# Path Category for Free

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## Abstract

There are different categorical approaches to variations of transition systems and their bisimulations. One is coalgebra for a functor G, where a bisimulation is defined as a span of G-coalgebra homomorphism. Another one is in terms of path categories and open morphisms, where a bisimulation is defined as a span of open morphisms. This similarity is no coincidence: given a functor G, fulfilling certain conditions, we derive a path-category for pointed G-coalgebras and lax homomorphisms, such that the open morphisms turn out to be precisely the G-coalgebra homomorphisms. The above construction provides path-categories and trace semantics for free for different flavours of transition systems: (1) non-deterministic tree automata (2) regular nondeterministic nominal automata (RNNA), an expressive automata notion living in nominal sets (3) multisorted transition systems. This last instance relates to Lasota’s construction, which is in the converse direction.

## Keywords

Coalgebra Open maps Categories Nominal sets## 1 Introduction

*Coalgebras*[25] and

*open maps*[16] are two main categorical approaches to transition systems and bisimulations. The former describes the branching type of systems as an endofunctor, a system becoming a coalgebra and bisimulations being spans of coalgebra homomorphisms. Coalgebra theory makes it easy to consider state space types in different settings, e.g. nominal sets [17, 18] or algebraic categories [5, 11, 20]. The latter, open maps, describes systems as objects of a category and the execution types as particular objects called paths. In this case, bisimulations are spans of open morphisms. Open maps are particularly adapted to extend bisimilarity to history dependent behaviors, e.g. true concurrency [7, 8], timed systems [22] and weak (bi)similarity [9]. Coalgebra homomorphisms and open maps are then key concepts to describe bisimilarity categorically. They intuitively correspond to functional bisimulations, that is, those maps between states whose graph is a bisimulation.

Two approaches to categorical (bi)simulations

We are naturally interested in the relationship between those two categorical approaches to transition systems and bisimulations. A reduction of open maps situations to coalgebra was given by Lasota using multi-sorted transition systems [19]. In this paper, we give the reduction in the other direction: from the category Open image in new window of pointed *TF*-coalgebras and lax homomorphisms, we construct the path-category Open image in new window and a functor Open image in new window such that Open image in new window -open morphisms coincide with strict homomorphisms, hence functional bisimulations. Here, *T* is a functor describing the branching behaviour and *F* describes the input type, i.e. the type of data that is processed (e.g. words or trees). This development is carried out with the case where *T* is a powerset-like functor, and covers transition systems allowing non-deterministic branching.

The key concept in the construction of Open image in new window are *F*-*precise maps*. Roughly speaking in set, a map \(f:X\mathbin {\longrightarrow }FY\) is *F*-precise if every \(y\in Y\) is used precisely once in *f*, i.e. there is a unique *x* such that *y* appears in *f*(*x*) and additionally *y* appears precisely once in *f*(*x*). Such an *F*-precise map represents one deterministic step (of shape *F*). Then a path Open image in new window is a finite sequence of deterministic steps, i.e. finitely many precise maps. *J* converts such a data into a pointed *TF*-coalgebra. There are many existing notions of paths and traces in coalgebra [4, 12, 13, 21], which lack the notion of *precise* map, which is crucial for the present work.

Once we set up the situation Open image in new window , we are on the framework of open map bisimulations. Our construction of Open image in new window using precise maps is justified by the characterisation theorem: Open image in new window -open morphisms and strict coalgebra homomorphisms coincide (Theorems 3.20 and 3.24). This coincidence relies on the concept of path-reachable coalgebras, namely, coalgebras such that every state can be reached by a path. Under mild conditions, path-reachability is equivalent to an existing notion in coalgebra, defined as the non-existence of a proper sub-coalgebra (Sect. 3.5). Additionally, this characterization produces a canonical trace semantics for free, given in terms of paths (Sect. 3.6).

We illustrate our reduction with several concrete situations: different classes of non-deterministic top-down tree automata using analytic functors (Sect. 4.1), Regular Nondeterministic Nominal Automata (RNNA), an expressive automata notion living in nominal sets (Sect. 4.2), multisorted transition systems, used in Lasota’s work to construct a coalgebra situation from an open map situation (Sect. 4.3).

*Notation.* We assume basic categorical knowledge and notation (see e.g. [1, 3]). The cotupling of morphisms \(f:A\rightarrow C\), \(g:B\rightarrow C\) is denoted by \([f,g]:A+B\rightarrow C\), and the unique morphsim to the terminal object is \(!:X\rightarrow 1\) for every *X*.

## 2 Two Categorical Approaches for Bisimulations

We introduce the two formalisms involved in the present paper: the open maps (Sect. 2.1) and the coalgebras (Sect. 2.2). Those formalisms will be illustrated on the classic example of Labelled Transition Systems (LTSs).

### Definition 2.1

Fix a set *A*, called the alphabet. A *labelled transition system* is a triple \((S,i,\varDelta )\) with *S* a set of *states*, \(i \in S\) the *initial state*, and \(\varDelta \subseteq S\times A\times S\) the *transition relation*. When \(\varDelta \) is obvious from the context, we write \(s\xrightarrow {~a}s'\) to mean \((s,a,s')\in \varDelta \).

For instance, the tuple \((\{0,\cdots ,n\},0,\{(k-1,a_k,k)~|~1\le k\le n\})\) is an LTS, and called the *linear system* over the word \(a_1\cdots a_n\in A^\star \). To relate LTSs, one considers functions that preserves the structure of LTSs:

### Definition 2.2

A *morphism of LTSs* from \((S,i,\varDelta )\) to \((S',i', \varDelta ')\) is a function \(f:S \longrightarrow S'\) such that \(f(i) = i'\) and for every \((s,a,s')\in \varDelta \), \((f(s),a,f(s'))\in \varDelta '\). LTSs and morphisms of LTSs form a category, which we denote by Open image in new window .

