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We compute the nuclear dimension of separable, simple, unital, nuclear, Z\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {Z}}$$\end{document}-stable C∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {C}^*$$\end{document}-algebras. This makes classification accessible from Z\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {Z}}$$\end{document}-stability and in particular brings large classes of C∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {C}^*$$\end{document}-algebras associated to free and minimal actions of amenable groups on finite dimensional spaces within the scope of the Elliott classification programme.


Introduction
Nuclear dimension is a non-commutative generalisation of topological covering dimension to C * -algebras, introduced in [79]. A unital abelian C * -algebra consists of continuous functions on a compact Hausdorff space X ; in this case the nuclear dimension recaptures the dimension of X . At the other extreme lie simple C * -algebras, where nuclear dimension divides the exotic examples of [25,54,65,69] from those accessible to K-theoretic classification. Indeed, through the work of generations of researchers ( [19,27,38,48,64,75] building on numerous works going back to [18]), we now have a complete classification of separable, simple, unital C * -algebras of finite nuclear dimension satisfying Rosenberg and Schochet's universal coefficient theorem (UCT) [56]. This provides the C * -analogue of the celebrated classification of amenable factors due to Connes and Haagerup [13,29].
A major task at this point is to identify simple nuclear C * -algebras of finite nuclear dimension, through establishing the Toms-Winter conjecture ( [21,72]; the precise statement is [79,Conjecture 9.3], and an overview is given in [77,Section 5]). This predicts that finite nuclear dimension is but one facet of a meta-notion of regularity for simple nuclear C * -algebras, with alternate descriptions of very different natures. Classification will then be accessed through regularity, the particular form depending on the example of interest.
In this paper we show that for separable, simple, unital, and nuclear C *algebras, finite nuclear dimension is entailed by the tensorial absorption condition of Z-stability, where Z is the Jiang-Su algebra of [34] (Z-stability will be described further below). Combining this with the main result of [74] gives the following theorem, which was predicted by the Toms-Winter conjecture. Both implications making up this equivalence have striking applications: (i)⇒(ii) allows classification to be accessed via localisation at strongly selfabsorbing algebras [75], while the implication (ii)⇒(i) proven here brings large classes of examples coming from topological dynamics within the scope of classification (see Corollaries E and F below).
Theorem A Let A be an infinite dimensional, separable, simple, unital, nuclear C * -algebra. Then the following statements are equivalent: (i) A has finite nuclear dimension; (ii) A is Z-stable, i.e., A ∼ = A ⊗ Z.
Let us put conditions (i) and (ii) into context. Tensorial absorption (or stability) phenomena are ubiquitous in operator algebras, originating with the characterisation of properly infinite von Neumann algebras M as those which tensorially absorb B(H), i.e., are isomorphic to the von Neumann tensor product M ⊗ B(H). A major step in Connes' seminal work [13] was to show that injective II 1 factors are McDuff, i.e., they absorb the hyperfinite II 1 factor tensorially, while Kirchberg famously characterised pure infiniteness for simple nuclear C * -algebras through tensorial absorption of the Cuntz algebra O ∞ [38].
In a precise sense, the Jiang-Su algebra Z is the smallest possible, and so from this perspective the most natural, C * -analogue of the hyperfinite II 1 factor R. It satisfies Z ⊗Z ∼ = Z; moreover, this isomorphism occurs in a particularly strong way, analogous to the corresponding statement for R (see [66]), and Z is the minimal non-trivial C * -algebra with this property [73]. Accordingly, Z-stability is the weakest non-trivial form of tensorial absorption. Moreover, Z-stability is to a vast extent necessary for K-theoretic classification: Z has the same K-theory as C-these algebras are KK-equivalent (think of this as a very weak kind of homotopy equivalence)-and a unique trace. Therefore, under natural restrictions, a simple C * -algebra and its Z-stabilisation share the same Elliott invariant (K-theory and traces).
While all known constructions of the Jiang-Su algebra are a little involved, Z-stability can be phrased entirely without reference to the algebra Z itself. Reminiscent of McDuff's characterisation of R-stability of a II 1 factor in terms of approximately central matrix algebras [45], Z-stability is equivalent to the existence of approximately central matrix cones which are large in a suitable sense [53]. Moreover, for simple, nuclear C * -algebras, via the groundbreaking work of Matui and Sato [43], this largeness can be measured in trace [32]. This criterion has enabled Z-stability to be established in a number of settings where a direct proof of finite nuclear dimension is unavailable [12,20,35,36].
Turning to (i) in Theorem A, one begins by noting that, through partitions of unity, a finite open cover of a compact Hausdorff space X gives rise to a finite dimensional approximation of C(X ). These provide an explicit realisation of the completely positive approximations of Choi-Effros and Kirchberg [9,37] which characterise nuclearity for C * -algebras. Covering dimension is determined by the existence of arbitrarily fine open covers which can be coloured so that no two sets of the same colour overlap; the number of colours needed determines the dimension. 1 A finite colouring of an open cover gives a decomposition of the associated finite dimensional model of C(X ). In the general setting, completely positive approximations can be viewed as noncommutative partitions of unity. The nuclear dimension (and its forerunner, the decomposition rank from [41]) is defined in terms of uniformly decomposable completely positive approximations; i.e., finitely colourable non-commutative partitions of unity. We recall the precise definition in Paragraph 1.1.
In principle, a system of completely positive approximations fully encodes a nuclear C * -algebra A, but extracting information is not straightforward without imposing additional structure on the approximations. 2 With finite nuclear dimension, one can use the uniform bound on the number of colours to transfer properties of the finite dimensional algebras in the approximation back to A, albeit at the cost of suitably loosening these properties, often in a dimensional way (see [52,72,74]). This strategy is at the heart of the construction of large approximately central matrix cones from nuclear dimension approximations in the proof of Theorem A (i)⇒(ii) given in [74].
The main result of this paper is the implication (ii)⇒(i) in Theorem A. We also determine the relationship between finite nuclear dimension and decomposition rank for simple C * -algebras in terms of quasidiagonality (see [2,4,41,44,50,64,70] for the history, and partial solutions to this problem).
Theorem B Let A be a separable, simple, unital, nuclear, Z-stable C *algebra. Then A has nuclear dimension at most 1. If A is also finite and all traces on A are quasidiagonal, then A has decomposition rank at most 1.
As C * -algebras whose nuclear dimension or decomposition rank is zero are necessarily approximately finite dimensional (AF for short) by [41,79], the previous theorem determines the nuclear dimension and decomposition rank of simple unital C * -algebras.

