GETCO 2022 – 11th International Conference on Geometric and Topological Methods in Computer Science

GETCO 2022 will take place in Paris from May 30th to June 3rd 2022.

The Geometric and Topological Methods in Computer Science (GETCO) conference series focuses on applications of algebraic topology in computer science with special emphasis on concurrency, distributed computing, networking and other situations related to systems of sequential computers that communicate with each other. It is aimed at mathematicians and computer scientists working in or interested in these subjects, including researchers and graduate students. The aim of the conference is to exchange ideas and to initiate or expand research collaborations.

Special emphasis will be on


The conference is hybrid. If are not registered, you can still have the links for the online conference by mailing the organizers.

Please fill in this poll if you are interested in participating in next GETCO (2023).


The conference takes place from Monday, April 30th to Friday, June 3rd. All times are given in French local time (GMT+2).

The schedule is indicative and still subject to changes.

Time Monday 30 Tuesday 31 Wednesday 1 Thursday 2 Friday 3
Dir. top. Dyn. sys. TDA Logics
9:00–9:45 Uli Fahrenberg Jonathan Barmak Magnus Botnan Eric Finster 9:00–10:00 Peter Giesl
10:00–10:45 Dmitry Kozlov Thomas Wanner Magnus Botnan Eric Finster 10:10–11:10 Paige North
10:45–11:15 break break break break
11:15–12:15 Krzysztof Ziemiański Claudia Landi free discussions Amar Hadzihasanovic 11:15–12:15 Gregory Ginot
12:15–13:50 lunch lunch lunch lunch lunch
13:50–14:30 Gabbay-Losa // Kahl Chocano // Dey-Mrozek-Slechta social event Costa-Goubault-Putot-Jaulin // Fernandez-Piterman Rajsbaum-Raventós Pujol // Woukeng-Sadowski-Leskiewicz-Mrozek
14:30–15:30 breakout discussions breakout discussions social event breakout discussions 14:45–15:25 Nishimura
15:30–16:10 Haucourt-Coursolle // Boutry Lipiński-Dey-Mrozek-Slechta // Lenzen social event Calk-Goubault-Malbos // Przybylski-Mrozek
16:30–17:30 Jeremy Ledent Georg Struth social event
later cocktail reception conference dinner

Color code is red for tutorials, green for invited talks and blue for contributed talks. Some sessions are in parallel (indicated by //).

Day-by-day schedule

Monday 30

9:00 – 9:45 Uli Fahrenberg: Directed Topology and Concurrency

I will introduce the basics of directed algebraic topology and how it is motivated by the theory of concurrent processes. I will cover directed topological spaces, directed homotopy, deadlocks and forbidden regions. Then I will proceed to the combinatorial setting of precubical sets and higher-dimensional automata, their geometric realizations, and combinatorial analogues of directed paths and homotopies. In the last part of the talk I will cover invariants of directed tpological spaces, such as component categories, directed homology, and path spaces, and their applications.


10:00–10:45 Dmitry Kozlov

In this short tutorial I will sketch the simplicial approach to theoretical distributed computing. Specifically, I will look at the notion of protocol complexes and open problems related to them and the standard task Weak Symmetry Breaking.


11:15–12:15 Krzysztof Ziemiański: Higher dimensional automata: between topology and concurrency

Higher dimensional automata (HDA) are precubical sets with some additional structure. They serve as a model for concurrency, which can be examined with topological methods. Executions of an HDA can be regarded both topologically, as certain (directed) paths on its geometric realization, and combinatorially, as sequences of its cells. In my talk I will compare these two approaches and present a nice “permutahedral” model that unifies them. This model can be applied to construct some invariants of HDA and to show connections between executions of HDAs and braid groups.


13:50–14:30 M. Gabbay and G. Losa: The (semi)topology of distributed consensus

An analysis of distributed consensus under heterogeneous agreement requirements reveals a novel mathematical structure which is closely related to topological spaces. Details can be found in the preprint The semitopology of heterogeneous consensus.