Some authors choose other notions of morphisms (e.g. [16]), allowing them to operate between LTSs with different alphabets for example. The usual way of comparing LTSs is by using simulations and bisimulations [23]. The former describes what it means for a system to have at least the behaviours of another, the latter describes that two systems have exactly the same behaviours. Concretely:

### Definition 2.3

A *simulation* from \((S,i,\varDelta )\) to \((S',i',\varDelta ')\) is a relation \(R \subseteq S\times S'\) such that (1) \((i,i') \in R\), and (2) for every \(s\xrightarrow {~a}t\) and \((s,s') \in R\), there is \(t'\in S'\) such that \(s'\xrightarrow {~a}t'\) and \((t,t') \in R\). Such a relation *R* is a *bisimulation* if \(R^{-1} = \{(s',s) \mid (s,s') \in R\}\) is also a simulation.

Morphisms of LTSs are functional simulations, i.e. functions between states whose graph is a simulation. So how to model (1) systems, (2) functional simulations and (3) functional bisimulations categorically? In the next two sections, we will describe known answers to this question, with open maps and coalgebra. In both cases, it is possible to capture similarity and bisimilarity of two LTSs *T* and \(T'\). Generally, a simulation is a (jointly monic) span of a functional bisimulation and a functional simulation, and a bisimulation is a simulation whose converse is also a simulation, as depicted in Table 1. Consequently, to understand similarity and bisimilarity on a general level, it is enough to understand functional simulations and bisimulations.

### 2.1 Open Maps

The categorical framework of open maps [16] assumes functional simulations to be already modeled as a category Open image in new window . For example, for Open image in new window , objects are LTSs, and morphisms are functional simulations. Furthermore, the open maps framework assumes another category Open image in new window of ‘paths’ or ‘linear systems’, together with a functor *J* that tells how a ‘path’ is to be understood as a system:

### Definition 2.4

[16]**.** An *open map situation* is given by categories Open image in new window (‘systems’ with ‘functional simulations’) and Open image in new window (‘paths’) together with a functor Open image in new window .

For example with Open image in new window , we pick Open image in new window to be the poset of words over *A* with prefix order. Here, the functor *J* maps a word \(w\in A^\star \) to the linear system over *w*, and \(w \le v\) to the evident functional simulation \(J(w\le v):Jw\mathbin {\longrightarrow }Jv\).

In an open map situation Open image in new window , we can abstractly represent the concept of a *run* in a system. A run of a path Open image in new window in a system Open image in new window is simply defined to be an Open image in new window -morphism of type \(Jw\mathbin {\longrightarrow }T\). With this definition, each Open image in new window -morphism \(h:T\mathbin {\longrightarrow }T'\) (i.e. functional simulation) inherently transfers runs: given a run \(x:Jw\mathbin {\longrightarrow }T\), the morphism \(h\cdot x:Jw\mathbin {\longrightarrow }T'\) is a run of *w* in \(T'\). In the example open map situation Open image in new window , a run of a path \(w=a_1\cdots a_n\in A^\star \) in an LTS \(T=(S,i,\varDelta )\) is nothing but a sequence of states \(x_0,\ldots ,x_n \in S\) such that \(x_0=i\) and \(x_{k-1}\xrightarrow {a_k} x_k\) holds for all \(1\le k \le n\).

*functional bisimulations*. For LTSs \(T=(S,i,\varDelta )\) and \(T'=(S',i',\varDelta ')\), an Open image in new window -morphism \(h:T\mathbin {\longrightarrow }T'\) is a functional bisimulation if the graph of

*h*is a bisimulation. This implies the following relationship between runs in

*T*and runs in \(T'\). Suppose that \(w\le w'\) holds in \(A^\star \), and a run

*x*of

*w*in

*T*is given as in (1); here

*n*,

*m*are lengths of \(w,w'\) respectively. Then for any run \(y'\) of \(w'\) in \(T'\) extending \(h\cdot x\) as in (2), there is a run \(x'\) of \(w'\) extending

*x*, and moreover its image by

*h*coincides with \(y'\) (that is, \(h\cdot x'=y'\)). Such \(x'\) is obtained by repetitively applying the condition of functional bisimulation.

*x*as \(x'\cdot J(w\le w')=x\). From these, we conclude that if an Open image in new window -morphism \(h:T\mathbin {\longrightarrow }T'\) is a functional bisimulation, then for any \(w\le w'\) in \(A^\star \) and run \(x:Jw\mathbin {\longrightarrow }T\) and \(y':Jw'\mathbin {\longrightarrow }T'\) such that \(y'\cdot J(w\le w')=h\cdot x\), there is a run \(x':Jw'\mathbin {\longrightarrow }T\) such that \(x'\cdot J(w\le w')=x\) and \(h\cdot x'=y'\) (the converse also holds if all states of

*T*are reachable). This necessary condition of functional bisimulation can be rephrased in any open map situation, leading us to the definition of open map.

### Definition 2.5

[16]**.** Let Open image in new window be an open map situation. An Open image in new window -morphism \(h:T \longrightarrow T'\) is said to be *open* if for every morphism Open image in new window making the square on the right commute, there is \(x'\) making the two triangles commute.

Open maps are closed under composition and stable under pullback [16].