Corollary C
The possible values of the nuclear dimension and decomposition rank of a simple, unital C * -algebra are 0, 1 and ∞. The value 0 occurs precisely for AF algebras.
When A has no traces in Theorem B, it is purely infinite, and the result is [3, Theorem G] (completing the line of research in [22,44,57,79]). With traces, partial results were first found in the unique trace case [44,58] and then when the trace space of A is a Bauer simplex [3], i.e., has compact extreme boundary. The major technical hurdle which must be overcome to go beyond this setting is a strategy for combining behaviour at individual traces to obtain global properties uniformly over the trace space while respecting the affine structure. 3 We resolve this by introducing a completely new techniquecomplemented partitions of unity-enabling us to perform partition of unity arguments inside a Z-stable nuclear C * -algebra. We discuss this in more detail below, but we first turn to the consequences of Theorem A for classification.
Combining the very recent stably finite results ( [19,27] and [64,Corollary D]) with the celebrated Kirchberg-Phillips classification theorem, simple, separable, unital, nuclear C * -algebras of finite nuclear dimension with the UCT are classified by their Elliott invariants. 4 A key consequence of Theorem A is that the classification theorem can now be accessed using Z-stability in place of finite nuclear dimension.
Corollary D Separable, simple, unital, nuclear, Z-stable C * -algebras in the UCT class are classified by their Elliott invariants.
Even if A is not itself Z-stable, this can be forced by tensoring with Z, at the cost of changing the algebra. So, in the language of [75], separable, simple, unital, nuclear C * -algebras in the UCT class are classified up to Z-stability.
Recently, new techniques have been introduced to verify Z-stability for C *algebras constructed out of infinite dimensional objects where a direct nuclear dimension computation seems out of reach. For example, Elliott and Niu show that crossed products associated to minimal Z-actions of mean dimension zero (in the sense of Gromov, Lindenstrauss, and Weiss) on infinite dimensional spaces are Z-stable, [20]. In contrast, direct estimates of the nuclear dimension, using Rokhlin dimension or similar techniques, only work for finite dimensional spaces [33,61].
Even more recently, the new tool of almost finiteness-which can be viewed as a dynamical analogue of the combination of Z-stability and amenabilityprovides a method for obtaining Z-stability of crossed products associated to free minimal actions of amenable groups [35]. This approach constructs approximately central matrix cones using tiling methods from [12,15], very much in the spirit of the Ornstein-Weiss Rokhlin lemma in the measurable setting. In [36], almost finiteness is obtained for all free actions on finite dimensional spaces of a countable discrete group whose finite subgroups have subexponential growth. This covers groups of intermediate growth, such as the Grigorchuk group [28]. In contrast, direct nuclear dimension computations via Rokhlin dimension methods seem to require serious conditions on the group, such as finite asymptotic dimension [62].
Corollary E Let G be a countably infinite, discrete group such that all finitely generated subgroups have subexponential growth. Then crossed products C(X ) G associated to free, minimal actions of G on a compact metrisable finite dimensional space X have finite nuclear dimension. All these crossed products are covered by the classification theorem.
Additionally, [12] shows that a generic free minimal action of a fixed amenable group G on a Cantor set has Z-stable crossed product. Accordingly, such crossed products are generically classifiable.
Corollary F Let G be a countably infinite, discrete, amenable group, and let X be the Cantor set. Then the set of all free, minimal actions α : G X, for which the crossed product C(X ) α G has finite nuclear dimension, and so is covered by the classification theorem, is comeagre in the set of all free minimal actions of G on X . 5 In the remainder of the introduction, we change gears and discuss the proof of our main results, isolating the key new technique-complemented partitions of unity-introduced in this paper. The reader may wish to return to the following two sections in parallel with the main body of the paper.