13:50–14:30 T. Kahl: The homology language of a higher-dimensional automaton

Higher-dimensional automata are a very expressive combinatorial-topological model of concurrency. Roughly speaking, an HDA is an ordinary automaton equipped with two- and higher-dimensional cubes representing independence or concurrency of actions. In this talk, I will introduce the homology language of an HDA, which is a graded submodule of the exterior algebra on the HDA’s alphabet and which may be used to reason about the independence of processes and subsystems of the modeled concurrent system. The homology language is a directed homotopy invariant in the sense that it is invariant under certain directed maps (called cubical dimaps) that are homotopy equivalences. It can be shown that the homology language is compatible with both the tensor product of HDAs, which models the parallel composition of independent concurrent systems, and the coproduct of HDAs, which corresponds to the nondeterministic sum of concurrent systems. Using software, the homology language can be computed for HDAs modeling shared-variable systems described in the Promela language.

15:30–16:10 E. Haucourt and P.-Y. Coursolle: Non-existing and ill-behaved coequalizers of locally ordered spaces

Categories of locally ordered spaces are especially well-adapted to the realization of most precubical sets [Fajstrup et. al., 2006], though their colimits are not so easy to determine (in comparison with colimits in the category of d-spaces for example [Grandis, 2009, 1.4.0]). We use the plural here, as the notion of a locally ordered space vary from an author to another, only differing according to seemingly anodyne technical details. As we explain in this article, these differences have dramatic consequences on colimits. In particular, we show that most categories of locally ordered spaces are not cocomplete, thus answering a question that was neglected so far. The strategy is the following: given a directed loop γ on a locally ordered space X, we try to identify the image of γ with a single point. If it were taken in the category of d-spaces, such an identification would be likely to create a vortex [Grandis, 2009, 1.4.7], while locally ordered space have no vortices. Concretely, the antisymmetry of local orders gets more points to be identified than in a mere topological quotient. However, the effect of this phenomenon is in some sense limited to the neighbourhood of (the image of) γ. So the existence and the nature of the corresponding coequalizer strongly depends on the topology around the image of γ. As an extreme example, if the latter forms a connected component, the coequalizer exists and its underlying space matches with the topological coequalizer.


15:30–16:10 N. Boutry: Introducing PC n-Manifolds and P-well-composedness in Partially Ordered Sets

In discrete topology, discrete surfaces are well-known for their strong topological and regularity properties. Their definition is recursive and checking if a poset is a discrete surface is tractable. However, a discrete surface has not any boundary point, in the sense that the neighborhood of an element of a discrete surface is also a discrete surface. In this paper, we propose then to introduce a new definition of boundary, called border, based on the definition of discrete surfaces, to allow us to introduce poset-based connected manifolds (shortly PC n-manifolds or n-PCMs), the extension of stellar/combinatorial manifolds with boundaries but in partially ordered sets. Some strong properties of this border and of PCMs are provided.

16:30–17:30 Jeremy Ledent: Simplicial Models for Multi-Agent Epistemic Logic

Epistemic Logic is the modal logic of knowledge. It allows to reason about a finite set of agents who can know facts about the world, and about what the other agents know. The traditional Kripke-style semantics for epistemic logic is based on graphs whose vertices represent the possible worlds, and whose edges indicate the agents that cannot distinguish between two worlds. In this talk, I will present an alternative semantics for epistemic logic, based on combinatorial topology. The idea is to replace the Kripke graph by a simplicial complex, allowing for higher-dimensional connectivity between the possible worlds. In fact, every Kripke model can be turned into an equivalent simplicial model, thus uncovering its underlying geometric structure.

Our notion of simplicial model is inspired from the “protocol complex” approach to distributed computing. I will show how our framework can be used to analyse distributed computing, where the agents are the processes, and the possible worlds are all the possible executions of the system. In order to prove impossibility results, one must find an epistemic logic formula representing the knowledge that the processes should acquire in order to solve a task; and argue that such knowledge cannot be achieved.

This is joint work with Éric Goubault and Sergio Rajsbaum.


Tusday 31

9:00 – 10:45 Jonathan Barmak and Thomas Wanner: From Discrete Morse Theory to Combinatorial Topological Dynamics

Morse theory establishes a celebrated link between classical gradient dynamics and the topology of the underlying phase space. It provided the motivation for two independent developments. On the one hand, Conley’s theory of isolated invariant sets and Morse decompositions, which is a generalization of Morse theory, is able to encode the global dynamics of general dynamical systems using topological information. On the other hand, Forman’s discrete Morse theory on simplicial complexes, which is a combinatorial version of the classical theory, and has found numerous applications in mathematics, computer science, and applied sciences.

