {M}}({\hat{X}}\times {\hat{X}})\). It is immediate to check that
defines a pseudometric on \(X_1\sqcup X_2\), coupling between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\).
Moreover, setting the Borel injective functions \(\Psi ^i: \psi ^1({\mathsf {supp}}(\mu _1))\cup \psi ^2({\mathsf {supp}}(\mu _2))\subset {\hat{X}} \rightarrow X_1\sqcup X_2\), \(i=1,2\), defined as
and using Lemma 4 it is immediate to check that \(\tilde{\varvec{\gamma }}:= (\Psi ^1,\Psi ^2)_{\sharp } \hat{\varvec{\gamma }}\in {\mathscr {M}}((X_1\sqcup X_2)\times (X_1\sqcup X_2))\) satisfies
where we have used the fact that \(\tilde{{\mathsf {d}}}(\Psi ^1(x),\Psi ^2(y))\le \hat{{\mathsf {d}}}(x,y)\) whenever \(x\in \psi ^1({\mathsf {supp}}(\mu _1))\) and \(y\in \psi ^2({\mathsf {supp}}(\mu _2)).\)
This yields that the infimum as in \(\mathrm{(i)}\) is less or equal to the infimum as in Definition 4.
To show that the infimum as in Definition 4 is less or equal to the infimum as in \(\mathrm{(i)}\), it is sufficient to notice that for every pseudometric coupling \({\hat{{\mathsf {d}}}}\), for every measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) and for every \(\epsilon >0\) there is \(\delta >0\) such that the complete and separable metric \(\hat{{\mathsf {d}}}_{\delta }\) defined in (32) is a coupling between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) satisfying
as a consequence of the finiteness of the measure \(\varvec{\gamma }\) and the continuity of \(\ell \).
\(\mathrm{(ii)}\) In case the infimum runs over the couples \((\varvec{\gamma },{\hat{{\mathsf {d}}}})\in C\) such that \({\hat{{\mathsf {d}}}}\) is a complete and separable metric, the inequality “\(\le \)” in (34) is a simple consequence of the explicit formulation of the EntropyTransport problem together with the fact that the superlinearity of F allows to consider measures \(\varvec{\gamma }\in {\mathscr {M}}((X_1\sqcup X_2)\times (X_1\sqcup X_2))\) with support contained in \(X_1\times X_2\). The fact that “\(\le \)” holds in (34) even if the infimum is taken over the larger set C is a consequence of (38).
The proof of the inequality “\(\ge \)” in (34) is analogous to the first part of the proof of \(\mathrm{(i)}\), see in particular (37). \(\square \)
In the next Lemma we collect some of the basic properties of the function .
Lemma 5
Let be a regular EntropyTransport distance induced by \((a,F,\ell )\).

(i)
For any \(M\ge 0\) it holds
(39) 
(ii)
If \((X_1,{\textsf {d} }_1)=(X_2,{\textsf {d} }_2)\) then
(40) 
(iii)
The set
$$\begin{aligned} \varvec{\mathrm {X}}_{*}:=\left\{ (X,{\textsf {d} },\mu )\in \varvec{\mathrm {X}}, \ {\textsf {supp} }(\mu )=\{x_1,\ldots ,x_n\}, \ n \in {\mathbb {N}}, \ \mu =M\sum _{i=1}^n \delta _{x_i}, \ M\in {\mathbb {R}}_{+}\right\} \end{aligned}$$(41)is dense in .

(iv)
If
$$\begin{aligned} \mu =M\sum _{i=1}^n \delta _{x_i} \quad \text {and} \quad \mu '=M\sum _{i=1}^n \delta _{x'_i}, \end{aligned}$$(42)then
(43)where we put \({\textsf {d} }_{ij}={\textsf {d} }(x_i,x_j)\) and \({\textsf {d} }'_{ij}={\textsf {d} }(x'_i,x'_j)\).

(v)
For any \(N>1\) there exists a constant C such that for every M, \(1/N<M<N\), we have
(44)
Proof

(i)
This is a consequence of the 1homogeneity of the cost (Proposition 1) and of the pushforward map together with the definitions of and .

(ii)
The result follows from the definition of , since \(({\hat{X}},{\hat{{\mathsf {d}}}})=(X_1,{\mathsf {d}}_1)\) with \(\psi _1=\psi _2=\mathrm {Id}\) is an admissible competitor for the infimum.

(iii)
The result follows by the point (ii) of the present Lemma, the fact that metrizes the weak convergence and the density in \({\mathscr {M}}(X)\) of the measures \(\mu \) of the form \(M\sum _{i=1}^n \delta _{x_i}\) with respect to weak convergence.

(iv)
Let assume without loss of generality that \(X=\{x_1,\ldots ,x_n\}\) and \(X'=\{x'_1,\ldots ,x'_n\}\). We put \(\delta =\sup _{i,j}{\mathsf {d}}_{ij}{\mathsf {d}}'_{ij}\). We construct the following pseudometric coupling: on \(X\times X\) we define \({\hat{{\mathsf {d}}}}={\mathsf {d}}\), on \(X'\times X'\) we put \({\hat{{\mathsf {d}}}}={\mathsf {d}}'\), on \(X\times X'\) we define
$$\begin{aligned} {\hat{{\mathsf {d}}}}(x_i,x'_j):=\inf _{k\in \{1,\ldots ,n\}}{\mathsf {d}}(x_i,x_k)+{\mathsf {d}}'(x'_k,x'_j)+\delta , \end{aligned}$$finally on \(X'\times X\) we put
$$\begin{aligned} {\hat{{\mathsf {d}}}}(x'_i,x_j):=\inf _{k\in \{1,\ldots ,n\}}{\mathsf {d}}(x_j,x_k)+{\mathsf {d}}'(x'_k,x'_i)+\delta , \end{aligned}$$so that \({\hat{{\mathsf {d}}}}(x_i,x'_i)={\hat{{\mathsf {d}}}}(x'_i,x_i)=\delta .\)
We then define the measure coupling
$$\begin{aligned} \varvec{\gamma }=M\sum _{i=1}^n\delta _{(x_i,x'_i)}. \end{aligned}$$It is straightforward to see that \({\hat{{\mathsf {d}}}}\) and \(\varvec{\gamma }\) are actually couplings between \({\mathsf {d}},{\mathsf {d}}'\) and \(\mu ,\mu '\), respectively. Then, using Proposition 2 and recalling that \(\ell \) is an increasing function we have that
and the thesis follows.

(v)
We can take \({\mathsf {d}}\) itself as metric coupling. Then, by the point (ii) of the present Lemma, we have
By replacing the cost \(\varvec{\mathrm c}\) with the cost
$$\begin{aligned} \varvec{\mathrm c}_{\infty }(x_1,x_2):={\left\{ \begin{array}{ll} 0 &{} \text {if } x_1=x_2\\ +\infty &{} \text {otherwise}, \end{array}\right. } \end{aligned}$$we obtain that
where we have denoted by the EntropyTransport problem induced by the entropy function F and the cost \(\varvec{\mathrm c}_{\infty }.\) Observe that every admissible entropy function satisfies
$$\begin{aligned} F(s)\le Cs1, \quad \text {for every} \ \ 1/N<s<N, \end{aligned}$$(45)where
$$\begin{aligned} C:=\max \left\{ \frac{F(1/N)}{1/N1},\frac{F(N)}{N1}\right\} . \end{aligned}$$The conclusion now follows from an explicit computation of together with the bound (45). Indeed, we have (see [29, Example E.5])
(46)
\(\square \)
The next Lemma shows the existence of the optimal couplings.
Lemma 6
Let be a regular EntropyTransport distance induced by \((a,F,\ell )\). Let \((X_1,{\textsf {d} }_1,\mu _1)\) and \((X_2,{\textsf {d} }_2,\mu _2)\) be two metric measure spaces. Then:

(i)
here exist a measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) and a pseudometric coupling \({\hat{\textsf {d} }}\) between \({\textsf {d} }_1\) and \({\textsf {d} }_2\) such that
(47) 
(ii)
There exist a complete and separable metric space \(({\tilde{X}},{\tilde{{\mathsf {d}}}})\) and isometric embeddings \(\psi ^1:{\textsf {supp} }(\mu _1)\rightarrow {\tilde{X}}\), \(\psi ^2:{\textsf {supp} }(\mu _2)\rightarrow {\tilde{X}}\) such that
(48)where we have denoted by the EntropyTransport distance computed in the space \(({\tilde{X}},{\tilde{{\mathsf {d}}}})\).
Proof