From Z-stability to finite nuclear dimension
For most of the rest of the introduction, we think of A as a separable, simple, unital, nuclear C * -algebra with at least one trace. At times we will allow more general A, and make this explicit.
The passage from Z-stability to finite nuclear dimension runs through the classification of order zero maps (equivalently * -homomorphisms out of cones) by traces. As usual, there are two required components: (i) existence, and (ii) uniqueness (up to approximate unitary equivalence). Somewhat more precisely, one should: (i) Produce a sequence of approximately order zero maps A → A ⊗ Z of nuclear dimension zero which uniformly approximates id A ⊗ 1 Z : A → A ⊗ Z on traces. (ii) Establish a uniqueness theorem for approximately order zero maps which uniformly approximate the two maps in (0.1) below on traces.
Using the fact that Z is strongly self-absorbing, to show that A⊗Z has nuclear dimension at most 1 it suffices to control the nuclear dimension of the map id A ⊗1 Z : A → A ⊗Z. By taking a positive contraction h ∈ Z with spectrum [0, 1], id A ⊗ 1 Z splits as the sum of the two order zero maps Now (i) and (ii) can be used to show that each map in (0.1) has nuclear dimension zero, and so id A ⊗ 1 Z , and hence A ⊗ Z has nuclear dimension at most 1.
To obtain (i) and (ii), we introduce the new technique of complemented partitions of unity. For ease of exposition, we will primarily focus on the existence component (i) in the discussion below. With hindsight, the strategy above was initiated in [44] in the case when A is quasidiagonal and has a unique trace, when the existence component (i) is an immediate consequence of the quasidiagonality assumption. An 'orderzero quasidiagonality' result was subsequently developed in [58] to handle the case where A is not assumed to be quasidiagonal (but still has unique trace). When A has many traces, these methods give approximately order zero maps of nuclear dimension zero each behaving well on an individual trace; the challenge is to combine them into a single map with the required behaviour on all traces simultaneously. Via compactness of the tracial state space, T (A), one can find an open cover U 1 , . . . , U k of T (A), and approximately order zero maps θ 1 , . . . , θ k : A → A ⊗ Z of nuclear dimension zero so that θ i models id A ⊗ 1 Z on the traces in U i . Given pairwise orthogonal approximately central positive contractions properties of the e i combine with those of the θ i to ensure that θ is approximately order zero and of nuclear dimension zero. The idea is that θ is obtained by 'gluing together' the θ i over the e i , which should be well chosen with respect to the open cover (U i ) k i=1 so θ has the global tracial behaviour required by (i). To do this it is necessary to control τ (e i θ i (·)) for τ ∈ T (A).
This was achieved in [3] when T (A) is a Bauer simplex, i.e., the extremal boundary ∂ e T (A) is compact. In this case, continuous affine functions on T (A) are in one-to-one correspondence with the continuous functions on the extreme boundary, and we only need to use extremal traces in the compactness argument to obtain the U i 's. The required (e i ) k i=1 arise via Ozawa's theory of W * -bundles [47]: one takes a partition of unity ( f i ) k i=1 in C(∂ e T (A)) subordinate to the cover (U i ) k i=1 and then one uses triviality of the W * -bundle associated to A⊗Z to produce the elements e i so that τ (e i ) ≈ f i (τ ). One can then show (see (0.4) below) that and then a partition of unity calculation shows that the map in (0.2) has the properties required by (i). The uniqueness ingredient (ii) was obtained in [3] under the same compactness assumption on the tracial extreme boundary of A. 6 This uses a detailed analysis of the structure of relative commutants, building on Matui and Sato's groundbreaking techniques for transferring structure between von Neumann and C * -algebras [43,44]. As with (i), a local-to-global argument at the level of traces is a crucial ingredient. Without getting into detail, this again boils down to the existence of approximately central pairwise orthogonal elements with uniform convergence when ∂ e T (A) is compact. 7 This is why (0.3) follows from τ (e i ) ≈ f i (τ ) in the Bauer simplex case. But without compactness, uniform convergence fails, even in straightforward cases (see [8,Example 3.3]). In general, one cannot obtain e i a automatically fromê i outside the Bauer setting.
The key novelty of this paper is a technique for producing the (e i ) k i=1 required for the gluing procedure of (0.2) taking into account the affine structure of T (A). The precise definition is given in Definition 3.1; for now, we give a relatively informal version of the characterisation from Proposition 3.2.

Definition G Let
is an open cover of T (A). The conclusion (a) says that theê i form an approximate partition of unity. Normally partitions of unity are constructed subordinate to a specified open cover or family of functions. But we need to control the affine functions e i a i not justê i . This is the role of condition (b). Note that it has the effect of ensuring thatê i is to some extent subordinate to the complement 1 −â i ofâ i : if τ ∈ T (A) satisfies τ (a i ) ≈ 1, then τ (e i ) ≈ τ (a i e i ) δτ (e i ), so that τ (e i ) must be small. 8 In applications, the a i in (0.5) will arise from compactness of the tracial state space. Returning to the sketch proof of existence above, we could set i.e., θ(1 A ) globally models 1 A ⊗ 1 Z in trace. With a bit more care, the same strategy can be used to produce a θ which globally models id A ⊗ 1 Z on an arbitrary finite subset of A in place of 1 A , establishing the required existence component (i). The uniqueness ingredient (ii) is also obtained by using complemented partitions of unity in place of the W * -bundle methods used in [3] for the special case of Bauer simplices. This gives the following result which we set out in Sects. 4 and 5.
Theorem H Let A be a separable, simple, unital, nuclear, Z-stable C *algebra with T (A) compact and non-empty, and with complemented partitions of unity. Then dim nuc (A) ≤ 1. If additionally all traces on A are quasidiagonal, then dr(A) ≤ 1.
Having isolated the abstract condition of complemented partitions of unity, the other main challenge of this paper is to verify this condition in the presence of Z-stability. This is the following theorem, which together with Theorem H gives Theorems A and B and their corollaries.

Theorem I Let A be a separable, nuclear, Z-stable C * -algebra with T (A) compact and non-empty. Then A has complemented partitions of unity.
Note that we establish CPoU in Theorem I without assuming A to be unital. In the main results of our paper, unitality enters through Theorem H, as the methods from [3] are only available in the presence of a unit. However, in [7] the first two authors develop the necessary machinery to extend Theorem H, and hence the main results of this paper to the non-unital setting. It should be noted that their result, although more general, requires Theorem I and the CPoU technology introduced in this paper.
The partitions of unity of Theorem I are also applicable in other situations, some of which are discussed in [8] (for example the equivalence of strict comparison and Z-stability under the assumption of uniform property ). Further, complemented partitions of unity will play a crucial role in the abstract approach to classification of C * -algebras being developed in [6].

Obtaining complemented partitions of unity
We end the introduction with an outline of the proof of Theorem I. Fix a 1 , . . . , a k and δ > 0 as in Definition G. The three main components of the proof are as follows: These steps produce elements satisfying (b) but they only cover approximately 1/k of the total trace. So we repeat the procedure 9 underneath the tracially approximate projection 1 A − k i=1 p i . Stage (2) is necessary at this point so that we have a (tracially approximate) orthogonal complement. Theorem I then follows using a maximality argument.
Our proof of step (1) uses nuclearity through the refined version of the completely positive approximation property obtained in [5]. This gives approx-imations of the identity map of the form where F is finite dimensional, ψ is completely positive, contractive and approximately order zero, and φ is a convex combination of order zero maps.
In particular, φ preserves traces, and ψ approximately does so. This gives an approximation of T (A) by T (F)-a finite dimensional simplex. The idea is to build complemented partitions of unity for ψ(a 1 ), . . . , ψ(a k ) in F (by taking the corresponding e i to be supported only on those full matrix summands of F where ψ(a i ) is small) and then push this back into A (at the cost of losing orthogonality).
Stages (2) and (3) use the extra space given by a tensor factor of Z (and do not need nuclearity). To get a feeling for (3) can use the additional space to redistribute the trace ofẽ i over the diagonal elements of M k to obtain a 1 k -approximate projection in trace. In fact, neither (2) nor (3) require the full strength of Z-stability; all that is needed is tracial divisibility of positive elements in an approximately central fashion reminiscent of Murray and von Neumann's property for II 1 factors. We introduce the concept of uniform property for C * -algebras in Sect. 2, and use it to obtain complemented partitions of unity in Sect. 3. The follow-up paper [8] further develops the theory of uniform property and complemented partitions of unity, giving other applications to the Toms-Winter conjecture.

