On monday, at 2pm - UPEC CMC - Room P2-131

April 12, 2021

Milner et Alur entrent dans un bar

Daniele Varacca (LACL)

Alternating bisimulation for Alternating transition systems has been defined by Alur et al. It bears some ressemblence to the well known concept devised by Milner and Park for classical transition systems. The first aim of this work is to make the connection more formal: how can we make them look more similar to each other? Milner’s bisimulation is also a sound (and often complete) proof technique for contextual equivalences. What does “contextual equivalence” mean for alternanting transition systems, when there is no syntax, and therefore no notion of context? We propose an answer to this question, and we show that our version of alternating bisimulation is sound and complete for this generalised notion of contextual equivalence.
This is joint work with Romain Demangeon (LIP6) and Catalin Dima (LACL)

Vidéo de l’exposé

March 29, 2021

Rewriting Theory for Biochemical Reaction Systems

Nicolas Behr (Université de Paris, CNRS, IRIF)

The biochemistry platform Kappa (https://kappalanguage.org) offers high-performance simulation and static analysis algorithms for complex and real-life scale biochemical reaction systems. At the core of the system, a form of rewriting theory (of so-called site graphs) provides the semantics based upon which the notion of biochemical reactions is implemented.

In this talk, I will present recent original developments that investigate further the mathematical properties of the Kappa rewriting language. I will demonstrate that Kappa constitutes a typical example of a so-called restricted rewriting theory. Objects from some ambient category of graph-like structures are subjected to certain structural constraints to identify patterns (i.e., observable sub-structures) and fully specified biomolecules.

One of the core advantages of this new approach to rewriting theory in the life sciences consists of the compositional associativity properties carried by such restricted rewriting theories, which permits extending the static analysis toolkit to cover a class of pattern-counting observable moment evolution equations. Concretely, while state-spaces for realistic biochemical systems are typically prohibitively complex, it is envisioned that the study of the evolution of pattern-counts (i.e., roughly of functional groups) might prove a fruitful avenue to gain insights into the high-level behaviors of such systems.

Finally, time permitting, I will briefly sketch the latest stage of this development, namely the first-of-its-kind fully specified rewriting semantics for organo-chemical reaction systems.

The talk is based upon joint work with J. Krivine (IRIF), J.L. Andersen and D. Merkle (U Odense).

video de l’exposé

March 22, 2021

À propos des réseaux d’automates booléens

Sylvain Sené (LIS, Aix Marseille University)

Les réseaux d’automates booléens constituent un modèle mathématique dans lequel des entités (les automates) à états booléens interagissent les unes avec les autres au cours du temps (discret). C’est un modèle qui est apparu au moment où l’informatique moderne est née. Il a été introduit sous une forme spécifique dans les années 1940 par McCulloch et Pitts avec leur travail sur les réseaux de neurones formels. Il a par la suite été largement développé en suivant plusieurs lignes d’intérêt, avec entre autres les modèles de calcul, les systèmes dynamiques et la modélisation des réseaux biologiques.
Dans ce séminaire, nous évoquerons les réseaux d’automates booléens sous un angle plutôt théorique, en laissant de côté les liens qu’ils entretiennent avec la biologie. Plus précisément, nous parlerons de certaines des lois générales historiques connues à leur sujet, avant de présenter quelques résultats plus récents relatifs à leurs ensembles limites et d’ouvrir la discussion sur leur sensibilité structurelle.

March 15, 2021

Left and Right Kan Extensions in Cellular Automata

Luidnel Maignan (LACL)

Paraphrasing the famous book of Saunders Mac Lane: “Everything is a Kan Extension”. However, depending on ones background of course, there might not be many examples that arise directly as Kan Extensions. In this talk, we show how the latter describes very naturally local to global definitions, as is the case for cellular automata for instance. Indeed the formal definition simply says “you have a local transformation and a way to compare arbitrary transformations? So simply choose the best global transformation possible !”.
This is based on my works with Antoine Spicher and Alexandre Fernandez on so-called “Global Transformations

Vidéo de l’exposé

March 8, 2021

Computability, universality and quantum automata

Pablo Arrighi (LRI - University of Paris-Saclay )

I will draw a distinction between finite-dimensional quantum evolutions (“automata“) and infinite-dimensional evolutions (“operators”), and then explore their consequences upon two well-established concepts in Computer Science : computability and universality. Most of the results I will mention will rely on a decomposition of quantum operators, into quantum cellular automata—which is based upon the tacit assumption of a fixed partial order. Time-allowing, I will try to touch on the topical question of quantum partial orders.