(i)
\(\mathbf {Step\, 1}\): tightness of the plans.
By Proposition 2 there exist a sequence \(\varvec{\gamma }_n\in {\mathscr {M}}(X_1\times X_2)\) and \(\hat{{\mathsf {d}}}_n\) pseudometric couplings of \({\mathsf {d}}_1,{\mathsf {d}}_2\) such that
(49)Since the entropy functionals with respect to the fixed measures \(\mu _1\) and \(\mu _2\) are bounded, we can apply Theorems 1 and Lemma 3 in order to obtain the existence of subsequences (from now on we will not relabel them) such that \((\gamma _n)_i\) converges weakly to some \(\gamma ^i\in {\mathscr {M}}(X_i)\), \(i=1,2\). Since \((\gamma _n)_i\) are marginals of the measure \(\varvec{\gamma }_n\), the tightness of \((\gamma _n)_i\) implies the tightness of \(\varvec{\gamma }_n\), so that the sequence \(\varvec{\gamma }_n\in {\mathscr {M}}(X_1\times X_2)\) is converging to some \(\varvec{\gamma }\). Moreover, by the continuity of the operator \(\pi ^i_{\sharp }\) with respect to the weak topology, the marginals of \(\varvec{\gamma }\) coincide with \(\gamma ^i\), \(i=1,2.\) We notice that if \(\varvec{\gamma }\) is the null measure the proof is concluded by taking any pseudometric coupling \(\hat{{\mathsf {d}}}\) between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\).
\(\mathbf {Step\, 2}\): precompactness of the pseudometric couplings.
Regarding the sequence \(\hat{{\mathsf {d}}}_n\), by the triangle inequality we have that
$$\begin{aligned} \hat{{\mathsf {d}}}_n(x_1,y_1)\hat{{\mathsf {d}}}_n(x_2,y_2)\le {\mathsf {d}}_1(x_1,x_2)+{\mathsf {d}}_2(y_1,y_2). \end{aligned}$$In particular, \(\hat{{\mathsf {d}}}_n\) is uniformly 1Lipschitz with respect to the complete and separable metric \({\mathsf {d}}_1+{\mathsf {d}}_2\) on \(X_1\times X_2\). We claim it is also uniformly bounded in a point. To see this, take \(({\bar{x}},{\bar{y}})\in {\mathsf {supp}}(\varvec{\gamma })\): since \(\varvec{\gamma }_n\) weakly converges to \(\varvec{\gamma }\) for every \(r,\epsilon >0\) and for all n sufficiently large we have
$$\begin{aligned} \varvec{\gamma }_n\left( B_r({\bar{x}})\times B_r({\bar{y}})\right) \ge \varvec{\gamma }\left( B_r({\bar{x}})\times B_r({\bar{y}})\right) \epsilon . \end{aligned}$$Fix \(r>0\) and suppose by contradiction that there exists a subsequence (not relabeled) such that \(2r\le \hat{{\mathsf {d}}}_n({\bar{x}},{\bar{y}})\rightarrow +\infty \). For \(\epsilon =\epsilon (r)\) small enough, from (49), the fact that \(({\bar{x}},{\bar{y}})\in {\mathsf {supp}}(\varvec{\gamma })\) and \(\ell \) is increasing we infer the existence of some positive constants C, c such that for all n sufficiently large
$$\begin{aligned}&C>\int _{X_1\times X_2}\ell \left( \hat{{\mathsf {d}}}_n(x,y)\right) {\mathrm d}\varvec{\gamma }_n(x,y)\\&\quad \ge \int _{B_r({\bar{x}})\times B_r({\bar{y}})}\ell \left( \hat{{\mathsf {d}}}_n({\bar{x}},{\bar{y}})2r\right) {\mathrm d}\varvec{\gamma }_n(x,y)\\&\quad \ge \ell \left( \hat{{\mathsf {d}}}_n({\bar{x}},{\bar{y}})2r\right) [\varvec{\gamma }(B_r({\bar{x}})\times B_r({\bar{y}}))\epsilon ]\ge c\ell \left( \hat{{\mathsf {d}}}_n({\bar{x}},{\bar{y}})2r\right) . \end{aligned}$$Since \(\ell \) has bounded sublevels, this implies that there exists a constant K such that \(\hat{{\mathsf {d}}}_n({\bar{x}},{\bar{y}})<K\) for every n that leads to a contradiction.
We can thus apply AscoliArzelà’s theorem to infer the existence of a limit function \({\mathsf {d}}:X_1\times X_2\rightarrow [0,\infty )\) such that \({\mathsf {d}}_n\) converges (up to subsequence) pointwise to \({\mathsf {d}}\) and the convergence is uniform on compact sets. We can extend \({\mathsf {d}}\) to \((X_1 \sqcup X_2)\times (X_1 \sqcup X_2)\) in order to get a limit pseudometric coupling, that we denote in the same way.
\(\mathbf {Step\, 3}\): passing to the limit.
Next, we pass to the limit in the following expression
$$\begin{aligned} \sum _{i=1}^2D_F((\gamma _n)_i\mu _i)+\int _{X_1\times X_2}\ell \left( \hat{{\mathsf {d}}}_n(x,y)\right) {\mathrm d}\varvec{\gamma }_n. \end{aligned}$$By Lemma 2, the entropy is jointly lower semicontinuous and thus
$$\begin{aligned} \liminf _n D_F((\gamma _n)_i\mu _i)\ge D_F(\gamma _i\mu _i). \end{aligned}$$So, it is sufficient to prove that
$$\begin{aligned} \liminf _n \int _{X_1\times X_2}\ell \left( \hat{{\mathsf {d}}}_n(x,y)\right) {\mathrm d}\varvec{\gamma }_n \ge \int _{X_1\times X_2}\ell \left( \hat{{\mathsf {d}}}(x,y)\right) {\mathrm d}\varvec{\gamma }. \end{aligned}$$(50)Using the equitightness of \(\{\varvec{\gamma }_k\}\) we can find a sequence of compact sets \(K_{1,n}\subset X_1\) and \(K_{2,n}\subset X_2\) such that
$$\begin{aligned} \varvec{\gamma }_k\left( X_1\times X_2 {\setminus } (K_{1,n}\times K_{2,n})\right) \le \frac{1}{n} \end{aligned}$$for every k. We define \(\ell _m(r):=\min (\ell (r),m),\) so that the sequence of functions \((x,y)\mapsto \ell _m({\mathsf {d}}_n(x,y))\) converges uniformly on compact subsets of \(X_1\times X_2\), as \(n\rightarrow \infty \). Possibly by taking a further subsequence via a diagonal argument, we can infer that \(\Vert \ell _m({\mathsf {d}})\ell _m({\mathsf {d}}_n)\Vert _{\infty ;n}\rightarrow 0\) when \(n\rightarrow \infty \), where we denote by \(\Vert \cdot \Vert _{\infty ;n}\) the supremum norm in the set \(K_{1,n}\times K_{2,n}.\) Let M be a positive constant such that \(\gamma _n(X_1\times X_2)\le M\) for every n. We can bound the integral on the left hand side of (50) in the following way:
$$\begin{aligned}&\int _{X_1\times X_2} \ell (\hat{{\mathsf {d}}}_n) {\mathrm d}\varvec{\gamma }_n \ge \int _{X_1\times X_2} \ell _m(\hat{{\mathsf {d}}}_n) {\mathrm d}\varvec{\gamma }_n \ge \int _{K^1_n\times K^2_n} \ell _m(\hat{{\mathsf {d}}}_n) {\mathrm d}\varvec{\gamma }_n \\&\quad \ge \int _{K^1_n\times K^2_n} \ell _m(\hat{{\mathsf {d}}}) {\mathrm d}\varvec{\gamma }_n  M \Vert \ell _m(\hat{{\mathsf {d}}})\ell _m(\hat{{\mathsf {d}}}_n)\Vert _{\infty ;n} \\&\quad \ge \int _{X_1\times X_2} \ell _m(\hat{{\mathsf {d}}}) {\mathrm d}\varvec{\gamma }_n  M \Vert \ell _m(\hat{{\mathsf {d}}})\ell _m(\hat{{\mathsf {d}}}_n)\Vert _{\infty ;n}  m /n. \end{aligned}$$Now we can pass to the limit with respect to n using the weak convergence of \(\{\varvec{\gamma }_n\}\), and we obtain
$$\begin{aligned} \liminf _{n} \int _{X_1\times X_2} \ell (\hat{{\mathsf {d}}}_n) {\mathrm d}\varvec{\gamma }_n\ge \int _{X_1\times X_2} \ell _m(\hat{{\mathsf {d}}}) {\mathrm d}\varvec{\gamma } \end{aligned}$$and then we conclude using the Beppo Levi’s monotone convergence theorem with respect to m.