Preliminaries
For a C * -algebra A, we write A + for the set of positive elements, A 1 for the closed unit ball, and A 1 + for the set of positive contractions.

Nuclear dimension and decomposition rank
Let A and B be C * -algebras. A completely positive map φ : A → B has order zero if it preserves orthogonality, i.e., φ(x)φ(y) = 0 whenever x, y ∈ A + satisfy x y = 0. Recall from [79], that A has nuclear dimension at most n, is a completely positive and contractive (c.p.c.) map, φ i : F → A is a completely positive map which is a sum of at most n +1 c.p.c. maps of order zero, and φ i (ψ i (a))−a i → 0 for all a ∈ A. Note that φ i ≤ n+1. The stronger (and historically earlier) condition from [41] that A has decomposition rank at most n is satisfied when in addition, each φ i can be taken to be contractive. Recall too that these definitions can be made at the level of maps; see [63, Definition 2.2].

Traces
In this paper a trace on a C * -algebra A means a tracial state, and we denote the collection of all traces by T (A). The set of traces T (A) is equipped with the weak * -topology it inherits from A * , and if X ⊆ T (A) then X refers to the closure in this topology. For a non-empty set X ⊆ T (A), define the seminorm

and so in particular when A is simple and
The (semi)norm · 2,T (A) plays a notable role in recent structural results [3,40,43,47], where it is often denoted · 2,u .

Ultrapowers
Throughout, ω will stand for a fixed free ultrafilter on N. Given a separable C * -algebra A, the ultrapower of A is When T (A) = ∅, the uniform tracial ultrapower is defined by where (abusing notation to use representative sequences in ∞ (A) to denote elements in A ω ), is the trace-kernel ideal. We will also use representative sequences to stand for elements in A ω . Notice that A embeds canonically into A ω as constant sequences. We will often regard A as a subalgebra of the ultrapower, for example to form the central sequence algebra A ω ∩ A . We shall adopt a similar notation in the case of uniform tracial ultrapowers. 10 We highlight two examples of the uniform tracial ultrapower.
(i) If A has a unique trace τ , then the GNS representation π τ generates a finite von Neumann factor M:=π τ (A) , which is II 1 when it is infinite dimensional. In this case, the Kaplansky density theorem canonically identifies A ω with the II 1 factor ultraproduct M ω .
where M is the W * -bundle obtained from the strict closure of A introduced by Ozawa in [47], and M ω is the W * -bundle ultraproduct defined in [3, Section 3.2]. This identification uses Kaplansky's density theorem in the stricttopology. 11

Limit traces
Any sequence of traces (τ n ) ∞ n=1 on A induces a trace on A ω by (a n ) ∞ n=1 → lim n→ω τ n (a n ). (1.5) A trace on A ω of this form is called a limit trace, and we will often denote a limit trace by a sequence (τ n ) ∞ n=1 that induces it. Evidently, every limit trace vanishes on J A , and thereby also induces a trace on A ω . We abuse notation, and write T ω (A) for the collection of these limit traces on both A ω and A ω ; context will make it clear to which ultrapower such a trace applies. With this notation, (1.6)

Kirchberg's -test
As with the norm ultrapower, the uniform tracial ultrapower lends itself to reindexing or diagonal sequence arguments which can be used to turn state-

Central surjectivity
The following central surjectivity result is a key tool in transferring structural properties from A ω to A ω via Matui-Sato's property (SI). It has its origins in Sato's work [59], and was established in general by Kirchberg Proof of (ii) Given y ∈ A ω ∩S withh y = y, lift y tox ∈ A ω ∩ S by (i). For each n ∈ N, choose a positive contraction f n ∈ C 0 ((0, 1]) + such that f n (1) = 1 and which is supported on (1−1/n, 1]. Then ash is a projection, f n (h)x maps toh y = y ∈ A ω for each n.

Compactness of T ( A)
Throughout the rest of the paper, our standard assumption will be that T (A) is non-empty and compact. We end this section by recording two consequences of this.

Lemma 1.10 Let A be a separable C * -algebra with T (A) non-empty and compact. Let
Proof Assume that x is a contraction, and for a contradiction that for some Since τ • ι, which a priori is a tracial functional on A of norm at most 1, is the limit as n → ω of the tracial states τ n , by compactness of T (A), τ • ι is a tracial state on A. Therefore there exists a ∈ A 1 + with τ (ι(a)) > 1 − . The hypothesis that x is orthogonal to A ensures that ι(a) + x * x is a contraction. Thus, Then (e − f )a = 0 for all a ∈ A, whence e = f by Lemma 1.10. It follows that ex = f x = x. Thus, e was already a unit for A ω . Conversely we have lim n→ω γ n = 1. Now suppose that some net (τ i ) in T (A) converges to a functional σ , so that σ is a tracial functional and σ ≤ 1. Then σ (e n ) = lim i τ i (e n ) ≥ γ n → 1, so σ is a state on A. This shows that T (A) is weak *closed in the (compact) unit ball of A * .

Uniform property for C * -algebras
Property for II 1 factors was introduced by Murray and von Neumann in [46] to prove the existence of non-hyperfinite II 1 factors. In modern language, a separably acting II 1 factor M has property if its central sequence algebra 14 M ω ∩ M is non-trivial. In this case Dixmier showed that M ω ∩ M is diffuse [14], and hence for each n ∈ N one can find orthogonal projections It is in this formulation that property has been most often used to obtain striking structural consequences; see [10,11,49].
In this section we introduce a uniform version of property for C * -algebras, motivated by Dixmier's formulation. 15 Recall our convention that A ω denotes the uniform tracial ultrapower from (1.3), which is unital when T (A) is nonempty and compact, and A ω ∩ A consists of the A-central elements of A ω (see footnote 10).