### 2.2 Coalgebras

The theory of G-coalgebras is another categorical framework to study bisimulations. The type of systems is modelled using an endofunctor Open image in new window and a system is then a coalgebra for this functor, that is, a pair of an object *S* of Open image in new window (modeling the state space), and of a morphism of type \(S \longrightarrow GS\) (modeling the transitions). For example for LTSs, the transition relation is of type \(\varDelta \subseteq S\times A\times S\). Equivalently, this can be defined as a function \(\varDelta :S \longrightarrow \mathcal {P} (A\times S)\), where \(\mathcal {P} \) is the powerset. In other words, the transition relation is a coalgebra for the Set-functor \(\mathcal {P} (A\times \_\!\_)\). Intuitively, this coalgebra gives the one-step behaviour of an LTS: *S* describes the state space of the system, \(\mathcal {P} \) describes the ‘branching type’ as being non-deterministic, \( A\times S\) describe the ‘computation type’ as being linear, and the function itself lists all possible futures after one-step of computation of the system. Now, changing the underlying category or the endofunctor allows to model different types of systems. This is the usual framework of coalgebra, as described for example in [25].

Initial states are modelled coalgebraically by a pointing to the carrier \(i:I\mathbin {\longrightarrow }S\) for a fixed object *I* in Open image in new window , describing the ‘type of initial states’ (see e.g. [2, Sec. 3B]). For example, an initial state of an LTS is the same as a function from the singleton set \(I:=\{*\}\) to the state space *S*. This object *I* will often be the final object of Open image in new window , but we will see other examples later. In total, an *I-pointed G-coalgebra* is a Open image in new window -object *S* together with morphisms \(\alpha :S\mathbin {\longrightarrow }GS\) and \(i:I\mathbin {\longrightarrow }S\). E.g. an LTS is an *I*-pointed *G*-coalgebra for \(I=\{*\}\) and \(GX = \mathcal {P} (A\times X)\).

In coalgebra, functional bisimulations are the first class citizens to be modelled as homomorphisms. The intuition is that those preserve the initial state, and preserve and reflect the one-step relation.

### Definition 2.6

An *I**-pointed* *G**-coalgebra homomorphism* from \(I \xrightarrow {\,i\,} S \xrightarrow {\,\alpha \,} GS\) to \(I \xrightarrow {\,i'\,} S' \xrightarrow {\,\alpha '\,} GS'\) is a morphism \(f:S \longrightarrow S'\) making the right-hand diagram commute.

For instance, when \(G=\mathcal {P} (A\times \_\!\_)\), one can easily see that a function *f* is a *G*-coalgebra homomorphism iff it is a functional bisimulation. Thus, if we want to capture functional simulations in LTSs, we need to weaken the condition of homomorphism to the inequality \(Gf(\alpha (s))\subseteq \alpha '(f(s))\) (instead of equality). To express this condition for general *G*-coalgebras, we introduce a partial order \(\sqsubseteq _{X,Y}\) on each homset Open image in new window in a functorial manner.

### Definition 2.7

A *partial order on* *G**-homsets* is a functor Open image in new window such that Open image in new window ; here, Open image in new window is the forgetful functor from the category Open image in new window of posets and monotone functions.

The functoriality of \(\sqsubseteq \) amounts to that \(f_1\sqsubseteq f_2\) implies \(Gh\cdot f_1\cdot g\sqsubseteq Gh\cdot f_2\cdot g\).

### Definition 2.8

Given a partial order on *G*-homsets, an *I**-pointed lax* *G**-coalgebra homomorphism* \(f:(S,\alpha ,i) \longrightarrow (S',\alpha ',i')\) is a morphism \(f:S\mathbin {\longrightarrow }S'\) making the right-hand diagram commute. The *I*-pointed *G*-coalgebras and lax homomorphisms form a category, denoted by Open image in new window .

### Conclusion 2.9

*I*and

*G*, such that the open morphisms are precisely the coalgebra homomorphisms.

## 3 The Open Map Situation in Coalgebras

Lasota’s construction [19] transforms an open map situation Open image in new window into a functor *G* (with a partial order on *G*-homsets), together with a functor Open image in new window that sends open maps to *G*-coalgebra homomorphisms (see Sect. 4.3 for details). In this paper, we provide a construction in the converse direction for functors *G* of a certain shape.

As exemplified by LTSs, it is a common pattern that *G* is the composition \(G=TF\) of two functors [12], where *T* is the branching type (e.g. partial, or non-deterministic) and *F* is the data type, or the ‘linear behaviour’ (words, trees, words modulo \(\alpha \)-equivalence). If we instantiate our path-construction to \(T=\mathcal {P} \) and \(F=A\times \_\!\_\), we obtain the known open map situation for LTSs (Conclusion 2.9).

Fix a category Open image in new window with pullbacks, functors Open image in new window , an object Open image in new window and a partial order \(\sqsubseteq ^T\) on *T*-homsets. They determine a coalgebra situation Open image in new window where \(\sqsubseteq \) is the partial order on *TF*-homsets defined by \({\sqsubseteq _{X,Y}}={\sqsubseteq _{X,FY}^T}\). Under some conditions on *T* and *F*, we construct a path-category Open image in new window and an open map situation Open image in new window where *TF*-coalgebra homomorphisms and Open image in new window -open morphisms coincide.

### 3.1 Precise Morphisms

While the path category is intuitively clear for \(FX=A\times X\), it is not for inner functors *F* that model tree languages. For example for \(FX=A + X\times X\), a \(\mathcal {P} F\)-coalgebra models transition systems over binary trees with leaves labelled in *A*, instead of over words. Hence, the paths should be these kind of binary trees. We capture the notion of tree like shape (“every node in a tree has precisely one route to the root”) by the following abstract definition:

### Definition 3.1

For a functor Open image in new window , a morphism \(s:S\mathbin {\longrightarrow }FR\) is called *F-precise* if for all *f*, *g*, *h* the following implication holds:

### Remark 3.2

If *F* preserves weak pullbacks, then a morphism *s* is *F*-precise iff it fulfils the above definition for \(g={\text { id }} \).