(ii)
Without loss of generality we assume \({\mathsf {supp}}(\mu _i)=X_i\). By the previous point we know the existence of an optimal measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) and an optimal pseudometric coupling \(\hat{{\mathsf {d}}}\) between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\). We consider the complete and separable metric space \(({\tilde{X}},{\tilde{{\mathsf {d}}}})\) constructed as in Lemma 1. Denoting by \(p: X_1\sqcup X_2\rightarrow {\tilde{X}}\) the projection to the quotient and using the identification
$$\begin{aligned} X_1\sqcup X_2=X_1\times \{0\}\cup X_2\times \{1\}, \end{aligned}$$we notice that \(X_1\times X_2 \hookrightarrow {\tilde{X}} \times {\tilde{X}}\) via the injective Borel map
$$\begin{aligned} \varvec{\psi }(x_1,x_2)=(\psi ^1(x_1),\psi ^2(x_2)):=(p(x_1,0),p(x_2,1)). \end{aligned}$$Moreover, we also have that \(\psi ^i\) is an isometry of \((X_i,{\mathsf {d}}_i)\) onto its image in \(({\tilde{X}},{\tilde{{\mathsf {d}}}})\), \(i=1,2\). Thus, denoting by \(\gamma _i\) the marginals of \(\varvec{\gamma }\), we can consider the measures \(\varvec{\psi }_{\sharp }\varvec{\gamma }\) whose projections are \((\psi ^1)_{\sharp }\gamma _1\) and \((\psi ^2)_{\sharp }\gamma _2\). Using Lemma 4 we know that
$$\begin{aligned} D_F(\gamma _i\mu _i)=D_F((\psi ^i)_{\sharp }\gamma _i\,\,(\psi ^i)_{\sharp }\mu _i), \qquad i=1,2. \end{aligned}$$(51)By recalling the definition of \({\tilde{{\mathsf {d}}}}\), we also have
$$\begin{aligned} \int _{X_1\times X_2}\ell \left( \hat{{\mathsf {d}}}(x,y)\right) {\mathrm d}\varvec{\gamma }=\int _{{\tilde{X}}\times {\tilde{X}}}\ell \left( {\tilde{{\mathsf {d}}}}(x,y)\right) {\mathrm d}(\varvec{\psi }_{\sharp }\varvec{\gamma }). \end{aligned}$$(52)Thus, as a consequence of (51), (52) and the optimality of \(\varvec{\gamma }\) and \({\hat{{\mathsf {d}}}}\), the equality (48) holds on \(({\tilde{X}},{\tilde{{\mathsf {d}}}})\) (with optimal measure \(\varvec{\psi }_{\sharp }\varvec{\gamma }\)).
\(\square \)
Remark 2
It is clear that the optimal coupling \(\hat{{\mathsf {d}}}\) whose existence is proven in the previous Lemma is in general only a pseudometric and not a metric on \(X_1\sqcup X_2\). To see this, it is sufficient to consider two isomorphic metric measure spaces \((X_1,{\mathsf {d}}_1,\mu _1)\), \((X_2,{\mathsf {d}}_2,\mu _2)\). If we denote by \(\psi :X_1\rightarrow X_2\) the isometry between \((X_1,{\mathsf {d}}_1)\) and \((X_2,{\mathsf {d}}_2)\), the optimal coupling \(\hat{{\mathsf {d}}}\) satisfies \({\hat{{\mathsf {d}}}}(x_1,\psi (x_1))=0\) for \(\mu _1\)a.e \(x_1\).
The next theorem is the main result of the paper.
Theorem 2
Let be a regular EntropyTransport distance induced by \((a,F,\ell )\). Then is a complete and separable metric space. It is also a length (resp. geodesic) space if is a length (resp. geodesic) metric.
Proof
\(\mathbf {Step\, 1}\): defines a metric.
It is clear that is symmetric, finite valued, nonnegative and
We claim that implies that the metric measure spaces \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\) are isomorphic. By Lemma 6 there exist a measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) and a pseudometric coupling \(\hat{{\mathsf {d}}}\) such that
All the terms are nonnegative, so that \(D_F\big (\gamma _i\mu _i\big )=0\) and thus \(\gamma _i=\mu _i\), \(i=1,2\). Moreover, since \(\ell (d)=0\) if and only if \(d=0\), it follows that \(\hat{{\mathsf {d}}}(x,y)=0\) for \(\varvec{\gamma }\)a.e (x, y). We also have
To see this, let \(({\bar{x}},{\bar{y}})\in {\mathsf {supp}}(\varvec{\gamma })\) so that for every \(r>0\) we have \(\varvec{\gamma }(B_r({\bar{x}},{\bar{y}}))>0\) where
We consider a sequence of balls of radius \(r_n:=1/n, n\in {\mathbb {N}},\) and use the fact that \(\hat{{\mathsf {d}}}(x,y)=0\) for \(\varvec{\gamma }\)a.e (x, y) to infer the existence of a sequence of points \((x_n, y_n)\in B_{r_n}(({\bar{x}},{\bar{y}}))\) such that \(\hat{{\mathsf {d}}}(x_n,y_n)=0\). Thus
Sending \(n\rightarrow +\infty \) and using the arbitrariness of \(({\bar{x}},{\bar{y}})\), the claim (53) follows.
Since \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) are metrics, we infer that for every \(x_{1}\in {\mathsf {supp}}(\mu _{1})\) there exists a unique \(x_{2}\in {\mathsf {supp}}(\mu _{2})\) such that \((x_{1},x_{2})\in {\mathsf {supp}}(\varvec{\gamma })\). Indeed, for any \(x_2,{\tilde{x}}_{2}\in {\mathsf {supp}}(\mu _{2})\) such that \((x_{1},x_{2}), (x_{1},{\tilde{x}}_{2})\in {\mathsf {supp}}(\varvec{\gamma })\) we have
and thus \(x_2={\tilde{x}}_2\). Switching the role of \(X_{1}\) and \(X_{2}\) in the argument above, we obtain the existence of a bijection \(\psi : {\mathsf {supp}}(\mu _{1}) \rightarrow {\mathsf {supp}}(\mu _{2})\) such that \(\varvec{\gamma }=(\mathrm {Id},\psi )_{\sharp }\mu _1\) and (in virtue of (53))
Let \(x,y\in {\mathsf {supp}}(\mu _{1}) \), from (54) and the triangle inequality it follows
which implies that \(\psi : {\mathsf {supp}}(\mu _{1}) \rightarrow {\mathsf {supp}}(\mu _{2})\) is an isometry.
Hence \((X_1,{\mathsf {d}}_1,\mu _1)\) and \((X_2,{\mathsf {d}}_2,\mu _2)\) are isomorphic, as claimed.
Regarding the triangle inequality, let \((X_i,{\mathsf {d}}_i,\mu _i)\), \(i=1,2,3\), be three metric measure spaces. From the definition of and Proposition 2, for every \(\epsilon >0\) we find a pseudometric coupling \({\mathsf {d}}_{12}\) between \({\mathsf {d}}_{1}\) and \({\mathsf {d}}_{2}\), and a pseudometric coupling \({\mathsf {d}}_{23}\) between \({\mathsf {d}}_{2}\) and \({\mathsf {d}}_{3}\) such that
where we have denoted by the EntropyTransport distance induced by the pseudometric \({\mathsf {d}}\). Set \(X:=X_1\sqcup X_2\sqcup X_3\) and define a pseudometric \({\mathsf {d}}\) on X in the following way
We notice that \({\mathsf {d}}\) coincides with \({\mathsf {d}}_i\) when restricted to \(X_i\). By applying Proposition 2, the point (ii) of Lemma 5 and the triangle inequality of we obtain
The conclusion follows since \(\epsilon >0\) is arbitrary.
\(\mathbf {Step\, 2}\): Completeness of .
In order to prove completeness, let \(\{(X_n,{\mathsf {d}}_n,\mu _n)\}_{n\in {\mathbb {N}}}\) be a Cauchy sequence in the space . In order to have convergence of the full sequence, it is enough to prove that there exists a converging subsequence. Let us consider a subsequence such that
By definition of and Proposition 2, we can find a measure \(\varvec{\gamma }_{k+1}\in {\mathscr {M}}(X_{n_k}\times X_{n_{k+1}})\) and a complete and separable metric coupling \(\hat{{\mathsf {d}}}_{k+1}\) between \({\mathsf {d}}_{X_{n_k}}\) and \({\mathsf {d}}_{X_{n_{k+1}}}\) such that
where \(\sigma _{n_k}\) (resp. \(\sigma _{n_{k+1}}\)) is the RadonNykodim derivative of the first (resp. second) marginal of \(\gamma _{k+1}\) with respect to \(\mu _{n_k}\) (resp. \(\mu _{n_{k+1}}\)).
Now we want to define a sequence \(\big \{(X'_k,{\mathsf {d}}'_k)\big \}_{k=1}^{\infty }\) of metric spaces such that \(X_{n_k}\subset X'_k\) and \(X'_k\subset X'_{k+1}\). We proceed in the following way: we set
where \(x\sim y\) if \({\mathsf {d}}'_{k+1}(x,y)=0\) and the latter is defined as
From the definition of \({\mathsf {d}}'_k\), it is clear that we can endow the space \(X':=\bigcup _{k=1}^{\infty } X'_k\) with a limit metric \({\mathsf {d}}'\). Now we consider the completion \((X,{\mathsf {d}})\) of \((X',{\mathsf {d}}')\) and we notice that \((X_{n_k},{\mathsf {d}}_{X_{n_k}})\) is isometrically embedded in this space for every k. Using the embedding, we can also define a measure \({\bar{\mu }}_{n_k}\) as the pushforward of the measure \(\mu _{n_k}.\) Combining the construction above with (55) gives
where is the regular EntropyTransport distance computed in the space \((X,{\mathsf {d}}).\) In particular, (56) implies that \(({\bar{\mu }}_{n_{k}})_{k\in {\mathbb {N}}}\) is a Cauchy sequence in . Since is complete, there exists \(\mu \in {\mathscr {M}}(X)\) such that .
Using again that \((X_{n_k},{\mathsf {d}}_{X_{n_k}})\) is isometrically embedded in \((X,{\mathsf {d}})\) and the point (ii) of Lemma 5, we can conlude that
\(\mathbf {Step\, 3}\): Separability of .
Thanks to (iii) of Lemma 5 it is enough to show that the set \(\varvec{\mathrm {X}}_*\), defined in (41), is separable. To this aim, we notice that \(\varvec{\mathrm {X}}_*\) can be written as \(\bigsqcup _{n\in {\mathbb {N}}} \tilde{{\mathcal {K}}}_n\) where
Since the set of all \((D,M)=(D_{ij},M)\in {\mathbb {R}}_+^{n\times n}\times {\mathbb {R}}_{+}\) such that
is separable (as a subset of the Euclidean space), using (iv) of Lemma 5 we get that
is separable for every fixed \(n\in {{\mathbb {N}}}, M>0\). The separability of \(\tilde{{\mathcal {K}}}_n\) follows by the separability of \(\tilde{{\mathcal {K}}}_{n,M}\) combined with (v) of Lemma 5.
\(\mathbf {Step\, 4}\): Length/geodesic property of .
Let us start by proving the length property. Let \((X_1,{\mathsf {d}}_1,\mu _1),\, (X_2,{\mathsf {d}}_2,\mu _2) \in \varvec{\mathrm {X}}\). By definition of , for every \(\varepsilon >0\) we can find a complete and separable metric space \((X, {\mathsf {d}})\) and isometric embeddings \(\psi ^i: {\mathsf {supp}}(\mu _i) \rightarrow X\), \(i=1,2\), such that
where, as before, we identify \({\mathsf {supp}}(\mu _i)\) with its isometric image \(\psi ^i({\mathsf {supp}} (\mu _i))\), and correspondingly \(\mu _i\) with \(\psi ^i_\sharp \mu _{i}\), \(i=1,2\), in order to keep notation short.
Recall that, by slightly modifying the classical Kuratowski embedding, one can show that every complete and separable metric space can be isometrically embedded in a complete, separable and geodesic metric space (see for instance [23, Exercise 1c. Ch. 3\(\frac{1}{2}.1\)] or [22, Proposition 1.2.12]). Thus, recalling also Lemma 4, without loss of generality we can assume that the complete and separable metric space \((X,{\mathsf {d}})\) above is also geodesic.
By assumption is a length distance on \({\mathscr {M}}(X)\) since \((X,{\mathsf {d}})\) is a length space, so that we can find a curve from \(\mu _1\) to \(\mu _2\) satisfying
Now, it is easy to check that the length of the curve of m.m.s. \(((X, {\mathsf {d}}, \mu _{t}))_{t\in [1,2]}\subset \varvec{\mathrm {X}}\) satisfies
Indeed the length of a curve is by definition the supremum of the sums of mutual distances over finite partitions (13), and for every partition \((t_{i})\) of [1, 2] it holds
The combination of (59), (60) and (61) gives
as desired.
To prove the geodesic property in the case is a geodesic distance, we notice that we can follow verbatim the argument given above with \(\varepsilon =0\). Here one has to notice that the existence of an optimal complete and separable metric space on which (59) holds with \(\varepsilon =0\) follows from (ii) of Lemma 6. \(\square \)
Remark 3
It is proved in [29, Proposition 8.3] that is a geodesic space when the underlying space \((X,{\mathsf {d}})\) is geodesic. In particular, the last claim of Theorem 2 can be applied to the HellingerKantorovich distance.
To the best of our knowledge, up to now this is the only known example of regular EntropyTransport geodesic distance (with the trivial exception of weighted variants of [28]).
Topology
Let us introduce a notion of convergence for sequences of (equivalence classes of) metric measure spaces (see [21, Definition 3.9] for the corresponding notion in the context of pointed metric measure spaces).
Definition 5
We say that a sequence \((X_n,{\mathsf {d}}_n,\mu _n)_{n\in {\mathbb {N}}}\) weakly measuredGromov converges to \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) if there exist a complete and separable metric space \((X,{\mathsf {d}})\) and isometric embeddings \(\iota _n:X_n\rightarrow X\), \(n\in \bar{{\mathbb {N}}}\), such that \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) weakly in \({\mathscr {M}}(X)\).
In the next Theorem we see that this notion of convergence actually coincides with the convergence induced by any SturmEntropyTransport distance.
Theorem 3
Let be a regular EntropyTransport distance induced by \((a,F,\ell )\). A sequence \((X_n,{\textsf {d} }_n,\mu _n)_{n\in {\mathbb {N}}}\) weakly measured Gromov converges to \((X_{\infty },{\textsf {d} }_{\infty },\mu _{\infty })\) if and only if
Proof
Let us suppose the validity of (62). By definition of we know that there exist a complete and separable metric space \((Y_n,{\mathsf {d}}_{Y_n})\) and isometric embeddings \(\psi _n,\psi _n^{\infty }\) of \((X_n,{\mathsf {d}}_n)\), \((X_{\infty },{\mathsf {d}}_{\infty })\) respectively, in \(Y_n\) such that
where is computed in the space \(Y_n\). We now define \(Y:=\sqcup _n X_n\), \(n\in \bar{{\mathbb {N}}}\) endowed with the pseudometric \({\mathsf {d}}_Y\)
We now consider the space \(Y/\sim \) defined as the quotient of Y with respect to the equivalence relation
and we then define the completion of this space, that we still denote by \((Y,{\mathsf {d}}_Y)\). It is easy to see that Y is separable. By construction we notice that the set
endowed with the distance \({\mathsf {d}}_{Y_n}\) is canonically isometrically embedded in \((Y,{\mathsf {d}}_Y)\), so that every space \(X_n\), \(n\in \bar{{\mathbb {N}}}\), is canonically isometrically embedded into Y by a map \(\psi _n'\). We claim now that Y and \(\psi _n'\) provide a realization of the weakly measured Gromov convergence. To see this, it is enough to notice that \((\psi _n')_{\sharp }\mu _n\rightarrow (\psi _{\infty }')_{\sharp }\mu _{\infty }\) weakly in \({\mathscr {M}}(Y)\) which is a consequence of the construction of \(\psi _n'\), (63) and the fact that induces the weak topology.
For the converse, let us suppose that \((X_n,{\mathsf {d}}_n,\mu _n)_{n\in {\mathbb {N}}}\) weakly measured Gromov converges to \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\). By definition we know that there exist a complete and separable metric space \((X,{\mathsf {d}})\) and isometric embeddings \(\iota _n:X_n\rightarrow X\), \(n\in \bar{{\mathbb {N}}}\), such that \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) weakly in \({\mathscr {M}}(X)\). Since metrizes the weak convergence on \({\mathscr {M}}(X)\) we know that
and the result follows by the very definition of , noticing that \((X,{\mathsf {d}})\) is a possible competitor. \(\square \)
Let us denote by \(\varvec{\mathrm {X}}(K,N,L,v,V)\) the family of isomorphism classes of metric measure spaces \((X,{\mathsf {d}},\mu ) \in {\mathsf {CD}}(K,N)\) such that
Let \(\varvec{\tilde{\mathrm {X}}}(K,N,L,v,V)\) be the family of isomorphism classes of spaces
such that \(\mu \) has full support.
Theorem 4
Fix \(K\in {\mathbb {R}}\), \(N\in (1,\infty )\), \(L\in (0,\infty )\) and \(0<v\le V<\infty \). Let be a regular EntropyTransport distance. Then