Definition 2.1 Let
A be a separable C * -algebra with T (A) non-empty and compact. We say that A has uniform property if for all n ∈ N, there exist projections p 1 , . . . , p n ∈ A ω ∩ A summing to 1 A ω , such that Note that the projections p 1 , . . . , p n in Definition 2.1 are necessarily pairwise orthogonal. While we insist in Definition 2.1 that every element a ∈ A can be 'tracially divided' in an approximately central fashion (as opposed to just requiring division of the unit), this does parallel the II 1 factor setting. If M is a II 1 factor and p ∈ M ω ∩ M , then τ M ω ( p ·) defines a tracial functional on M, and hence by uniqueness of the trace on M, So in the II 1 factor setting, p divides arbitrary elements in trace, in just the same way it divides the unit. This is not true for C * -algebras whose trace space is non-Bauer; [8,Example 3.3] gives an example.
Kirchberg's -test or other reindexing methods allow us to replace the central sequence algebra with the relative commutant of an arbitrary separable subset. The argument is standard and we omit the proof (cf. [74,Proposition 4.4] for an example of such a reindexing argument).

Lemma 2.2 Let
A be a separable C * -algebra with T (A) non-empty and compact. Suppose A has uniform property . Then for n ∈ N and a · 2,T ω (A)separable subset S of A ω , there exist projections p 1 , . . . , p n ∈ A ω ∩ S summing to 1 A ω , such that In this paper, Z-stable C * -algebras are the most important instances of uniform property ; for later use we record a stronger statement.

Proposition 2.3 Suppose A is a separable, Z-stable C * -algebra with T (A)
non-empty and compact. Then for any · 2,T ω (A) -separable subset S of A ω and any n ∈ N, there is a unital embedding M n → A ω ∩ S . In particular, A has uniform property .
To get uniform property , take equivalent, orthogonal projections q 1 , . . . , q n ∈ M n which sum to 1 M n , and define p i :=ψ(q i ). As M n has a unique trace, for any τ ∈ T (A ω ) and any a ∈ A, τ (ψ(·)a) must be a scalar multiple of the trace on M n , so that We now turn to one of the key applications of uniform property .

Lemma 2.4 (Tracial projectionisation) Let A be a separable C * -algebra with T (A) non-empty and compact, and with uniform property . Let S, T ⊆
Lemma 2.2 provides pairwise orthogonal projections p 1 , . . . , p n ∈ A ω ∩ S ∩ {b} satisfying Then for a ∈ T and τ ∈ T ω (A), Moreover, for τ ∈ T ω (A), using properties of the p i in the first line, Since p − p 2 is a positive contraction, we get as required.

Complemented partitions of unity
In this section we perform the main technical argument of the paper, obtaining complemented partitions of unity for nuclear C * -algebras with uniform property . Recall our standard convention from (1.3) that A ω denotes the uniform tracial ultrapower, which, by Proposition 1.11, is unital when T (A) is non-empty and compact, and A ω ∩ A consists of the A-central elements of A ω (see footnote 10). For later use, we note the following stability result regarding CPoU.
Then We now turn towards the proof of Theorem I, beginning by recording the strong form of the completely positive approximation property we require. In the case whenŜ = A, and working with approximate statements rather than ultrapowers, the following is a consequence of [5, Theorem 3.1] (which is stated in terms of finite sets and 's, and obtains a stronger convexity conclusion in (ii), which we do not need). The version stated here is obtained from [5, Theorem 3.1] via standard reindexing or applying Kirchberg's -test. We omit the details.

c. order zero maps, (iii) the map ψ restricts to an order zero map onŜ.
In our use of these approximations to perform the first step in the proof of Theorem I (as outlined in the introduction), the key role of (ii) and (iii) is that the order zero maps preserves traciality ([78, Corollary 4.4]). Consequently, τ •φ m is a tracial functional on F m if τ ∈ T (A), and τ •ψ is a tracial functional onŜ if τ ∈ T ( ω F m ).
Fix 0 < < μ/2. By Kirchberg's -test, it suffices to find b 1 , . . . , b k ∈ (A ω ∩ S ) 1 + satisfying the weaker conditions be as in Lemma 3.5. For each m, decompose F m as λ∈ m F m,λ , where m is a finite set and each F m,λ is a full matrix algebra. Correspondingly, ψ m decomposes as a direct sum λ∈ m ψ m,λ . We denote the tracial state on F m,λ by τ F m,λ . As T (A) is compact, applying Dini's theorem to an approximate unit for A gives We first claim that {m ∈ N :˜ m = ∅} ∈ ω. (3.20) If this is not the case, then for any τ = (τ m ) ∞ m=1 ∈ T ω (A), as above, using that φψ(eq) = eq, we have (3.21) which combines with (3.16) to contradict the choice of < μ/2. Inductively define for i = 1, . . . , k,

(3.22)
We next claim that If this is not the case, then there exists λ m ∈ m for each m, such that Since each φ m is a contractive map, note that k i=1 b i has norm at most 1, and thus τ ( k i=1 b i q) ≤ τ (q) = μ for τ ∈ T ω (A). By Lemma 3.5(i), it follows by the Stinespring argument of [41,Lemma 3.5], that ψ mapsŜ into the multiplicative domain of φ, and so for x ∈Ŝ and x ∈ S its image, (3.30) In particular, as f i is central in ω F m we havē Since the image ofŜ is · 2,T ω (A) -dense in S, it follows that b i ∈ A ω ∩ S . For i = 1, . . . , k, and τ = (τ m ) ∞ m=1 ∈ T ω (A), we have proving (3.14). Likewise, we have establishing (3.13).
We now show how the conclusion of the previous lemma combines with uniform property to give CPoU. The strategy has the spirit of geometric series arguments used in [74,76], but as we work only with projections of constant value on (limit) traces we can use a maximality argument directly. Note that we do not require A to be nuclear for this step, only that the conclusion of Lemma 3.6 has been obtained.