### Example 3.3

Intuitively speaking, for a polynomial Open image in new window -functor *F*, a map \(s:S\rightarrow FR\) is *F*-precise iff every element of *R* is mentioned precisely once in the definition of the map *f*. For example, for \(FX = A\times X + \{\bot \}\), the case needed later for LTSs, a map \(f:X\mathbin {\longrightarrow }FY\) is precise iff for every \(y \in Y\), there is a unique pair \((x,a) \in X\times A\) such that \(f(x) = (a,y)\). For \(FX=X\times X +\{\bot \}\) on Open image in new window , the map \(f:X\mathbin {\longrightarrow }FY\) in Fig. 1 is not *F*-precise, because \(y_2\) is used three times (once in \(f(x_2)\) and twice in \(f(x_3)\)), and \(y_3\) and \(y_4\) do not occur in *f* at all. However, \(f':X\mathbin {\longrightarrow }FY'\) is *F*-precise because every element of \(Y'\) is used precisely once in \(f'\), and we have that \(Fh\cdot f' = f\). Also note that \(f'\) defines a forest where *X* is the set of roots, which is closely connected to the intuition that, in the *F*-precise map \(f'\), from every element of \(Y'\), there is precisely one edge up to a root in *X*.

So when transforming a non-precise map into a precise map, one duplicates elements that are used multiple times and drops elements that are not used. We will cover functors *F* for which this factorization pattern provides *F*-precise maps. If *F* involves unordered structure, this factorization needs to make choices, and so we restrict the factorization to a class \(\mathcal {S} \) of objects that have that choice-principle (see Example 4.5 later):

### Definition 3.4

Fix a class of objects Open image in new window closed under isomorphism. We say that *F* *admits precise factorizations w.r.t.* \(\mathcal {S} \) if for every \(f:S\rightarrow FY\) with \(S\in \mathcal {S} \), there exist \(Y'\in \mathcal {S} \), \(h:Y'\rightarrow Y\) and \(f':S\rightarrow FY'\) *F*-precise with \(Fh\cdot f' = f\).

For Open image in new window , \(\mathcal {S} \) contains all sets. However for the category of nominal sets, \(\mathcal {S} \) will only contain the strong nominal sets (see details in Subsect. 4.2).

### Remark 3.5

Precise morphisms are essentially unique. If \(f_1:X\mathbin {\longrightarrow }FY_1\) and \(f_2:X\mathbin {\longrightarrow }FY_2\) are *F*-precise and if there is some \(h:Y_1\mathbin {\longrightarrow }Y_2\) with \(Fh\cdot f_1 = f_2\), then *h* is an isomorphism. Consequently, if \(f:S\mathbin {\longrightarrow }FY\) with \(S\in \mathcal {S} \) is *F*-precise and *F*-admits precise factorizations, then \(Y\in \mathcal {S} \).

Functors admitting precise factorizations are closed under basic constructions:

### Proposition 3.6

- 1.
Constant functors, if Open image in new window has an initial object 0 and \(0 \in \mathcal {S} \).

- 2.
\(F\cdot F'\) if Open image in new window and Open image in new window do so.

- 3.
\(\prod \limits _{i\in I} F_i\), if all \((F_i)_{i\in I}\) do so and \(\mathcal {S} \) is closed under

*I*-coproducts. - 4.
\(\coprod \limits _{i\in I} F_i\), if all \((F_i)_{i\in I}\) do so, Open image in new window is

*I*-extensive and \(\mathcal {S} \) is closed under*I*-coproducts. - 5.
Right-adjoint functors, if and only if its left-adjoint preserves \(\mathcal {S} \)-objects.

### Example 3.7

When Open image in new window is infinitary extensive and \(\mathcal {S} \) is closed under coproducts, every polynomial endofunctor Open image in new window admits precise factorizations w.r.t. \(\mathcal {S} \). This is in particular the case for Open image in new window . In this case, we shall see later (Sect. 4.1) that many other Open image in new window -functors, e.g. the bag functor \(\mathcal {B} \), where \(\mathcal {B} (X)\) is the set of finite multisets, have precise factorizations. In contrast, \(F=\mathcal {P} \) does not admit precise factorizations, and if \(f:X\mathbin {\longrightarrow }\mathcal {P} Y\) is \(\mathcal {P} \)-precise, then \(f(x) = \emptyset \) for all \(x\in X\).

### 3.2 Path Categories in Pointed Coalgebras

We define a path for *I*-pointed *TF*-coalgebras as a tree according to *F*. Following the observation in Example 3.3, one layer of the tree is modelled by a *F*-precise morphism and hence a path in a *TF*-coalgebra is defined to be a finite sequence of \((F+1)\)-precise maps, where the \(\_\!\_+1\) comes from the dead states w.r.t. *T*; the argument is given later in Remark 3.23 when reachability is discussed. Since the \(\_\!\_+1\) is not relevant yet, we define Open image in new window in the following and will use Open image in new window later. For simplicity, we write \(\varvec{X}_n\) for finite families \((X_k)_{0\le k < n}\).