video de l’exposé

March 1, 2021

Rational Players in Committee-based Blockchains

Yackolley Amoussou Guenou (CEA)

In most committee-based blockchains, participants propose blocks and then exchange messages, so that a block is accepted if a majority of participants sent the corresponding message.
In this presentation, we will present a game theoretic analysis of committee-based blockchains (which run distributed Byzantine fault tolerant consensus) when participants exhibit strategic or adversarial behaviour. All participants are rational and play best responses. Strategic participants maximize expected net rewards, while adversaries goal is to prevent the consensus.
We show that the prescribed protocol is not an equilibrium, and that there exist equilibria without consensus, even when there are few adversaries. However, under given conditions, there exists an equilibrium satisfying the consensus properties.

Vidéo de l’exposé

February 22, 2021

Threshold dot-depth one languages.

Nathan Grosshans (Theoretische Informatik / Komplexe Systeme, Universität Kassel)

Dot-depth one languages are regular languages for which membership of words
basically depends on the sequences of factors they contain. This class of
languages does also correspond to the Boolean closure of the fragment of first
order logic with successor where only existential quantification is allowed.

I this talk, I will present a new class of regular languages that is a
restriction of the class of dot-depth one languages where factor-detection
only works up to a certain threshold. The main motivation behind its
definition is to capture the effect of intersecting the class of dot-depth one
languages with another well-known class of languages: the one of unambiguous
polynomials. I shall explain why considering such a new class is interesting,
give some properties of it and establish its links with other classes of
languages. I will finish with some research avenues suggested by my work on
threshold dot-depth one languages.

February 15, 2021

A type theoretic approach to weak omega-categories

Thibaut BENJAMIN (CEA LIST)

Weak omega-categories are a very challenging concept to study rigorously in mathematics, as the axioms defining them are very intricate and complicated to verify. In this talk I will present a way of dealing with this complexity algorithmically, by designing a language relying on dependent type theory. This language allows for the implementation of an interactive theorem prover “CaTT” dedicated to work with weak omega-category. I will then present the semantics of this language and formally relate its models with anterior notions of weak omega-categories, which is the main contribution of my PhD thesis.

video de l’exposé

February 15, 2021

Proof Theory on Graphs

Matteo Acclavio (Université du Luxembourg)

Exceptionnellement deux séminaires aujourd’hui : celui-ci est à 15h15.

Logic and proof theory provide methods and tools used in areas such as formal verification and model checking. However, when we model processes-as-formulas, formulas are not expressive enough: they can model only some processes, those admitting a series-parallel decomposition, but they cannot handle the more general cases.
To overcome this limitation and develop new proof-theoretical tools for process analysis, in this talk I present a proof system based on unoriented graphs instead of formulas.

After showing how this system can find applications in process analysis, I will show some basic properties of the system.

Vidéo de l’exposé

February 8, 2021

Un cadre formel pour la conception correcte par construction de systèmes hybrides

Guillaume Dupont (INP Toulouse)

Les systèmes hybrides sont des systèmes qui intègrent des comportements discrets (ordinateurs, calculateurs) et des comportements continus (phénomènes physiques à contrôler). Les systèmes hybrides sont variés et versatiles, et de ce fait occupent une place de plus en plus importante dans notre vie quotidienne, que ce soit dans le cadre d’objets connectés ou pour des applications critiques telles que des voitures autonomes ou des avions.

La nature hybride de ces systèmes les rend particulièrement difficiles à vérifier : les méthodes formelles classiques sont adaptées à leurs parties discrètes, et leurs parties continues est l’objet d’étude de la théorie du contrôle, mais intégrer ces deux aspects au même niveau dans le processus de conception et de vérification de ces systèmes est encore un défi majeur du domaine.

Dans le cadre de nos travaux, nous avons mis au point un cadre formel dont le but est la conception correcte par construction de systèmes hybrides. Ce cadre se base sur la méthode Event-B, dont il exploite le mécanisme d’extension ainsi que l’opération de raffinement, afin de mettre à disposition un ensemble de patrons de conception formels, qui peuvent être utilisés et réutilisés pour concevoir tout type de système hybride, de manière incrémentale. Ces patrons ont été formellement prouvés en utilisant les outils de preuve de Event-B. Ce cadre sert par ailleurs de support à une méthode générique de conception formelle de ces systèmes, que nous illustrons sur divers études de cas.