Lemma 3.7 Let A be a separable C * -algebra with T (A) non-empty and compact. Suppose that A has uniform property and that the conclusion of Lemma 3.6 holds. Then A has CPoU.
Proof As in Remark 3.3, it suffices to verify CPoU for positive elements a 1 , . . . , a k ∈ A and δ > 0 such that min{τ (a 1 ), . . . , τ (a k )}. (3.34) Let S ⊂ A ω be a · 2,T ω (A) -separable subset.

Theorem 3.8 Let A be a separable, nuclear C * -algebra with T (A) non-empty and compact, and with uniform property . Then A has CPoU.
Proof This follows by combining Lemma 3.6 with Lemma 3.7.

Structural results for relative commutants and classification of maps out of cones
With the key ingredient of complemented partitions of unity in place, we now turn to showing how to use it to obtain our main results. In this section we obtain the key structural results for B ω (and its relative commutants), and transfer these to B ω (and its relative commutants) leading to classification results for order zero maps, which will be used to obtain finite nuclear dimension in Sect. 5. The strategy closely follows the arguments of [3]. The key difference is our use of CPoU to remove the hypothesis that the traces form a Bauer simplex from [3]. In comparing the results of this section with [3], recall from Sect.

1.3(ii) that when T (B)
is a Bauer simplex, B ω is canonically identified with the ultraproduct M ω of the W * -bundle M obtained from B, and the results of [3,Section 3] are all expressed in terms of this W * -bundle. 19 To make it transparent how we adapt the proofs of Sections 4 and 5 of [3], we start by showing how CPoU can be used to obtain a version of the key localto-global transfer lemma ([3, Lemma 3.18]), used (there) to glue fibrewise properties of trivial W * -bundles to global properties. In the following lemma, think of the equation h n (a, y) = 0 as a condition we want y to satisfy, with a a fixed constant. For example, h(a, y) = ay − ya describes the condition y ∈ {a} . The idea is that if for each trace there is an approximate solution to a family of conditions, then there are exact solutions in B ω (i.e., · 2,T (B)approximate solutions in B). be a * -polynomial in r m + s m non-commuting variables. Let (a i ) ∞ i=1 be a sequence from B ω . Suppose that, for every > 0, k ∈ N, and τ ∈ T (B ω ) in the closure of T ω (B), there exist contractions y τ 1 , y τ 2 , · · · ∈ π τ (B ω ) (where π τ is the GNS representation associated to τ ) such that h m (a 1 , . . . , a r  Using Kaplansky's density theorem, we may in fact assume each y τ i is in B ω while retaining (4.6). Set (a 1 , . . . , a r m , y so that a 1 , . . . , a m , y τ  1 , . . . , y τ m ) 2 2,τ < 2 , τ ∈ T ω (B). (4.8) By continuity and compactness, there are τ 1 , . . . , τ n ∈ T ω (B) such that By CPoU, there exist pairwise orthogonal projections Using this, we obtain h m (a 1 , . . . , a r m , y 1 , . . . , y s m ) 2 2,τ = τ (|h m (a 1 , . . . , a r m , y 1 , . . . , y s m )| 2 ) for all τ ∈ T ω (B), as required.
Our first use of Lemma 4.1 is to obtain strict comparison for relative commutants in B ω . Proof This is obtained by following the proof [3,Lemma 3.20] replacing M ω by B ω and using Lemma 4.1 in place of [3,Lemma 3.18]. There are two things to note.
Firstly, strict comparison is a property defined at the level of matrix amplifications. The first paragraph of the proof of Lemma 3.20 of [3], notes that without loss of generality we do not need to perform this matrix amplification; this is equally valid in our setting as M k (B ω ) ∼ = (M k (B)) ω , and M k (B) inherits CPoU from B by Lemma 3.4.
Secondly, to verify the existence of approximate local solutions, (4.2) as in the last two paragraphs of the proof of [3, Lemma 3.20], we only require that π τ (B ω ) is a finite von Neumann algebra, 20 so the additional traces required in the hypothesis of Lemma 4.1 (as discussed in Remark 4.2 (iii)) cause no difficulties.
Our next goal is to use CPoU to determine all traces on relative commutants B ω ∩ φ(A) , where A is a separable, nuclear C * -algebra and φ is a  Then the weak * -closed convex hull of T 0 is T (A).
We now turn to the fibrewise statement for traces on relative commutants. The key difference between this and the original statement of [3,Lemma 3.21] is that we also need to consider elements which are uniformly small in trace, not just those which are zero in all traces. Suppose that δ > 0 and z ∈ N is a contraction such that |ρ(z)| ≤ δ for all ρ ∈ T 0 . Set K :=12 · 12 · (1 + δ). Then for any finite subset F of B and > 0, there exist contractions w, x 1 , . . . , x 10 , y 1 , . . . , If (M, τ M ) is an ultraproduct of tracial von Neumann algebras, 22 then one can take w, x 1 , . . . , x 10 , y 1 , . . . , y 10 ∈ N and have the equality Moreover, in this case, the weak * -closed convex hull of T 0 is T (N ).
Proof Throughout the proof, we will write · 2 as shorthand for · 2,τ M . Assume without loss of generality that F consists of contractions. As B is hyperfinite (by Connes' theorem [13]), we may find a finite dimensional sub- We claim that |σ (z )| ≤ δ for every trace σ ∈ T (M ∩ B ∩ (1 M − 1 B ) ⊥ ). By convexity and density, it suffices to prove this for a normal trace σ that is concentrated in a single direct summand p k (M ∩ B ). Since p k B is a unital matrix algebra in p k M p k , we have and so such a σ extends uniquely to a (necessarily normal) trace on p k M p k , also denoted σ . Since E(q k ) is bounded away from 0 on Y k , the tracial functional σ (q k ·) on q k Mq k extends to a bounded normal tracial functional on 22 This means that we have a sequence (M n ) ∞ n=1 of finite von Neumann algebras with a distinguished faithful trace τ n ∈ T (M n ) for each n, and M:= ω M n is the tracial von Neumann ultrapower, i.e. the quotient of ∞ n=1 M n by {(x n ) ∞ n=1 ∈ ∞ n=1 M n : lim n→ω τ n (x * n x n ) = 0}. Then M is a finite von Neumann algebra and carries the distinguished faithful normal trace τ M arising from the sequence (τ n ) ∞ n=1 .
χ Y k M, 23 and then to a bounded normal tracial functional σ on M. Note σ ( p k z) = σ ( p k χ Y k z) = σ (q k z), and σ ( p k ) = σ (q k ). Accordingly, using the hypothesis on z for the first inequality, we have  We now use CPoU to obtain a version of the previous result for relative commutants in B ω . Proposition 4.6 (cf. [3,Proposition 3.22]) Let B be a separable, unital C *algebra with T (B) non-empty, and with CPoU. Let A be a separable, unital, nuclear C * -algebra and φ : Define T 0 to be the set of all traces on C of the form τ (φ(a) ·) where τ ∈ T (B ω ) and a ∈ A + satisfies τ (φ(a)) = 1.
Since w is a contraction, we have |τ (z)| ≤ δ for all τ ∈ T (C). Hence, the closure of T 0 is T (C) by Lemma 4.4.
With the above results in place, we can now use property (SI) as set up in [3, Section 4.1] (without any tracial boundary assumption) and central surjectivity to lift the structural results for relative commutants in B ω back to the C * -level.
In particular, using the results above in place of the corresponding results from [3], the proofs in [3] give the following omnibus lemma (which was shown in [3] in the compact boundary case). As in [3], from this point on we need the hypotheses that B is simple, and all quasitraces on B are traces (written QT (B) = T (B), and famously automatic when B is exact by Haagerup's work [30]). Lemma 4.7 (cf. [3,Theorem 4.1]) Let B be a separable, simple, unital, Zstable C * -algebra with QT (B) = T (B) = ∅ and with CPoU. Let A be a separable, unital, nuclear C * -algebra and π : A → B ω a c.p.c. order zero map such that π(a) is full for each nonzero a ∈ A, and which induces a * -homomorphismπ (4.26)