### Definition 3.8

The category Open image in new window consists of the following. An object is \((\varvec{P}_{n+1},\varvec{p}_n)\) for an Open image in new window with \(P_0 = I\) and \(\varvec{p}_n\) a family of *F*-precise maps \((p_k:P_k\mathbin {\longrightarrow }FP_{k+1})_{k<n}\). We say that \((\varvec{P}_{n+1},\varvec{p}_n)\) is a *path of length* *n*. A morphism \(\varvec{\phi }_{n+1}:(\varvec{P}_{n+1},\varvec{p}_{n}) \mathbin {\longrightarrow }(\varvec{Q}_{m+1},\varvec{q}_{m})\), \(m\ge n\), is a family \((\phi _k:P_k \mathbin {\longrightarrow }Q_k)_{k\le n}\) with \(\phi _0={{\text { id }} _{I}}\) and \(q_k\cdot \phi _k = F\phi _{k+1}\cdot p_k\) for all \(0 \le k \le n\).

### Example 3.9

Paths for \(FX = A\times X + 1\) and \(I = \{*\}\) singleton are as follows. First, a map \(f:I \longrightarrow FX\) is precise iff (up-to isomorphism) either \(X = I\) and \(f(*) = (a, *)\) for some \(a \in A\); or \(X = \varnothing \) and \(f(*) = \bot \). Then a path is isomorphic to an object of the form: \(P_i = I\) for \(i \le k\), \(P_i = \varnothing \) for \(i > k\), \(p_i(*) = (a_i,*)\) for \(i < k\), and \(p_k(*) = \bot \). A path is the same as a word, plus some “junk”, concretely, a word in \(A^\star .\bot ^\star \). For LTSs, an object in Open image in new window with \(FX=A\times X\) is simply a word in \(A^\star \). For a more complicated functor, Fig. 2 depicts a path of length 4, which is a tree for the signature with one unary, one binary symbol, and a constant. The layers of the tree are the sets \(\varvec{P}_4\). Also note that since every \(p_i\) is *F*-precise, there is precisely one route to go from every element of a \(P_k\) to \(*\).

### Remark 3.10

The inductive continuation of Remark 3.5 is as follows. Given a morphism \(\varvec{\phi }_{n+1}\) in Open image in new window , since \(\phi _0\) is an isomorphism, then \(\phi _k\) is an isomorphism for all \(0\le k \le n\). If *F* admits precise factorizations and if \(I\in \mathcal {S} \), then for every path \((\varvec{P}_{n+1},\varvec{p}_{n})\), all \(P_k\), \(0\le k\le n\), are in \(\mathcal {S} \).

### Remark 3.11

If in Definition 3.4, the connecting morphism \(h:Y'\mathbin {\longrightarrow }Y\) uniquely exists, then it follows by induction that the hom-sets of Open image in new window are at most singleton. This is the case for all polynomial functors, but not the case for the bag functor on sets (discussed in Subsect. 4.1).

### Definition 3.12

The *path poset* Open image in new window is the set Open image in new window equipped with the order: for \(u:I\mathbin {\longrightarrow }F^n1\) and \(v:I\mathbin {\longrightarrow }F^m1\), we define \(u\le v\) if \(n \le m\) and \(F^n(!)\cdot v = u\).

So \(u\le v\) if *u* is the truncation of *v* to *n* levels. This matches the morphisms in Open image in new window that witnesses that one path is prefix of another:

### Proposition 3.13

1. The functor Open image in new window defined by \( I= P_0 \overset{p_0}{\rightarrow } FP_1\cdots \rightarrow F^{n}P_{n}\overset{F^{n}!}{\rightarrow }F^{n}1 \text { on }(\varvec{P}_{n+1},\varvec{p}_{n}) \) is full, and reflects isos.

2. If *F* admits precise factorizations w.r.t. \(\mathcal {S} \) and \(I\in \mathcal {S} \), then Open image in new window is sujective. 3. If additionally *h* in Definition 3.4 is unique, then Open image in new window has a right-inverse.

In particular, \(\mathsf {PathOrd} (I,F)\) is Open image in new window up to isomorphism. In the instances, it is often easier to characterize \(\mathsf {PathOrd} (I,F)\). This also shows that Open image in new window contains the elements – understood as morphisms from *I* – of the finite start of the final chain of *F*: \( 1 \xleftarrow {!} F1 \xleftarrow {F!} F^21 \xleftarrow {F^2!} F^31 \xleftarrow {}\cdots . \)

### Example 3.14

When \(FX = A\times X + 1\), \(F^n1\) is isomorphic to the set of words in \(A^\star .\bot ^\star \) of length *n*. Consequently, \(\mathsf {PathOrd} (I,F)\) is the set of words in \(A^\star .\bot ^\star \), equipped with the prefix order. In this case, Open image in new window is an equivalence of categories.

### 3.3 Embedding Paths into Pointed Coalgebras

The paths \((\varvec{P}_{n+1},\varvec{p}_n)\) embed into Open image in new window as one expects it for examples like Fig. 2: one takes the disjoint union of the \(P_k\), one has the pointing \(I=P_0\) and the linear structure of *F* is embedded into the branching type *T*.

During the presentation of the results, we require *T*, *F*, and *I* to have certain properties, which will be introduced one after the other. The full list of assumptions is summarized in Table 2:

(Ax1) – The main theorem will show that coalgebra homomorphisms in Open image in new window are the open maps for the path category Open image in new window . So from now on, we assume that Open image in new window has finite coproducts and to use the results from the previous sections, we fix a class Open image in new window such that \(F+1\) admits precise factorizations w.r.t. \(\mathcal {S} \) and that \(I\in \mathcal {S} \).

(Ax2) – Recall, that a family of morphisms \((e_i:X_i \mathbin {\longrightarrow }Y)_{i\in I}\) with common codomain is called jointly epic if for \(f,g:Y\mathbin {\longrightarrow }Z\) we have that \(f\cdot e_i = g\cdot e_i~\forall i\in I\) implies \(f=g\). For Open image in new window , this means, that every element \(y\in Y\) is in the image of some \(e_i\). Since we work with partial orders on *T*-homsets, we also need the generalization of this property if \(f\sqsubseteq g\) are of the form \(Y\mathbin {\longrightarrow }TZ'\).