In this tutorial, we introduce recent work on combinatorial topological dynamics, which combines both of the above theories and leads as a special case to a dynamical Conley theory for Forman vector fields, and more general, for multivectors. This theory has been developed using the general framework of finite topological spaces, which contain simplicial complexes as a special case.

The tutorial consists of two parts:

slides for part 1

slides for part 2

11:15–12:15 Claudia Landi: Computing the matching distance of multi-parameter persistence from Morse critical values

Multi-parameter persistence modules are promising tools in topological data analysis of multivariate data that still need to maintain their promises due to their heavy computational and theoretical intricacies. They are defined by applying homology to multi-filtered cell complexes. On the other hand, discrete Morse theory is a well-understood theory that permits reducing a cell complex to the critical cells of a gradient vector field. Critical cells carry all the relevant homological information about the input data. A connection between the two theories comes from the fact that the entrance values of critical cells are sufficient to determine the fibered rank invariant of multi-parameter persistence. In this talk, after reviewing such a connection, I’ll consider the matching distance to compare the rank invariant of two persistence modules. I’ll show recent results about the exact computation of the matching distance using only the geometric patterns of the entrance values of the critical cells in the filtrations. This is joint work with Asilata Bapat, Robyn Brooks, Celia Hacker, and Barbara Mahler.


13:50–14:30 P. J. Chocano: Approximating Discrete Dynamical Systems

Discrete dynamical systems have been proved to be a very useful tool to model different situations, but a direct study of them may be difficult. For this reason, it is important to develop computational methods to get some of their relevant information. The goal of this contributed talk is to present topological methods to study fixed points. Finite topological spaces, that are combinatorial objects (partially ordered sets), have the same homology and homotopy groups of polyhedra and can be used to reconstruct them. Therefore, the idea is to use finite spaces to approximate discrete dynamical systems given by homeomorphisms f : K → K where K is a compact polyhedron. For this purpose, we first discuss the notion of dynamical system in this combinatorial setting and then introduce a class of multivalued maps inducing morphisms in homology groups. From this, we deduce a Lefschetz fixed point theorem. Finally, we use the theory developed to study fixed points of discrete dynamical systems defined on polyhedra and give some lines of future work.


13:50–14:30 T. Dey, M. Mrozek and R. Slechta: Capturing Changes in Dynamical Systems with Persistent Homology

One of the major achievements of topological data analysis is persistent homology or persistence, which is a tool that summarizes the changing homology of a sequence of simplicial complexes. In this talk, we show how to use persistence to capture the changing behavior of a sequence of combinatorial dynamical systems. Formally, we begin by computing the persistence of the so-called Conley index. The output of our method is a barcode that summarizes the evolving structure of a combinatorial dynamical system. Typically, changes in the barcode correspond to bifurcation points or a loss of isolation. We also show how to compute the persistence of the Conley-Morse graph, which permits one to capture the changing structure of a combinatorial dynamical system at different resolutions. Finally, we conclude by discussing directions for future work.


15:30–16:10 M. Lipiński, T. Dey, M. Mrozek and R. Slechta: Tracking Dynamical Features via Continuation and Persistence

In recent years, combinatorial dynamics have become an important subject of interest due to their potential in computational methods. Forman’s combinatorial vector fields theory became a cornerstone of combinatorial models for continuous-time dynamical systems. Recently, Mrozek extended Forman’s idea by proposing a much more flexible theory of multivector fields. The theory was enriched with the combinatorial theory of the Conley index and other mathematical objects allowing an extensive study of combinatorial systems. In the following works, a prominent topological data analysis tool, the persistent homology, has been used to study the robustness of combinatorial isolated invariant sets or to track changes in the Conley index and Morse-Conley graph. Our current work focuses on the idea of continuation. Two isolated invariant sets are said to be related by a continuation if one can be transformed into the other by a continuous deformation of a dynamical system. In particular, the Conley index of the isolated invariant set stays intact throughout the transformation. Our first goal was to adapt the concept of continuation into combinatorial settings of multivector fields. To this end, we had to introduce a combinatorial counterpart of a continuous deformation of a system. Due to the finite realm of our settings, the natural choice for the minimal perturbation of a system is an atomic refinement, i.e., splitting a multivector into two smaller multivectors. This imposes a topology on the space of all multivector fields and facilitates the construction resembling the classical definition of continuation. Secondly, we propose a tracking protocol, a canonical way of studying the evolution of an isolated invariant set. With the method, one can follow a chosen isolated invariant set, observe how it travels in phase space, and get hints where the set is passing through a bifurcation (at least on the level given by a data or a resolution). Moreover, we present the construction in the spirit of persistent homology. In particular, we show that the continuation is a special case of the persistence of the Conley index.