Then
(i) all traces on C factor throughC, (ii) C has strict comparison of positive elements by traces, and (iii) the traces on C are the closed convex hull of traces of the form τ (π(a) ·) for τ ∈ T (B ω ) and a ∈ A + with τ (π(a)) = 1.
Proof (i) is just a repeat of [3, Theorem 4.1(i)], and CPoU is not required. For (ii), we use the proof from [3] with the following small modifications. In place of the paragraph following Eq. (4.57) in [3], use Lemma 4.3 to obtain strict comparison forC; the embedding of M k into B ω ∩π(A) ∩ {c} in Eq. (4.58) of [3] is obtained from Proposition 2.3 in place of [3, Remark 3.13]); and Lemma 1.8 is used in place of [3, Lemma 3.10] to justify Eq. (4.59) of [3].
We now have all the tools in place to obtain the main classification lemma for order zero maps required to obtain our nuclear dimension estimates. The following removes the tracial boundary hypothesis from [3, Theorem 5.5] (though for simplicity we stick to a single algebra B rather than the sequence of algebras (B n ) ∞ n=1 given in [3]). Lemma 4.8 (cf. [3, Theorem 5.5]) Let B be a separable, simple, unital, Zstable C * -algebra with QT (B) = T (B) = ∅ and with CPoU. Let A be a separable, unital, nuclear C * -algebra, let φ 1 : A → B ω be a * -homomorphism such that φ 1 (a) is full in B ω for each nonzero a ∈ A, and let φ 2 : A → B ω a c.p.c. order zero map such that 25 Recall too, that under the hypotheses of the theorem which follows, B has CPoU when it is nuclear by Theorem I. The proofs from [3, Section 6] work verbatim for Theorem 4.9, using Lemma 4.8 in place of [3, Theorem 5.5]. Theorem 4.9 (cf. [3, Corollary 6.5]) Let A be a separable, unital, nuclear C *algebra, and let B be a separable, simple, unital, Z-stable C * -algebra such that QT (B) = T (B) = ∅, and with CPoU. Let φ 1 , φ 2 : A → B be unital * -homomorphisms such that φ 1 is injective. Then the following are equivalent: (ii) φ 1 and φ 2 are approximately n-coloured equivalent for some n ∈ N with n ≥ 2; (iii) φ 1 and φ 2 are approximately 2-coloured equivalent.