\(\varvec{\mathrm {X}}(K,N,L,v,V)\) is compact with respect to .

\(\varvec{\tilde{\mathrm {X}}}(K,N,L,v,V)\) is compact with respect to mGH. Moreover on such family the topology and the mGHtopology coincide.
Proof
By [21, Corollary 3.22] we have precompactness of \(\varvec{\mathrm {X}}(K,N,L,v,V)\) with respect to the weakly measured Gromov convergence and thus precompactness with respect also to the convergence by Theorem 3. From [44, Theorem 3.1] (see also [21, Theorem 4.9]) we know that the condition \({\mathsf {CD}}(K,N)\) is stable with respect to the weakly measured Gromov convergence and thus the first statement follows. For the second statement we observe that the spaces in \(\varvec{\tilde{\mathrm {X}}}(K,N,L,v,V)\) are uniformly doubling and thus the weakly measured Gromov convergence is equivalent to the mGHconvergence (see [21, Theorem 3.30 and 3.33]). \(\square \)
Corollary 1
It follows:

(i)
The stability of \({\textsf {CD} }(K,N)\) with \(N\in (1,\infty ]\) under convergence.

(ii)
The convergence of heat flows under convergence of \({\textsf {CD} }(K,\infty )\) spaces.

(iii)
The stability of \({\textsf {RCD} }(K,N)\) with \(N\in (1,\infty ]\) under convergence.