(Ax3) – In this section, we encode paths as a pointed coalgebra by constructing a functor Open image in new window . For that we need to embed the linear behaviour \(FX+1\) into *TFX*. This is done by a natural transformation \([\eta ,\bot ]:{\text { Id }} +1\mathbin {\longrightarrow }T\), and we require that \(\bot :1\mathbin {\longrightarrow }T\) is a bottom element for \(\sqsubseteq \).

### Example 3.15

For the case where *T* is the powerset functor \(\mathcal {P} \), \(\eta \) is given by the unit \(\eta _X(x) = \{x\}\), and \(\bot \) is given by empty sets \(\bot _X(*) = \varnothing \).

### Definition 3.16

*I*-pointed

*TF*-coalgebra on \(\coprod \varvec{P}_{n+1} := \coprod _{0\le k\le n} P_k\). The pointing is given by \(\mathsf {in} _0:I =P_0\mathbin {\longrightarrow }\coprod \varvec{P}_{n+1} \) and the structure by:

### Example 3.17

In the case of LTSs, a path, or equivalently a word \(a_1...a_k.\bot ...\bot \in A^\star .\bot ^\star \), is mapped to the finite linear system over \(a_1...a_k\) (see Sect. 2.1), seen as a coalgebra (see Sect. 2.2).

### Proposition 3.18

Given a morphism \([x_k]_{k\le n}:\coprod \varvec{P}_{n+1} \mathbin {\longrightarrow }X\) for some system \((X,\xi ,x_0)\) and a path \((\varvec{P}_{n+1},\varvec{p}_{n})\), we have

Also note that the pointing \(x_0\) of the coalgebra is necessarily the first component of any run in it. In a run \([x_k]_{k\le n}\), \(p_k\) corresponds to an edge from \(x_k\) to \(x_{k+1}\).

### Example 3.19

For LTSs, since the \(P_k\) are singletons, \(x_k\) just picks the *k*th state of the run. The right-hand side of this lemma describes that this is a run iff there is a transition from the *k*th state and the \((k+1)-\)th state.

### 3.4 Open Morphisms Are Exactly Coalgebra Homomorphisms

In this section, we prove our main contribution, namely that Open image in new window -open maps in Open image in new window are exactly coalgebra homomorphisms. For the first direction of the main theorem, that is, that coalgebra homomorphisms are open, we need two extra axioms:

(Ax4) – describing that the order on Open image in new window is point-wise. This holds for the powerset because every set is the union of its singleton subsets.

(Ax5) – describing that Open image in new window admits a choice-principle. This holds for the powerset because whenever \(y \in h[x]\) for a map \(h:X\mathbin {\longrightarrow }Y\) and \(x\subseteq X\), then there is some \(\{x'\}\subseteq x\) with \(h(x') = y\).

### Theorem 3.20

Under the assumptions of Table 2, a coalgebra homomorphism in Open image in new window is Open image in new window -open.

Main assumptions on Open image in new window , \(\sqsubseteq ^T\), Open image in new window

The converse is not true in general, because intuitively, open maps reflect runs, and thus only reflect edges of reachable states, as we have seen in Sect. 2.1. The notion of a state being reached by a path is the following:

### Definition 3.21

A system \((X,\xi ,x_0)\) is *path-reachable* if the family of runs \([x_k]_{k\le n}:J(\varvec{P}_{n+1},\varvec{p}_n)\mathbin {\longrightarrow }(X,\xi ,x_0)\) (of paths from Open image in new window ) is jointly epic.

### Example 3.22

For LTSs, this means that every state in *X* is reached by a run, that is, there is a path from the initial state to every state of *X*.

### Remark 3.23

In Definition 3.21, it is crucial that we consider Open image in new window and not Open image in new window for functors incorporating ‘arities \(\ge 2\)’. This does not affect the example of LTSs, but for \(I=1\), \(FX=X\times X\) and \(T=\mathcal {P} \) in Open image in new window , the coalgebra \((X,\xi ,x_0)\) on \(X=\{x_0,y_1,y_2,z_1,z_2\}\) given by \( \xi (x_0) = \{ (y_1,y_2) \}, ~~ \xi (y_1) = \{ (z_1,z_2) \}, ~~ \xi (y_2) = \xi (z_1) = \xi (z_2) = \emptyset \) is path-reachable for Open image in new window . There is no run of a length 2 path from Open image in new window , because \(y_2\) has no successors, and so there is no path to \(z_1\) or to \(z_2\).

### Theorem 3.24

Under the assumptions of Table 2, if \((X,\xi ,x_0)\) is path-reachable, then an open morphism \(h:(X,\xi ,x_0) \mathbin {\longrightarrow }(Y,\zeta ,y_0)\) is a coalgebra homomorphism.

### 3.5 Connection to Other Notions of Reachability

There is another concise notion for reachability in the coalgebraic literature [2].

### Definition 3.25

A *subcoalgebra* of \((X,\xi ,x_0)\) is a coalgebra homomorphism \(h:(Y,\zeta ,y_0) \mathbin {\longrightarrow }(X,\xi ,x_0)\) that is carried by a monomorphism \(h:X\rightarrowtail Y\). Furthermore \((X,\xi ,x_0)\) is called *reachable* if it has no proper subcoalgebra, i.e. if any subcoalgebra *h* is an isomorphism.

Under the following assumptions, this notion coincides with the path-based definition of reachability (Definition 3.21).