Nuclear dimension
The final ingredient we need to prove Theorem B and its consequences, is the existence result (i) discussed in the outline 'From Z-stability to finite nuclear dimension' in the introduction. The corresponding result with a compact tracial boundary assumption is [3,Lemma 7.4]. In order to use CPoU to extend this result to general trace simplices, we first handle the case of a single trace using a strategy from [5].
We now glue the previous lemma over T (A) using CPoU to obtain our existence result. Note that the conclusion of Lemma 5.2 is stronger than its counterpart in [3], since it exactly factorises the map A → A ω (as opposed to producing a factorisation which agrees on traces).
with θ n c.p.c. and η n c.p.c. order zero in such a way that the induced map (θ n ) ∞ n=1 : A → ω F n is order zero and the induced map If all traces on A are quasidiagonal, then we may arrange that (θ n ) ∞ n=1 is unital.
Proof By Kirchberg's -test (similar to the application of the -test in the proof of [3, Lemma 7.4]), it suffices to show that for a finite set F ⊂ A and a tolerance > 0, there is a sequence of c.p.c. maps φ n : A → A which factor through finite dimensional algebras F n as in (5.8) with θ n c.p.c. and η n c.p.c. order zero such that θ n (x)θ n (y) < for x, y ∈ F satisfying x y = 0, and (5.9) (x) − x 2,T ω (A) ≤ for x ∈ F, (5.10) where : A → A ω is the map induced by (η n • θ n ) ∞ n=1 . In fact, we will arrange for all the F n to be the same finite dimensional algebra F, and all the θ n to be the same map θ . Fix a finite set F ⊂ A and > 0. For each τ ∈ T (A), by Lemma 5.1, there are a finite dimensional algebra F τ , a c.p.c. map θ τ : A → F τ satisfying (5.9), and a c.p.c. order zero map η τ : F τ → A such that η τ • θ τ (x) − x 2 2,τ < 2 /|F|, x ∈ F. (5.11) Define a τ := x∈F |η τ (θ τ (x)) − x| 2 ∈ A + , (5.12) so that τ (a τ ) < 2 . Continuity and compactness give τ 1 , . . . , τ k ∈ T (A) such that min{τ (a τ 1 ), . . . , τ (a τ k )} < 2 , τ ∈ T (A). (5.13) Viewing a τ 1 , . . . , a τ k as elements of A ω , we apply CPoU 28 to get a partition of unity consisting of projections e 1 , . . . , e k ∈ A ω ∩ A such that τ (e i a τ i ) ≤ 2 τ (e i ), τ ∈ T ω (A), i = 1, . . . , k. (5.14) Since the e i are orthogonal projections commuting with the images of the c.p.c order zero maps η τ i , it follows that η is c.p.c. order zero. By projectivity of c.p.c. order zero maps with finite dimensional domains, 29 η may be lifted to a sequence of c.p.c. order zero maps η n : F → A. It is evident that θ n = θ satisfies (5.9), since each map θ τ i has the same property. Next, for x ∈ F and τ ∈ T ω (A), we have τ (e i a τ i ) (5.14) ≤ k i=1 τ (e i ) 2 = 2 . (5.16) Therefore, (x) − x 2,T ω (A) ≤ , for all x ∈ F, as required. If all traces are quasidiagonal, then the above argument (using the last line of Lemma 5.1) yields the stronger conclusion that the induced map (θ n ) ∞ n=1 : A → ω F n is a * -homomorphism. Hence by cutting down by the image of 1 A under this map (which can be lifted to a projection in ∞ n=1 F n ), we can arrange it to be unital.
We now have all the pieces in place to obtain Theorem H, and deduce its consequences. This is a matter of using the existence and uniqueness results established above using CPoU in place of the versions of these results with a compact tracial boundary assumption in [3].
Proof of Theorem H The proof is almost exactly as in [3,Theorem 7.5] (the corresponding result with compact boundary), using Lemma 4.8 and Lemma 5.2 in place of [3,Theorem 5.5] and [3,Lemma 7.4]. 30 For the reader's convenience, we spell out a proof of the nuclear dimension component of the theorem explicitly. Let (φ n : A → A) ∞ n=1 be the sequence of maps produced by Lemma 5.2. By construction, each φ n has nuclear dimension zero, 31 and the induced map˜ : A → A ω is c.p.c. order zero. Following˜ with the quotient map q : A ω → A ω , gives the inclusion A → A ω .
Let h ∈ Z 1 + have spectrum [0, 1]. We apply Lemma 4.8 with B = A to the diagonal embedding A → A ω and˜ : A → A ω in place of φ 1 and φ 2 , once with k:=h and once with k:=1 Z − h. Simplicity of A ensures that the diagonal embedding satisfies the fullness requirement, while the condition (4.27) follows from the form of q •˜ . Accordingly there are unitaries u (0) , u (1) ∈ (A ⊗ Z) ω , which lift to sequences of unitaries (u (0 n ) ∞ n=1 and (u (1) n ) ∞ n=1 in A ⊗ Z such that for all a ∈ A, we have n * + u (1) n (φ n (a) ⊗ (1 Z − h))u (1) n * ).

(5.17)
Noting that each of the maps u (0) n (φ n (·) ⊗ h)u (0) n * and u (1) n (φ n (·) ⊗ (1 Z − h))u (1) n * has nuclear dimension zero, it follows that id A ⊗ 1 Z : A → A ⊗ Z has nuclear dimension at most 1. As A is Z-stable, and Z is strongly selfabsorbing, dim nuc (A) ≤ 1 by [63, Proposition 2.6]. 30 Recall that the maps produced in Lemma 5.2 satisfy a stronger conclusion than those of [3,Lemma 7.4]. 31 Strictly speaking the definition of nuclear dimension in [63, Definition 2.2] is only made for * -homomorphisms; for the purpose of this proof, we use exactly the same definition for c.p.c.
order zero maps.
In the presence of traces, Theorem B is a consequence of Theorems H and I (established at the end of Sect. 3). In the absence of traces, Theorem B is [3, Corollary 9.9] (as in this case A is a Kirchberg algebra by [55,Corollary 5.1]).
Theorem A is then a combination of our Theorem B for (ii)⇒(i) and the main result of [74] for (i)⇒(ii).
Proof of Corollary C Firstly recall that a C * -algebra is AF 32 if and only if it has nuclear dimension zero if and only if it has decomposition rank zero ([79, Remark 2.2(iii)] and [41, Example 4.1]), so this part of the statement is well known. For the rest of the proof we exclude finite dimensional C * -algebras, so that Winter's Z-stability theorem applies.
For separable A, these statements are consequences of Theorem B and Winter's Z-stability theorem (encompassed in Theorem A): if the nuclear dimension of A is finite, then it is Z-stable, so A has nuclear dimension at most 1. Likewise for decomposition rank, as in this case A is finite with all traces quasidiagonal by [3,Proposition 8.5].
For the general case, if A has finite nuclear dimension, then for any finite subset F of A there exists a separable, simple, unital C * -subalgebra A 0 of A containing F with finite nuclear dimension. 33 Thus A 0 has nuclear dimension at most 1 by Theorem B. Since F was arbitrary, it follows that A has nuclear dimension at most 1. The same argument works for decomposition rank.
Corollary D is a direct replacement of finite nuclear dimension by Z-stability (using Theorem B) as the regularity hypothesis in the classification theorem (see [19,27] and [64,Corollary D]).
In Corollaries E and F, the crossed products C(X ) G are simple when the actions are free and minimal ([17, Corollary 5.16]), and when G is amenable they have the UCT by work of Tu [68]. Thus finite nuclear dimension is the remaining condition which must be checked to obtain classifiability, and so Corollary F is a combination of Theorem B and [12,Theorem 5.4].
Proof of Corollary E In [36, Theorem 8.1], Kerr and Szabó show that every free action of a group G as in Corollary E on a finite dimensional space is almost finite (building on the zero dimensional case of this result in [16]), and hence if the action is also minimal, the crossed product is Z-stable by [35,Theorem 12.4]. Finite nuclear dimension for these crossed products is a consequence of Theorem B, and classifiability follows from Corollary D.