(iv)
The stability of the spectrum of the Laplacian under convergence of \({\textsf {CD} }(K,\infty )\) spaces.
Proof
The proof is a direct consequence of Theorem 3 and the results contained in [21], to which we refer for the precise statements. In particular, for (i) we use [21, Theorem I and pp. 2930], (ii) follows from [21, Theorem 5.7], for (iii) we take advantage of [21, Theorem IV], and (iv) is a consequence of [21, Theorem V]. \(\square \)
Limiting cases
Pure entropy distances
In the setting of EntropyTransport problems, we call pure entropy problems the ones induced by the choices
In this situation one can prove (see [29, Example E.5]) that for any \( \mu _1,\mu _2\in {\mathscr {M}}(X)\) we have
where \(\lambda \in {\mathscr {M}}(X)\) is any dominating measure of \(\mu _1\) and \(\mu _2\) and \(H_0\) is defined as the lower semicontinuous envelope of the function
In particular, the functional corresponds in this situation to the Csiszár’s divergence induced by the function \(s\mapsto H_0(1,s)\in \Gamma _0({\mathbb {R}}_+)\) (see [14, Lemma 3]), justifying the name of pure entropy problem.
For some entropy functions F one can prove that a power a of the induced pure entropy cost is a distance. For instance, when \(a=1\) and \(F(s)=s1\) we obtain the celebrated total variation (denoted by \(\mathsf {TV}\) in the sequel), a distance in the space of measures inducing a strong topology. Actually, thanks to the result proved in [14, Lemma 8] and the explicit bounds contained in [31, Theorem 2.5], we know that every pure entropy distance induces the same topology of the total variation.
As shown in [14, Propositions 2, 3], we obtain another class of pure entropy distances by choosing \(a=1/2\) and the powerlike entropy \(F=U_p\), \(p\ge 1\), defined in example 3. In this situation we have
and we recognize some wellknown functionals like the 2Hellinger distance (case \(p=1\)) and the triangular discrimination (case \(p=2\)). We will denote these distances by \(\mathsf {PL}_{p}\).
We start with a useful lemma, valid for any pure entropy problem.
Lemma 7
Fix \(a\in (0,1]\) and let us consider the functions
Let us denote by \({\textsf {PE} }\) the power a of the EntropyTransport cost induced by F and \(\varvec{\mathrm c}\).
For any \((X_1,{\textsf {d} }_1,\mu _1),(X_2,{\textsf {d} }_2,\mu _2)\in \varvec{\mathrm {X}}\) let us define
where the infimum in the right hand side is taken over all complete and separable metric spaces \(({\hat{X}},{\hat{\textsf {d} }})\) with isometric embeddings \(\psi ^1:{\textsf {supp} }(\mu _1)\rightarrow {\hat{X}}\) and \(\psi ^2:{\textsf {supp} }(\mu _2)\rightarrow {\hat{X}}\).
Then
where
Proof
Setting
we can prove that the infimum of
over the set
is less or equal to the infimum in the right hand side of (69) by reasoning as in the first part of the Proof of Proposition 2. The fact that the power a of the right hand side of (70) is less or equal to the infimum as in (69) follows by noticing that
For the converse inequality, we reason in a similar way as in the proof of the point (ii) of Lemma 6. For any pseudometric coupling \({\hat{{\mathsf {d}}}}\) of \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) let us consider the space \(((X_1\sqcup X_2)/\sim ,{\hat{{\mathsf {d}}}})\), where \(x_1\sim x_2 \Longleftrightarrow {\hat{{\mathsf {d}}}}(x_1,x_2)=0.\) It is a complete and separable metric space as proved in Lemma 1. Denoting by \(q: X_1\sqcup X_2\rightarrow (X_1\sqcup X_2)/\sim \) the projection to the quotient and using the identification
we notice that
via the injective map
Moreover, we also have that \(\psi ^i\) is an isomorphism of \((X_i,{\mathsf {d}}_i)\) into its image in \(((X_1\sqcup X_2)/\sim ,{\hat{{\mathsf {d}}}})\), \(i=1,2\). Thus, for any measure \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) such that \({\mathsf {supp}}(\varvec{\gamma })\subset \{{\hat{{\mathsf {d}}}}=0\}\), denoting by \(\gamma _i\) the marginals of \(\varvec{\gamma }\), we can consider the measures \(\varvec{\psi }_{\sharp }\varvec{\gamma }\) whose projections are \((\psi ^1)_{\sharp }\gamma _1\) and \((\psi ^2)_{\sharp }\gamma _2\). Using Lemma 4 we know that
and the proof is completed by noticing that \({\mathsf {supp}}(\varvec{\psi }_{\sharp }\varvec{\gamma })\) is contained in the diagonal of the metric space \(((X_1\sqcup X_2)/\sim ,{\hat{{\mathsf {d}}}})\). \(\square \)
In the next theorem we prove that some pure entropy problems, specifically the ones generated by powerlike entropies \(U_p\), \(p\ge 1\), can be recovered as a limiting case of regular EntropyTransport problems.
Theorem 5
Fix \(p\ge 1\) and let us consider the sequence of cost functions \(\varvec{\mathrm c}_n=n{\mathsf {d}}\) and the entropy function \(F:=U_p\). Let us denote by \({\textsf {D} }_{p,n}\) the EntropyTransport distance induced by \(a=1/2\), \(\varvec{\mathrm {c}}_n=n{\textsf {d} }\) and \(F:=U_p\).
Then, for every metric measure spaces \((X_1,{\textsf {d} }_1,\mu _1)\), \((X_2,{\textsf {d} }_2,\mu _2)\in \varvec{\mathrm {X}}\) the limit
where \({\mathbf {D}}_{p,n}\) denotes the function defined as in Definition 4 upon replacing by \({\textsf {D} }_{p,n}\).
Moreover, \({\textsf {D} }_{p,n}\) is a regular EntropyTransport distance and \({\mathbf {PL}}_p\) defines a metric on \(\varvec{\mathrm {X}}\) such that
where the infimum in the right hand side is taken over all complete and separable metric spaces \(({\hat{X}},{\hat{\textsf {d} }})\) with isometric embeddings \(\psi ^1:{\textsf {supp} }(\mu _1)\rightarrow {\hat{X}}\) and \(\psi ^2:{\textsf {supp} }(\mu _2)\rightarrow {\hat{X}}\).
Proof
The first assertion follows by noticing that for any metric \({\mathsf {d}}\) we have
The fact that \({\mathsf {D}}_{p,n}\) is a regular EntropyTransport distance is a consequence of [14, Theorem 6], noticing the obvious fact that \(n{\mathsf {d}}\) is a complete and separable metric for any fixed n.
In particular, since for every fixed n we know that \({\mathbf {D}}_{p,n}\) is a metric on \(\varvec{\mathrm {X}}\) by Theorem 2, we have that \({\mathbf {PL}}_p\) is nonnegative, symmetric, it satisfies the triangle inequality and
We claim that
only if \((X_1,{\mathsf {d}}_1,\mu _1)=(X_2,{\mathsf {d}}_2,\mu _2)\) (as equivalence classes). Indeed, since \({\mathbf {D}}_{p,n}\) is nonnegative and nondecreasing, the fact that
implies
and the result follows because \({\mathbf {D}}_{p,n}\) is a distance on \(\varvec{\mathrm {X}}\).
At this level we do not know that \({\mathbf {PL}}_p\) is finite valued, which is a consequence of (73) together with the fact that \(\mathsf {PL}_p\) is a (finite valued) distance on the space of measures as recalled above.
In order to prove (73), we first notice that the monotonicity (74) easily implies that
giving the finiteness of \({\mathbf {PL}}_p\left( (X_1,{\mathsf {d}}_1,\mu _1),(X_2,{\mathsf {d}}_2,\mu _2)\right) \).
For the converse inequality, thanks to Lemma 6 we know that for every \((X_1,{\mathsf {d}}_1,\mu _1)\), \((X_2,{\mathsf {d}}_2,\mu _2)\) and for every \(n \in {{\mathbb {N}}}\) there exist a measure \(\varvec{\gamma }_n\in {\mathscr {M}}(X_1\times X_2)\) and a pseudometric coupling \(\hat{{\mathsf {d}}}_n\) between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) such that:
By the superlinearity of the entropy functionals we can infer the existence (up to subsequence) of a weak limit \({\varvec{\gamma }}\in {\mathscr {M}}(X_1\times X_2)\) of the sequence \(\{{\varvec{\gamma }_n}\}_{n\in {\mathbb {N}}}\). We also know that
where we have used the usual notation for the marginal measures. If \(\varvec{\gamma }\) is the null measure the result follows trivially. Otherwise, since the integral
is bounded from above we can argue as in the step 2 of the Proof of Lemma 6 and we deduce the existence of a pseudometric coupling \(\hat{{\mathsf {d}}}\) between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) such that \(\hat{{\mathsf {d}}}_n\) converges (up to subsequence) pointwise to \(\hat{{\mathsf {d}}}\) and the convergence is uniform on compact sets.
By recalling the explicit formulation of the right hand side of (73) given in Lemma 7, the proof is completed if we show that \({\mathsf {supp}}(\varvec{\gamma })\subset \{{\hat{{\mathsf {d}}}}=0\}\). Let us suppose by contradiction the existence of a point \(({\bar{x}},{\bar{y}})\in {\mathsf {supp}}(\varvec{\gamma })\) such that \({\hat{{\mathsf {d}}}}({\bar{x}},{\bar{y}})=k>0.\) Fix \(k/2>r>0\): for every \(\epsilon >0\) sufficiently small we know that there exist \(m\in {\mathbb {N}}\) such that for every \(n>m\) we have
Starting from the bound in (75) we have
that leads to a contradiction for n sufficiently large thanks to (77). \(\square \)
Definition 6
We say that a sequence of metric measure spaces \((X_n,{\mathsf {d}}_n,\mu _n)_{n\in {\mathbb {N}}}\) strongly measuredGromov converges to the metric measure space \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) if there exist a complete and separable metric space \((X,{\mathsf {d}})\) and isometric embeddings \(\iota _n:X_n\rightarrow X\), \(n\in \bar{{\mathbb {N}}}\), such that \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) in \({\mathscr {M}}(X)\) with respect to the total variation topology.
In the next Theorem we see that this notion of convergence coincides with the convergence induced by the distance \({\mathbf {PL}}_p\) for every \(p\ge 1\).
Theorem 6
Let \(p\ge 1\). A sequence \((X_n,{\textsf {d} }_n,\mu _n)_{n\in {\mathbb {N}}}\) strongly measuredGromov converges to \((X_{\infty },{\textsf {d} }_{\infty },\mu _{\infty })\) if and only if
Proof
The proof is analogous to the one of Theorem 3. Let us suppose the validity of (78). By definition of \({\mathbf {PL}}_p\) we know that there exist a complete and separable metric space \((Y_n,{\mathsf {d}}_{Y_n})\) and isometric embeddings \(\psi _n,\psi _n^{\infty }\) of \((X_n,{\mathsf {d}}_n)\) and \((X_{\infty },{\mathsf {d}}_{\infty })\) in \(Y_n\) such that
where \(\mathsf {PL}_p\) is computed in the space \(Y_n\). We now define \(\displaystyle Y:=\sqcup _{n\in \bar{{\mathbb {N}}}} X_n\) endowed with the pseudometric \({\mathsf {d}}_Y\)
We consider the space \(Y/\sim \) defined as the quotient of Y with respect to the equivalence relation
and we then define the completion of this space, that we still denote by \((Y,{\mathsf {d}}_Y)\). It is easy to see that Y is separable. By construction we notice that the set
endowed with the distance \({\mathsf {d}}_{Y_n}\) is canonically isometrically embedded in \((Y,{\mathsf {d}}_Y)\), so that every space \(X_n\), \(n\in \bar{{\mathbb {N}}}\), is canonically isometrically embedded into Y by a map \(\psi _n'\). We claim now that Y and \(\psi _n'\) provide a realization of the strong measuredGromov convergence. To see this, it is enough to notice that \((\psi _n')_{\sharp }\mu _n\rightarrow (\psi _{\infty }')_{\sharp }\mu _{\infty }\) in \({\mathscr {M}}(Y)\) with respect to the topology induced by the total variation, which is a consequence of the construction of \(\psi _n'\), (79) and the fact that \(\mathsf {PL}_p\) induces the topology of the total variation.
For the converse, let us suppose that \((X_n,{\mathsf {d}}_n,\mu _n)_{n\in {\mathbb {N}}}\) strongly measuredGromov converges to the metric measure space \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\). By definition we know that there exist a complete and separable metric space \((X,{\mathsf {d}})\) and isometric embeddings \(\iota _n:X_n\rightarrow X\), \(n\in \bar{{\mathbb {N}}}\), such that \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) in \({\mathscr {M}}(X)\) with respect to the topology of the total variation. Since \(\mathsf {PL}_p\) metrizes this topology on \({\mathscr {M}}(X)\) we know that
and the result follows noticing that \((X,{\mathsf {d}})\) is a possible competitor in the characterization of \({\mathbf {PL}}_p\) given in Theorem 5. \(\square \)
In the next easy proposition we show that the strong measuredGromov convergence implies the weak measuredGromov convergence.
Proposition 3
Let \((X_n,{\textsf {d} }_n,\mu _n)_{n\in {\mathbb {N}}}\) be a sequence of metric measure spaces strong measuredGromov converging to \((X_{\infty },{\textsf {d} }_{\infty },\mu _{\infty })\). Then \((X_n,{\textsf {d} }_n,\mu _n)_{n\in {\mathbb {N}}}\) weakly measuredGromov converges to \((X_{\infty },{\textsf {d} }_{\infty },\mu _{\infty })\).
Proof
By definition there exist a complete and separable metric space \((X,{\mathsf {d}})\) and isometric embeddings \(\iota _n:X_n\rightarrow X\), \(n\in \bar{{\mathbb {N}}}\), such that \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) in \({\mathscr {M}}(X)\) with respect to the total variation topology, which implies that \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) with respect to the weak convergence. The result follows by the very definition of weak measuredGromov convergence. \(\square \)
We conclude the section with a list of examples of convergences.
Examples 2