15:30–16:10 F. Lenzen: Efficient computation of two-parameter persistent cohomology

Among the many developments that have led to efficient algorithms in one-parameter persistence, “clearing” has been a rather effective tool to increase efficiency. The mechanism uses the fact that in a (co)chain complex, the composition of two consecutive (co)boundary maps is zero; a basis for the (co)boundaries thus can be extended to a basis of the (co)cycles. This extension is often much less work than a computation of a basis of the (co)cycles from scratch. For many filtered simplicial complexes of practical interest, such as Vietoris-Rips complexes, this mechanism is only feasible in combination with the computation of persistent cohomology, rather than homology; an approach that also increases the efficiency of other optimization schemes, such as implicit matrix representations. The computation of persistent cohomology instead of homology, possibly with clearing, has not been employed successfully in the computation of two-parameter persistent (co)homology. This seems to be mainly due to the more involved structure of the underlying algebraic objects. We show an approach to deal with this algebraic difficulties, and develop an algorithm for the efficient computation of persistent cohomology that allows for clearing. We explain how a correspondence between two-parameter persistent homology and cohomology can be established, similar to what is known for one-parameter persistence. We show experimental results of an implementation of our approach.

16:30–17:30 Georg Struth: Catoids as a Basis for Algebras of Programs

Algebras of programs, such as variants of Kleene algebras, relation algebras or quantales, are useful for program correctness and verification. They provide abstractions for various concrete semantics of programs and computing systems, qualitative or quantitative. Such semantics typically model program behaviours in terms of relations or predicate transformers, execution traces or dependency orders – or they form quantitative extensions of these.

I will argue that many of these algebras and their relationships with models arise from a uniform construction of convolution algebras from catoids, which generalise categories to (ternary) relational structures, and from modal correspondences between catoids and convolution algebras. The approach combines ideas from representation theory, group and category algebras, with the duality theory for ternary relations and boolean algebras with operators. Using catoids may streamline the development of algebras of programs and their models. Only a few simple catoid axioms may need to be checked to construct a model of a much richer algebraic structure, and lifting properties from models via catoids to algebras is often compositional. To construct a Kleene algebra or quantale for a given application, or a model for a given algebra, it is therefore worth asking what the underlying catoid might be.

I will explain the approach through two examples: modal Kleene algebras, which yield algebras of predicate transformers akin to dynamic logics, and concurrent Kleene algebras, which capture interleaving and non-interleaving semantics for concurrent systems. For both cases I will describe the catoids, and often categories, that formalise the standard semantics.

Combining these two examples, I will aim to explain how the approach helps justifying the globular n-Kleene algebra axioms, which have recently been proposed for higher-dimensional rewriting, in terms of globular n-dioids, and thus (strict) n-categories and n-polygraphs.

This talk is based on joint work with many colleagues I would like to thank Cameron Calk, James Cranch, Simon Doherty, Brijesh Dongol, Uli Fahrenberg, Ian Hayes, Christian Johansen, Philippe Malbos, Damien Pous and Krzysztof Ziemianski for their collaboration.


Wednesday 1

9:00 – 10:45 Magnus Botnan: A mini-survey of multiparameter persistent homology

Persistent homology associates a family of barcodes to a filtered topological space. The best-known example arises when applying homology in low dimensions to the filtered Rips complex built on a finite metric space. There are, however, many situations in which it is desirable to filter a topological space by multiple functions, and while one can apply homology to multifiltration in a straightforward way, the notion of a barcode does not generalize from the single-parameter setting. Given that nearly all the theory in the single-parameter setting is built upon the idea of a barcode, the non-existence of a barcode has been a major obstacle in the development of multiparameter persistent homology. However, in recent years there has been a surge in activity resulting in a range of new invariants and applications in the multiparameter setting. In this talk, I will discuss some of these. The talk will be based on a recent survey on multiparameter persistent homology that I have coauthored with Michael Lesnick.