### Assumption 3.26

For the present Subsect. 3.5, let Open image in new window be cocomplete, have (epi,mono)-factorizations and wide pullbacks of monomorphisms.

The first direction follows directly from Theorem 3.20:

### Proposition 3.27

Every path-reachable \((X,\xi ,x_0)\) has no proper subcoalgebra.

For the other direction it is needed that *TF* preserves arbitrary intersections, that is, wide pullbacks of monomorphisms. In Open image in new window , this means that for a family \((X_i\subseteq Y)_{i\in I}\) of subsets we have \(\bigcap _{i\in I} TFX_i = TF\bigcap _{i\in I} X_i\) as subsets of *TFY*.

### Proposition 3.28

If, furthermore, for every monomorphism \(m:Y\mathbin {\longrightarrow }Z\), the function Open image in new window Open image in new window reflects joins and if *TF* preserves arbitrary intersections, then a reachable coalgebra \((X,\xi ,x_0)\) is also path-reachable.

All those technical assumptions are satisfied in the case of LTSs, and will also be satisfied in all our instances in Sect. 4.

### 3.6 Trace Semantics for Pointed Coalgebras

*trace semantics*of a system \((X,\xi ,x_o)\) as the set:Since Open image in new window -open maps preserve and reflect runs, we have the following:

### Corollary 3.29

Open image in new window is a functor and if \(f:(X,\xi ,x_0) \longrightarrow (Y,\zeta ,y_0)\) is Open image in new window -open, then Open image in new window .

Let us look at two LTS-related examples (we will describe some others in the next section). First, for \(FX = A\times X\). The usual trace semantics is given by all the words in \(A^\star \) that are labelled of a run of a system. This trace semantics is obtained because \(\mathsf {PathOrd} (I,F) = \coprod _{n\ge 0} A^n\) and because Open image in new window maps every path to its underlying word. Another example is given for \(FX = A\times X + \{\checkmark \}\), where \(\checkmark \) marks final states. In this case, a path in Open image in new window of length *n* is either a path that can still be extended or encodes less than *n* steps to an accepting state \(\checkmark \). This obtains the trace semantics containing the set of accepted words, as in automata theory, plus the set of possibly infinite runs.

## 4 Instances

### 4.1 Analytic Functors and Tree Automata

In Example 3.7, we have seen that every polynomial Open image in new window -functors, in particular the functor \(X \mapsto A\times X\), has precise factorizations with respect to all sets. This allowed us to see LTSs, modelled as \(\{*\}\)-pointed \(\mathcal {P} (A\times \_\!\_)\)-coalgebra, as an instance of our theory. This allowed us in particular to describe their trace semantics using our path category in Sect. 3.6. This can be extended to tree automata as follows. Assume given a signature \(\varSigma \), that is, a collection Open image in new window of disjoint sets. When \(\sigma \) belongs to \(\varSigma _n\), we say that *n* is the *arity of* \(\sigma \) or that \(\sigma \) is a *symbol of arity n*. A top-down non-deterministic tree automata as defined in [6] is then the same as a \(\{*\}\)-pointed \(\mathcal {P} F\)-coalgebra where *F* is the polynomial functor \(X \mapsto \coprod _{\sigma \in \varSigma _n} X^n\). For this functor, \(F^n(1)\) is the set of trees over \(\varSigma \sqcup \{*(0)\}\) of depth at most \(n+1\) such that a leaf is labelled by \(*\) if and only if it is at depth \(n+1\). Intuitively, elements of \(F^n(1)\) are partial runs of length *n* that can possibly be extended. Then, the trace semantics of a tree automata, seen as a pointed coalgebra, is given by the set of partial runs of the automata. In particular, this contains the set of accepted finite trees as those partial runs without any \(*\), and the set of accepted infinite trees, encoded as the sequence of their truncations of depth *n*, for every *n*.

In the following, we would like to extend this to other kinds of tree automata by allowing some symmetries. For example, in a tree, we may not care about the order of the children. This boils down to quotient the set \(X^n\) of *n*-tuples, by some permutations of the indices. This can be done generally given a subgroup *G* of the permutation group \(\mathfrak {S}_n\) on *n* elements by defining \(X^n/G\) as the quotient of \(X^n\) under the equivalence relation: \((x_1, \ldots , x_n) \equiv _G (y_1, \ldots , y_n)\) iff there is \(\pi \in G\) such that for all *i*, \(x_i = y_{\pi (i)}\). Concretely, this means that we replace the polynomial functor *F* by a so-called *analytic functor*:

### Definition 4.1

[14, 15]**.** An *analytic* Open image in new window -functor is a functor of the form \(FX = \coprod _{\sigma \in \varSigma _n} X^{n}/G_\sigma \) where for every \(\sigma \in \varSigma _n\), we have a subgroup \(G_\sigma \) of the permutation group \(\mathfrak {S}_n\) on *n* elements.

### Example 4.2

*F*is given by \(\varSigma \) and \(G_\sigma \) as above, then \(\alpha _X\) is given by

### Proposition 4.3

For an analytic Open image in new window -functor *F*, the following are equivalent (1) a map \(f:X\mathbin {\longrightarrow }FY\) is *F*-precise, (2) \(\alpha _Y\cdot f\) is \(\mathcal {B} \)-precise, (3) every element of *Y* appears precisely once in the definition of *f*, i.e. for every \(y \in Y\), there is exactly one *x* in *X*, such that *f*(*x*) is the equivalence class of a tuple \((y_1, \ldots , y_n)\) where there is an index *i*, such that \(y_i = y\); and furthermore this index is unique. So every analytic functor has precise factorizations w.r.t. Open image in new window .