(1)
Let us consider the metric measure space \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) defined as the unit interval \(X_{\infty }=[0,1]\) endowed with the Euclidean distance and the Lebesgue measure. We know that \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) can be approximated in the weak measuredGromov convergence by a sequence of discrete spaces: take for instance \(X_n=\{m/n\}_{m=0}^{n1}\) endowed with the distance \({\mathsf {d}}_n\) inherited from the ambient 1dimensional Euclidean space and the measure \(\mu _n\) such that \(\mu _n(m/n)=1/n\) for every \(m=0,\ldots ,n1.\)
We next claim that \((X_n,{\mathsf {d}}_n,\mu _n)\) does not converge to \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) in the strong measuredGromov convergence. Indeed, for any metric space \((X,{\mathsf {d}})\) such that \(X_n\) is isometrically embedded in X via \(\iota _n\), \(n\in \bar{{\mathbb {N}}}\), we have
$$\begin{aligned} \begin{aligned} \mathsf {TV}((\iota _n)_{\sharp }\mu _n,(\iota _{\infty })_{\sharp }\mu _{\infty })&=\sup _{A\in {\mathscr {B}}(X)}(\iota _{\infty })_{\sharp }\mu _{\infty }(A)(\iota _n)_{\sharp }\mu _n(A)\\&\ge \mu _{\infty }\left( [0,1]{\setminus } \left\{ \bigcup _{m=0}^{n1}\,m/n\right\} \right) =1 \qquad \text {for any} \ n\in {\mathbb {N}}. \end{aligned} \end{aligned}$$ 
(2)
Let us consider the metric measure space \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) defined as the unit interval \(X_{\infty }=[0,1]\) endowed with the Euclidean distance and the measure \(\mu _{\infty }=f{\mathrm d}{\mathcal {L}}_{[0,1]}\), where \({\mathcal {L}}_{[0,1]}\) is the Lebesgue measure on [0, 1]. Let us define the sequence of metric measure spaces \((X_n,{\mathsf {d}}_n,\mu _n)\) where \(X_n=[0,11/n]\), \({\mathsf {d}}_n\) is the Euclidean distance and \(\mu _n=f_n{\mathrm d}{\mathcal {L}}_{[0,11/n]}.\) Let us suppose that \({\tilde{f}}_n\rightarrow f\) in \(L^1([0,1])\), where
$$\begin{aligned} {\tilde{f}}_n(x)={\left\{ \begin{array}{ll} f_n(x) &{}\text {if} \quad 0\le x\le 11/n\\ 0 &{}\text {if} \quad 11/n< x\le 1. \end{array}\right. } \end{aligned}$$Then, \((X_n,{\mathsf {d}}_n,\mu _n)\rightarrow (X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })\) in the strong measuredGromov convergence. To see this, it is enough to notice that for every \(n\in \bar{{\mathbb {N}}}\) the maps \(\iota _n:X_n\rightarrow X_{\infty }\) defined as \(\iota _n(x)=x\) provides an isometric embedding such that the convergence \((\iota _n)_{\sharp }\mu _n \rightarrow (\iota _{\infty })_{\sharp }\mu _{\infty }\) with respect the total variation distance is exactly equivalent to \({\tilde{f}}_n\rightarrow f\) in \(L^1([0,1])\).