full paper

Thursday 2

9:00 – 10:45 Eric Finster: Introduction to Homotopy Type Theory

In this talk, I will summarize some of the main intuitions of the homotopical interpretation of dependent type theory, introduced by Awodey and Warren and independently by Voevodsky around 15 years ago. Roughly speaking, this perspective reinterprets type theory not as a theory of constructive sets, but rather as a theory of constructive spaces up to homotopy equivalence. In doing so, it explains many of the counterintuitive aspects of the behavior of equality in type theory and has led to the introduction of many new ideas, both meta-theoretical, for example in the form of previously unknown models, as well as practical in the form of new computational principles which have since then been incorporated into modern proof assistants.


11:15–12:15 Amar Hadzihasanovic: Diagrammatic sets and homotopically sound rewriting

In the polygraph approach to rewriting, the fundamental structures of rewriting theory are “directed complexes” whose cells have an orientation in all higher dimensions. Unfortunately, the polygraph model of “directed spaces” does not admit a sound “orientation-forgetting” interpretation in a standard model of spaces, which would turn rewrites into homotopies in the expected way. In this tutorial talk, I will give an introduction to diagrammatic sets, an alternative model that is provably sound for homotopical algebra, while remaining close in practice to the polygraph model. I will briefly discuss its combinatorial underpinning and interpretation in topological spaces, then focus on practical aspects and differences from the polygraph model. If there is time, I will also discuss the “smash Gray product” construction of pointed diagrammatic sets and its intriguing application to higher algebra.


13:50–14:30 M. L. Costa Vianna, E. Goubault, S. Putot and L. Jaulin: A Geometric Approach to the Coverage Measure of the Area Explored by a Robot

13:50–14:30 X. Fernandez and K. Piterman: The persistent fundamental group of point clouds

We present an efficient algorithm based on discrete Morse theory to describe the persistent fundamental group of a finite metric space, in terms of group presentations. Our technique provides a tool to infer properties of the fundamental group of a topological space from a (possibly noisy) sample of data points, extending the scope of existing methods in topological data analysis such as persistent homology.


15:30 – 16:10 C. Calk, E. Goubault and P. Malbos: Persistence and the classification of directed spaces

In this work, we explore links between natural homology and persistent homology. The former is an algebraic invariant of directed spaces, which constitute a semantic model of concurrent programs. The latter was developed in the context of topological data analysis, and extracts topological properties of point-cloud data sets while eliminating noise. In both approaches, the evolution homological properties are tracked through a sequence of inclusions of usual topological spaces. Exploiting this similarity, we show that natural homology may be considered a persistence object, and may be calculated as a colimit of uni-dimensional persistent homologies along traces. Finally, we suggest further links and avenues of future work in this direction.

15:30–16:10 M. Przybylski and M. Mrozek: The Szymczak functor on the category of finite relations

The Szymczak functor is a tool used to construct Conley index for discrete-time dynamical systems. Due to a certain key property, it enables the correct definition of the index. Moreover, the functor is universal in the sense that any other functor with this property factorizes through the Szymczak functor. The universality of the Szymczak functor shows its generality, but is also responsible for its computational weakness, because there is no general method to tell whether two objects in the Szymczak category (i.e. target category of the functor) are isomorphic or not.

In this talk, I will present an algorithmizable classification of isomorphism classes in the Szymczak category over the category of finite sets with arbitrary relations as morphisms. Such a classification may provide a new method to study multivalued dynamical systems represented by relations. These multivalued dynamical systems arise naturally from a dynamics given by the data.

This is joint work with Marian Mrozek.

Friday 3

9:00–10:00 Peter Giesl: Computation of Lyapunov functions and contraction metrics for dynamical systems

We consider a general dynamical system, either continuous-time, given by solutions of an ordinary differential equation, or discrete-time, given by the iteration of a map. The long-term behaviour can be characterised by attractors and their corresponding basins of attraction. Examples of attractors are equilibria or periodic orbits, and the corresponding basins of attraction consist of all initial points which converge towards them. Tools to determine the basin of attraction include (complete) Lyapunov functions and contraction metrics. A Lyapunov function is a scalar-valued function which decreases along solutions; attractors are local minima and their basin of attraction can be determined using sublevel sets. A contraction metric is a metric such that the distance between adjacent solutions decreases with respect to the metric, and thus they share the same long-term behaviour. In this talk I will discuss the numerical construction of Lyapunov functions and contraction metrics. In particular, I will present computational methods using meshfree collocation with Radial Basis Functions (RBF) as well as Continuous Piecewise Affine (CPA) functions, compare them and illustrate them with examples.