### 4.2 Nominal Sets: Regular Nondeterministic Nominal Automata

We derive an open map situation from the coalgebraic situation for *regular nondeterministic nominal automata* (*RNNAs*) [26]. They are an extension of automata to accept *words with binders*, consisting of literals Open image in new window and binders \({|}_{a}\) for Open image in new window ; the latter is counted as length 1. An example of such a word of length 4 is \(a{|}_{c}bc\), where the last *c* is bound by \({|}_{c}\). The order of binders makes difference: \({|}_{a}{|}_{b}ab \ne {|}_{a}{|}_{b}ba\). RNNAs are coalgebraically represented in the category of nominal sets [10], a formalism about atoms (e.g. variables) that sit in more complex structures (e.g. lambda terms), and gives a notion of *binding*. Because the choice principles (Ax4) and (Ax5) are not satisfied by every nominal sets, we instead use the class of *strong nominal sets* for the precise factorization (Definition 3.4).

### Definition 4.4

[10, 24]**.** Fix a countably infinite set Open image in new window , called the set of *atoms*. For the group Open image in new window of finite permutations on the set Open image in new window , a *group action* \((X,\cdot )\) is a set *X* together with a group homomorphism Open image in new window , written in infix notation. An element \(x\in X\) is *supported by* Open image in new window , if for all Open image in new window with \(\pi (a) =a \ \forall a\in S\) we have \(\pi \cdot x = x\). A *nominal set* is a group action for Open image in new window such that every \(x\in X\) is finitely supported, i.e. supported by a finite Open image in new window . A map \(f:(X,\cdot )\mathbin {\longrightarrow }(Y,\star )\) is *equivariant* if for all \(x\in X\) and Open image in new window we have \(f(\pi \cdot x) = \pi \star f(x)\). The category of nominal sets and equivariant maps is denoted by Open image in new window . A nominal set \((X,\cdot )\) is called *strong* if for all \(x\in X\) and Open image in new window with \(\pi \cdot x = x\) we have \(\pi (a) = a\) for all Open image in new window .

Intuitively, the support of an element is the set of free literals. An equivariant map can forget some of the support of an element, but can never introduce new atoms, i.e. Open image in new window . The intuition behind strong nominal sets is that all atoms appear in a fixed order, that is, Open image in new window is strong, but Open image in new window (the finite powerset) is not. We set \(\mathcal {S} \) to be the class of strong nominal sets:

### Example 4.5

The Open image in new window -functor of unordered pairs admits precise factorizations w.r.t. strong nominal sets, but not w.r.t. all nominal sets.

In the application, we fix the set Open image in new window of distinct *n*-tuples of atoms (\(n\ge 0\)) as the pointing. The hom-sets Open image in new window are ordered point-wise.

### Proposition 4.6

Uniformly finitely supported powerset Open image in new window satisfies (Ax2-5) w.r.t. \(\mathcal {S} \) the class of strong nominal sets.^{1}

As for *F*, we study an LTS-like functor, extended with the *binding functor* [10]:

### Definition 4.7

For a nominal set *X*, define the \(\alpha \)-equivalence relation Open image in new window on Open image in new window by: Open image in new window Denote the quotient by Open image in new window . The assignment Open image in new window extends to a functor, called the *binding functor* Open image in new window .

RNNA are precisely \(\mathcal {P}_{\mathsf {ufs}} F\)-coalgebras for Open image in new window [26]. In this paper we additionally consider initial states for RNNAs.

### Proposition 4.8

The binding functor Open image in new window admits precise factorizations w.r.t. strong nominal sets and so does Open image in new window .

An element in Open image in new window may be regarded as a word with binders under a context \(\varvec{a}\vdash w\), where Open image in new window , all literals in *w* are bound or in \(\varvec{a}\), and *w* may end with \(\checkmark \). Moreover, two word-in-contexts \(\varvec{a}\vdash w\) and \(\varvec{a}'\vdash w'\) are identified if their closures are \(\alpha \)-equivalent, that is, \({|}_{a_1}\cdots {|}_{a_n}w={|}_{a'_1}\cdots {|}_{a'_n}w'\). The trace semantics of a RNNA *T* contains all the word-in-contexts corresponding to runs in *T*. This trace semantics distinguishes whether words are concluded by \(\checkmark \).

### 4.3 Subsuming Arbitrary Open Morphism Situations

*I*for the initial state pointing. As usual in open map papers, we require that Open image in new window and Open image in new window have a common initial object Open image in new window . Observe that we have Open image in new window whereLasota considers coalgebras without pointing, but one indeed has a canonical pointing as follows. For Open image in new window , define the characteristic family Open image in new window by \(\chi ^P_Q = 1\) if \(P=Q\) and \(\chi ^P_Q = \emptyset \) if \(P\ne Q\). With this, we fix the pointing Open image in new window .

### Proposition 4.9

*T*, *F* and *I* satisfy the axioms from Table 2, with Open image in new window .

The path category in Open image in new window from our theory can be described as follows.

### Proposition 4.10

An object of Open image in new window is a sequence of composable Open image in new window -mor-

phisms Open image in new window .

## 5 Conclusions and Further Work

We proved that coalgebra homomorphisms for systems with non-deterministic branching can be seen as open maps for a canonical path-category, constructed from the computation type *F*. This limitation to non-deterministic systems is unsurprising: as we have proved in Sect. 4.3 on Lasota’s work [19], every open map situation can been encoded as a coalgebra situation with a powerset-like functor, so with non-deterministic branching. As a future work, we would like to extend this theory of path-categories to coalgebras for further kinds of branching, especially probabilistic and weighted. This will require (1) to adapt open maps to allow those kinds of branching (2) adapt the axioms from Table 2, by replacing the “+1” part of (Ax1) to something depending on the branching type.

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