(3)
Let \((X_n,{\mathsf {d}}_n,\mu _n)\) be the sequence of collapsing flat tori \(S^1\times \frac{1}{n} S^1\subset {{\mathbb {R}}}^4\) endowed with the normalized measures \(\mu _n:= n/(4\pi ^2)\, \mathrm{dvol_{S^1\times \frac{1}{n} S^1}}.\) It is a standard fact that \((X_n,{\mathsf {d}}_n,\mu _n)\) converges to \((X_{\infty },{\mathsf {d}}_{\infty },\mu _{\infty })=(S^1, {\mathsf {d}}_{S^1}, (2\pi )^{1} {{\mathcal {L}}}^1)\) in the weak measuredGromov sense (this a standard example of a collapsing sequence).
We claim that the convergence cannot be improved to strong measuredGromov. Indeed, for any metric space \((X,{\mathsf {d}})\) such that \(X_n\) is isometrically embedded in X via \(\iota _n\), \(n\in \bar{{\mathbb {N}}}\), we have
$$\begin{aligned} \begin{aligned} \mathsf {TV}((\iota _n)_{\sharp }\mu _n,(\iota _{\infty })_{\sharp }\mu _{\infty })&=\sup _{A\in {\mathscr {B}}(X)}(\iota _{\infty })_{\sharp }\mu _{\infty }(A)(\iota _n)_{\sharp }\mu _n(A)\\&\ge \mu _n \left( \left( S^1\times \frac{1}{n} S^1 \right) {\setminus } \gamma _n\left( S^1\right) \right) =1 \qquad \text {for any} \ n\in {\mathbb {N}}, \end{aligned} \end{aligned}$$where \(\gamma _n:S^1\rightarrow S^1\times \frac{1}{n} S^1\) is an arbitrary isometric immersion.
Sturm’s distances
We notice that the classical pWasserstein distance \({\mathcal {W}}_p\), \(p\ge 1\), can be recovered as a particular case of EntropyTransport problem with the choices
It is clear that \({\mathcal {W}}_p\) is not a regular EntropyTransport distance, however we show now that we can recover the \({\mathbf {D}}_p\)distance of Sturm (defined in Definition 3) as a limiting case of our framework.
Theorem 7
Fix \(p\ge 1\) and let us consider the cost function \(\ell (d):=d^p\), the entropy function \(F:=U_1\), and the power \(a:=1/p\). Let us denote by the power a of the EntropyTransport cost induced by \(F_n:=nU_1\) and \(\varvec{\mathrm c}=\ell (d)\). Then, for every metric measure spaces \((X_1,{\textsf {d} }_1,\mu _1)\), \((X_2,{\textsf {d} }_2,\mu _2)\in \varvec{\mathrm {X}}\)
where denotes the function defined as in Definition 4 upon replacing by . Moreover, for every metric measure spaces \((X_1,{\textsf {d} }_1,\mu _1)\), \((X_2,{\textsf {d} }_2,\mu _2)\in \varvec{\mathrm {X}}_{1,p}\) we have
Proof
We start by proving that the limit (82) exists on the set \(\varvec{\mathrm {X}}\). To see this, we notice that \(nF(s)\uparrow I_{1}(s)\) for every \(s\in [0,\infty )\). In particular, using the explicit formulation of proved in Proposition 2 (we remark that we have not used the fact that \({\mathbf {D}}\) is a distance in the proof of the proposition), we can infer that is nondecreasing so that the limit exists.
It remains to prove that for every \(p\ge 1\) we have \({\mathcal {D}}_p={\mathbf {D}}_p\) on the set \(\varvec{\mathrm {X}}_{1,p}\). Since for every \(n\in {\mathbb {N}}\), for every complete and separable metric space \((X,{\mathsf {d}})\) and for every \(\mu ,\gamma \in {\mathscr {M}}(X)\) we have
it is clear that \({\mathcal {D}}_p\le {\mathbf {D}}_p\). For the converse inequality, we know that for every \((X_1,{\mathsf {d}}_1,\mu _1)\), \((X_2,{\mathsf {d}}_2,\mu _2)\in \varvec{\mathrm {X}}_{1,p}\) we have
for some \(\varvec{\gamma }_n\in {\mathscr {M}}(X_1\times X_2)\) and metric coupling \(\hat{{\mathsf {d}}}_n\) of \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\), whose existence is a consequence of Lemma 6 (notice that we have only used the properties of the cost and the entropy in the proof of the lemma, while the fact that is a metric plays no role). Since \(D_{F_n}\) is bounded from above by the superlinear entropy \(D_{I_{1}}\), by using Lemma 2 and Lemma 3 we can infer that \(\varvec{\gamma }_n\) is weakly converging (up to subsequence) to a limit \(\varvec{\gamma }\in {\mathscr {M}}(X_1\times X_2)\) and
By reasoning as in step 2 and step 3 of Lemma 6, we know that there exists a pseudometric coupling \(\hat{{\mathsf {d}}}\) of \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\) such that \(\hat{{\mathsf {d}}}_n\) converges (up to subsequence) pointwise to \(\hat{{\mathsf {d}}}\) and the convergence is uniform on compact sets. Moreover, we have that
and the result follows since \(\hat{{\mathsf {d}}}\) and \(\varvec{\gamma }\) are competitors in the explicit formulation of \({\mathbf {D}}_p\) (see [43, Lemma 3.3]) as a consequence of the fact that
\(\square \)
Remark 4
We point out that we are not claiming that the sequence defined in Theorem 7 is a sequence of regular EntropyTransport distances. Actually, this is the case for \(p=2\) as a conseguence of [29, Theorem 7.25], noticing that the cost of the EntropyTransport problem induced by \((nU_1,{\mathsf {d}}^2)\) is n times the cost of the EntropyTransport problem induced by \((U_1,({\mathsf {d}}/\sqrt{n})^2)\) and \({\mathsf {d}}/\sqrt{n}\) is trivially a complete and separable distance.
In this situation, one can show that \({\mathbf {D}}_2\) defines a metric possibly attaining the value \(+\infty \) on the whole set \(\varvec{\mathrm {X}}\), by reasoning as in the Proof of Theorem 5.
PiccoliRossi distance
A natural extension of the \({\mathcal {W}}_1\)metric in the context of EntropyTransport problem is the PiccoliRossi generalized Wasserstein distance \(\mathsf {BL}\) [35, 36], induced by the choices
We notice that the entropy function is not superlinear.
It is proved in [35] that \(\mathsf {BL}\) is a complete distance on \({\mathscr {M}}(X)\) for every Polish space \((X,{\mathsf {d}})\) ( [35] is in the Euclidean setting, however the proof for a Polish space can be performed verbatim).
By exploiting the dual formulation of this distance, we know that \(\mathsf {BL}\) corresponds to the socalled flat metric or bounded Lipschitz distance (see [36, Theorem 2]), namely
We also recall this useful lemma, which is proved in [35, Proposition 1].
Lemma 8
Given \(\mu _1,\mu _2\in {\mathscr {M}}(X)\), let us consider the EntropyTransport problem induced by \((F,\varvec{\mathrm c})\) defined in (86). Then the infimum of the problem (26) is attained by a measure \(\varvec{\gamma }\in {\mathscr {M}}(X\times X)\) such that \(\gamma _i:=(\pi ^i)_{\sharp }\varvec{\gamma }\le \mu _i\), \(i=1,2\).
We have the following:
Theorem 8
Fix \(a=1\), \(\ell (d):=d\) and let us consider the sequence \((F_n)_{n\ge 2}\) defined by
Let us denote by the EntropyTransport cost induced by a, \(F_n\) and \(\varvec{\mathrm c}=\ell (d)\).
Then, for every metric measure spaces \((X_1,{\textsf {d} }_1,\mu _1)\), \((X_2,{\textsf {d} }_2,\mu _2)\in \varvec{\mathrm {X}}\) the quantity
where denotes the function defined as in Definition 4 upon replacing by .
Moreover, \({\mathbf {BL}}\) defines a complete metric on \(\varvec{\mathrm {X}}\) such that
where the infimum in the right hand side is taken over all complete and separable metric spaces \(({\hat{X}},{\hat{\textsf {d} }})\) with isometric embeddings \(\psi ^1:{\textsf {supp} }(\mu _1)\rightarrow {\hat{X}}\) and \(\psi ^2:{\textsf {supp} }(\mu _2)\rightarrow {\hat{X}}\).
Proof
We notice that \((F_n)_{n\ge 2}\) is a sequence of continuous superlinear entropy functions. We also know that \(F_n(s)=s1\) in [0, 1] and \(F_n(s)\ge s1\) in \([0,\infty )\) for every \(n\ge 2\), which implies that coincide with \(\mathsf {BL}\) thanks to Lemma 8. In particular we see that does not depend on n and also the identity (89) follows.
The fact that \({\mathbf {BL}}\) is a complete distance on \(\varvec{\mathrm {X}}\) is a consequence of the completeness of \(\mathsf {BL}\) (and thus ) on the set of measures \({\mathscr {M}}(X)\), and can be proved along the lines of Step 2 in the Proof Theorem 2. \(\square \)
Remark 5
We observe that the sequence defined in Theorem 8 is not a sequence of regular EntropyTransport distances. The problem here is that the topology induced by the distance \(\mathsf {BL}\) does not coincide with the weak topology, but it requires an additional tightness condition (see [35, Theorem 3] for all the details).
Bounds between distances
The aim of this last short section is to give some explicit bounds between the distances discussed in the paper.
Proposition 4
Let us denote by , , \({\textsf {QPL} }_p\) (for \(1<p\le 3\)) and \({\textsf {LPL} }_p\) (for \(p>1\)) the regular EntropyTransport distances defined in examples (1), (2), (3) and (4) respectively. Accordingly, we denote by , , \({\mathbf {D}}_{{\textsf {QPL} }_p}\) and \({\mathbf {D}}_{{\textsf {LPL} }_p}\) the induced SturmEntropyTransport distances. The following inequalities hold:

(1)
.