10:10–11:10 Paige North: Directed homotopy type theory

In this talk, I will describe the development of a directed homotopy type theory. The aim is to capture (higher) categories and directed topological spaces as models of the theory, and thus use it to study phenomena (such as concurrency and rewriting) of interest to this community. To get this type theory, the identity type of Martin-Löf is modified to produce a “directed” identity type. The terms of this new directed identity type behave analogously to those of the usual Martin-Löf identity type: they can be composed, but not inverted. This has semantics in the category of categories in which the directed identity type is interpreted by hom-sets. I will also talk about work-in-progress in which techniques of modal type theory are used to modify the underlying syntax to allow for different kinds of transport: forward along directed identities, backward along directed identities, and along (usual, undirected) identities. These are reflected in the semantics, for example in Cat, as the lifting properties of Grothendieck opfibrations, fibrations, and isofibrations.


11:15–12:15 Gregory Ginot: Homotopical and sheaf theoretic point of view on multi-parameter persistence

A first goal of the talk will be to highlight the relationship between level sets persistence theory (which are sheaf theoretic naturally) with a certain type of 2-parameter persistence theory; precisely an equivalence between the first one and Mayer-Vietoris systems in 2-parameter theoy. Then we will explain how to generalize this idea in higher parameter persistence introducing a specific kind of higher parameter persistence with homotopical properties. This is based on joint work with Nicoals Berkouk and Steve Oudot.

13:50–14:30 S. Rajsbaum and A. Raventós Pujol: Arrow’s Impossibility theorem: topological perspectives

Full coverage of an area of interest is a common task for an autonomous robot. Estimating the area explored by the robot is indeed essential for determining if path-planning algorithms lead to complete coverage. In the presentation we are applying for, using a set membership approach, we propose a method for a guaranteed estimation of the area explored by an autonomous robot. The proposed algorithm is able to determine how many times each portion of the space has been sensed by the robot using a novel approach based on topological properties of the environment that has been scanned, and more precisely an estimation of certain winding numbers. This property is useful for localization inside homogeneous environments, e.g. the underwater environment, and assessment for potential revisiting missions. We demonstrate the efficiency of the presented approach on a real dataset acquired by an autonomous underwater robot.

13:50–14:30 D. Woukeng Feudjio, D. Sadowski, J. Leskiewicz and M. Mrozek: Computational approach to dynamics based on combinatorial multivector fields

A computational study of dynamical systems either given explicitly by a formula or only via a finite sample requires combinatorial tool. Among such tools is the concept of combinatorial multivector field, an extension of Forman’s concept of combinatorial vector field which may be studied by algorithmic means. The construction of a combinatorial multivector field combined with transversality may lead to computer-assisted proofs. However, the construction itself is a challenge that we intend to address. In particular, the construction of a multivector enclosing a stationary point, if not taken care of by special means, may lead to a loss of information at the end of the computation of our combinatorial multivector field, due to some properties that multivectors must satisfy. Hence we will first introduce an algorithm for the construction of a transversal polytope around each stationary point of some 2-D dynamical systems. Then, we will introduce an algorithm constructing combinatorial multivector fields from a 2-D vector field on a triangulation of a compact subset of 2, with certain transversality relation with respect to the flow. A similar construction but without transversality is also possible in the case of a dynamical system known only from a sample. We will proceed with the computation of Morse sets to show the features we can extract from our systems or from the data using those algorithms. The transversal polytope will be necessary while computing the Morse sets since it will allow us to easily separate all the stationary points with other features such as periodic orbits during the computation of Morse sets. This will lead us to computer-assisted proof of the existence of periodic orbits in some dynamical systems. Some examples of computation will be given at the end of the presentation for the case of 2-D dynamical systems such as the Van der Pol system for some fixed parameters and some sampled datasets.