(2)
.

(3)
\({\mathbf {D}}_{{\textsf {LPL} }_p}\le {\mathbf {PL}}_p \qquad p>1.\)
Moreover, for every regular entropy transport distance induced by \((1/p, F, \ell )\) where \(p\ge 1\), \(F\in \Gamma _0({\mathbb {R}}_+)\), \(\ell (d)=d^p\) we have:

(4)
.
Proof

(1)
is a consequence of the bound proved in [29, Sect. 7.8].

(2)
follows by the corresponding inequality proved in [14, Proposition 7].

(3)
has been shown along the lines of the Proof of Theorem 5 (notice that \({\mathbf {D}}_{{\mathsf {LPL}}_p}\) equals \({\mathbf {D}}_{p,1}\) in the notation of that Theorem).

(4)
is a consequence of the explicit formulations of \({\mathbf {D}}_p\) and , by noticing that for any \(F\in \Gamma _0({\mathbb {R}}_+)\) we have \(F\le I_{1}\) where \(I_{1}\) has been defined in (81).
\(\square \)
Comparison with conic Gromov–Wasserstein
Let be a regular entropy transport distance induced by \((a,F,\ell )\). Recalling the construction introduced in Remark 1, given F and a number \(c\ge 0\) we can associate to the EntropyTransport problem a function \(H_c:[0,+\infty )\times [0,+\infty )\rightarrow [0,+\infty ]\) called marginal perspective function. Moreover, for any complete and separable metric space \((X,{\mathsf {d}})\) the function
is such that \(H^a\) is a distance on \({\mathfrak {C}}(X)\). In particular, we have
and for any \(w_1,w_2,w_3\in [0,\infty )\) such that there exists a complete and separable metric space \((X,{\mathsf {d}})\) with \(w_1={\mathsf {d}}(x_1,x_2)\), \(w_2={\mathsf {d}}(x_2,x_3)\), \(w_3={\mathsf {d}}(x_1,x_3)\), \(x_1,x_2,x_3\in X\).
We also recall that H is positively 1homogeneous in the scalar variables, i.e.
Let X be a metric space and fix \({\bar{x}}\in X\). We define the canonical projection \({\mathfrak {p}}:X\times [0,\infty )\rightarrow {\mathfrak {C}}(X)\) as \({\mathfrak {p}}(x,r)=[x,r].\) We also introduce the maps
We denote by \(\varvec{{\mathfrak {y}}}=({\mathfrak {y}}_1,{\mathfrak {y}}_2)=([x_1,r_1],[x_2,r_2])\) a point on \({\mathfrak {C}}(X_1)\times {\mathfrak {C}}(X_2)\), and we set \({\mathsf {r}}_i(\varvec{{\mathfrak {y}}}):={\mathsf {r}}({\mathfrak {y}}_i)\), \({\mathsf {x}}_i(\varvec{{\mathfrak {y}}}):={\mathsf {x}}({\mathfrak {y}}_i)\).
Given \(p\ge 1\), the phomogeneous marginals of a measure \(\varvec{\alpha }\in {\mathscr {M}}({\mathfrak {C}}(X_1)\times {\mathfrak {C}}(X_2))\) are defined as
Following the approach of [39], we define the conic Gromov–Wasserstein distance \(\mathrm {CGW}^a\) between two metric measure spaces \((X_1,{\mathsf {d}}_1,\mu _1)\), \((X_2,{\mathsf {d}}_2,\mu _2)\), as the power a of
where
and
In [39], the following main result has been obtained:
Theorem 9
([39, Theorem 1]) If \(H^a\) is a distance on the cone then the conic Gromov–Wasserstein distance \(\mathrm {CGW}^a\) is a metric on \(\varvec{\mathrm {X}}.\)
As we will see, it is possible to prove an inequality between the conic Gromov–Wasserstein distance and the SturmEntropyTransport distance. We start with a lemma.
Lemma 9
Let be a regular entropy transport distance induced by \((a,F,\ell )\) and let \(p\ge 1\). Let \((X_1,{\mathsf {d}}_1)\), \((X_2,{\mathsf {d}}_2)\) be two complete and separable metric spaces and let \({\hat{{\mathsf {d}}}}\) be a pseudometric coupling between \({\mathsf {d}}_1\) and \({\mathsf {d}}_2\). For any \(\mu _1\in {\mathscr {M}}(X_1)\) and \(\mu _2\in {\mathscr {M}}(X_2)\) we have
In particular, for any pair of metric measure spaces \((X_1,{\textsf {d} }_1,\mu _1)\), \((X_2,{\textsf {d} }_2,\mu _2)\) there exists \(\varvec{\alpha }\in {\mathcal {U}}_p(\mu _1,\mu _2)\) such that
where \({\hat{{\mathsf {d}}}}\) is an optimal pseudometric coupling for .
Proof
Notice that under our assumptions we are in the basic coercive setting described in [29, Sect. 3.1]. The equality stated in (96), corresponding to the equivalence between the homogeneous formulation based on the function H and the primal EntropyTransport formulation, is thus a consequence of [29, Theorem 5.8 (iii)] and the use of the projection map
for passing to the cone (see [29, Sect. 7] for all the details in the case of the HellingerKantorovich distance, the general case follows straightforwardly).
Once (96) has been proved, (97) can be deduced by recalling the characterization of given in the point (i) of Lemma 6. \(\square \)
We can now state the main result of this section. It should be compared with [32, Theorem 5.1], [45, Proposition 2.6].
Proposition 5
Let be a regular entropy transport distance induced by \((a,F,\ell )\) and let \(\mathrm {CGW}^a\) be the associated conic Gromov–Wasserstein distance. For any pair of metric measure spaces \((X_1,{\textsf {d} }_1,\mu _1)\), \((X_2,{\textsf {d} }_2,\mu _2)\), it holds
Proof
By taking advantage of Lemma 6, let us consider an optimal pseudometric coupling \({\hat{{\mathsf {d}}}}\) for . Thanks to Lemma 9, let us also consider \(\varvec{\alpha }\in {\mathcal {U}}_p(\mu _1,\mu _2)\) satisfying
To shorten the notation, let us set \(\varvec{{\mathfrak {y}}}:=([x,r],[y,s])\) and \(\varvec{{\mathfrak {y}}}':=([x',r'],[y',s'])\). By the triangle inequality for \({\hat{{\mathsf {d}}}}\), and the fact that \(\ell (\cdot )\) and \(H_{\cdot }(r,t)\) are increasing for any r, t, we have
where we have also used the Minkowski inequality and the homogeneity of H in the last passage, and we have taken advantage of (90) with \(w_3={\hat{{\mathsf {d}}}}(x,y)+{\hat{{\mathsf {d}}}}(x',y')\), \(w_1={\hat{{\mathsf {d}}}}(x,y)\) and \(w_2={\hat{{\mathsf {d}}}}(x',y')\). To justify the use of (90) we can argue as follows: if \({\hat{{\mathsf {d}}}}(x,y)=0\) or \({\hat{{\mathsf {d}}}}(x',y')=0\) there is nothing to prove, otherwise we notice that the three points metric space \((\{A,B,C\},{\tilde{{\mathsf {d}}}})\) with mutual distances between different points defined as \({\tilde{{\mathsf {d}}}}(A,B):={\hat{{\mathsf {d}}}}(x,y)\), \({\tilde{{\mathsf {d}}}}(B,C):={\hat{{\mathsf {d}}}}(x',y')\), \({\tilde{{\mathsf {d}}}}(A,C):={\hat{{\mathsf {d}}}}(x,y)+{\hat{{\mathsf {d}}}}(x',y')\) is indeed a complete and separable metric space for any \(x,x'\in X_1\), \(y,y'\in X_2\).
Using the definition of \(\varvec{\alpha }\in {\mathcal {U}}_p(\mu _1,\mu _2)\), we can now perform the integrals in (100) obtaining
and similarly
We thus reach the desired conclusion. \(\square \)
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Acknowledgements
The project started when N.D.P. was visiting A.M. in the fall 2018 at the Mathematics Institute of the University of Warwick, and took advantage of a second visit of N.D.P. to the Mathematical Institute of the University of Oxford in March 2020. The authors wish to thank both the institutions for the inspiring atmosphere and the excellent working conditions. A.M. acknowledges the support of the EPSRC First Grant EP/R004730/1 “Optimal transport and geometric analysis” and of the European Research Council (ERC), under the European’s Union Horizon 2020 research and innovation programme, via the ERC Starting Grant “CURVATURE”, grant agreement No. 802689. The authors wish to warmly thank Giuseppe Savaré for valuable discussions on the topics of the paper and are grateful to the anonymous reviewers, for their suggestions and comments that helped to improve a previous version of the manuscript.
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De Ponti, N., Mondino, A. EntropyTransport distances between unbalanced metric measure spaces. Probab. Theory Relat. Fields (2022). https://doi.org/10.1007/s00440022011594
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DOI: https://doi.org/10.1007/s00440022011594
Mathematics Subject Classification
 Primary 54E35
 Secondary 49J45
 53C23
 60B10