14:45 – 15:25 S. Nishimura: Proving Unsolvability of Set Agreement Task with Epistemic mu-Calculus

This paper shows, in the framework of the logical method, the unsolvability of k-set agreement task by devising a suitable formula of epistemic logic. The unsolvability of k-set agreement task is a well-known fact, which is a direct consequence of Sperner’s lemma, a classic result from combinatorial topology. However, Sperner’s lemma does not provide a good intuition for the unsolvability, hiding it behind the elegance of its combinatorial statement. The logical method has a merit that it can account for the reason of unsolvability by a concrete formula, but no epistemic formula for the general unsolvability result for k-set agreement task has been presented so far.

We employ a variant of epistemic μ-calculus, which extends the standard epistemic logic with distributed knowledge operators and propositional fixpoints, as the formal language of logic. With these extensions, we can provide an epistemic μ-calculus formula that mentions higher-dimensional connectivity, which is essential in the original proof of Sperner’s lemma, and thereby show that k-set agreement tasks are not solvable even by multi-round protocols. Furthermore, we also show that the same formula applies to establish the unsolvability for k-concurrency, a submodel of the 2-round protocol.



The conference will take place at EPITA School of Engineering and Computer Science located 14-16 rue Voltaire 94270 Le Kremlin-Bicêtre (just at the south border of Paris):

Some recommendations of nearby hotels are

Social event

A visit of the musée du quai Branly is organized on Wednesday, June 1st. The bus leaves at 15h30 from EPITA, there is some free time (to visit gardens, the banks of Seine or see the Eiffel tower), and then the guided tour of the museum starts at 17h20. The guided tour will be in two groups, one in English and one in French. The bus will finally leave at 19h to the restaurant.

The dinner will take place in the Zeyer restaurant located 62 rue d’Alesia 75014 Paris, starting from 19h30.


A special issue of a journal will be dedicated to selected papers from the conference. The deadline is the 31st of January 2023. Submissions can be made at

The last two decades have witnessed a fruitful interplay between certain qualitative ideas and methods from geometric and topological areas on one side and from aspects of theoretical computer science on the other. Simplicial and cubical models in distributed computing and in concurrency theory are amenable to investigations using methods from algebraic topology – with a twist! Similar developments have emerged with point of departure in logics and rewriting, often through the lens of higher category theory and/or connected to Homotopy Type Theory. In a different line of research, the analysis and investigation of (hybrid variants of) control systems and robotics profit from developments in classical and combinatorial versions of Conley and Morse theory. Moreover, topological methods significantly contributed to the emergence of rigorous numerics as part of computer science with fundamental implications for control problems. It turns out that quite different aspects and topics from Computer Science lead to similar models and lines of topological investigations.

Topics of interest include, but are not limited to

Guest editors:

APCT editor:


The first GETCO conference was held in Aalborg in 1999. Applications of algebraic topology in concurrency was a new subject, fostered by seminal papers such as those by Vaughn Pratt in ACM POPL 1991 and Eric Goubault at CONCUR 1992 and CONCUR 1993, on the formal methods side, and the ACM STOC 1993 papers by Herlihy-Shavit and Saks-Zaharoglou, on the distributed computing side; brought to attention in the Workshop on New Connections between Mathematics and Computer Science, organized by Jeremy Gunawardena in Cambridge Nov. 1995.

The following seven GETCO workshops were held as satellites to CONCUR or DISC, the main conferences on concurrency and distributed computing. The 2nd was held at Penn State University, in 2000, then in Aalborg in 2001, Toulouse 2002, Marseille 2003, Amsterdam 2004, San Francisco 2005, and Bonn 2006.

The 2010 workshop had a broader scope and included further applications of algebraic topology including robotics and shape analysis. GETCO was back in Aalborg for its 9th edition in 2015, expanding to topics such as data analysis. By then, an ESF network ACAT, Applied and Computational Algebraic Topology, had been established, and two books had been published, Distributed Computing Through Combinatorial Topology and Directed Algebraic Topology and Concurrency; some of many indications that applications of algebraic topology to concurrent systems is now a mature subject, widespread and with impact in many fields. The 10th edition, which took place in Oaxaca, Mexico in 2018, expanded further to neuroscience and learning applications. The 11th edition was originally planned in 2020 in Palaiseau, but had to be delayed due to the pandemia.


Scientific committee

Organizing committee