Every new/old PhD student is invited to come and talk about her/his research interests.

Each talk will be 2-5 minutes long, without any slides, and should be understandable by everyone

Un arbre binaire compacté est un graphe acyclique dirigé qui représente un arbre binaire de façon sans redondances, dans le sens que tous les sous-arbres isomorphes sont partagés. Nous montrons que le nombre des arbres binaires compactés de tailles n croît asymptotiquement en

\Theta(n! 4^n e^{3 a_1 n^{1/3}} n^{3/4}).

Ici, a_1 ≈ -2,338 est la plus grande racine de la fonction d'Airy. Ce résultat est obtenu à partir d'une nouvelle récurrence des nombres de ces arbres compactés, avec une nouvelle méthode qui, inspirée par des estimations empiriques suffissament précises, prouve des bonnes bornes par induction. Ce travail donne aussi des nouvelles bornes sur le nombre d'automates minimaux qui reconnaissent un langage fini d'un alphabet binaire, qui possède aussi un exponentiel étiré. Par sa simplicité, notre méthode s'applique potentiellement aux autres objets.

http://chocola.ens-lyon.fr/events/

IRIF Cake^TM is an amazing opportunity to meet people while simultaneously eating cakes baked by your fellow colleagues! Join us every Thursday, at 5pm, in front of room 3052 (Sophie Germain 3rd floor) for a weekly feast. You can also express your cooking skills and volunteer to bake a cake by sending an email to cake@irif.fr.

Il y a plusieurs caractérisations algébriques (dites “axiomatiques”) de l'entropie discrète de Shannon. Je vais présenter ici une caractérisation analogue pour l'entropie différentielle, qui apparaît dans la théorie de compression des signaux continus: le cadre utilisé est la “cohomologie de l'information”, associé aux préfaisceaux sur certaines structures combinatoires. Si le temps le permet, je vais présenter d'autres théories cohomologiques des faisceaux très proches, définies sur les graphes (Friedman) et les complexes simpliciaux (Abramsky et al.), qui correspondent aussi à la cohomologie des topos.

I will talk about two fun constructions for reachability analysis of one-clock timed automata, which lead to concise logical characterizations in existential Linear Arithmetic.

The first one describes “punctual” reachability relations: reachability in exact time t. It uses a coarse interval abstraction and counting of resets via Parikh-Automata. The other is a “sweep line” construction to compute optimal time to reach in reachability games played on one-clock TA.

Together, these can be used to derive a (tight) NP complexity upper bound for the coverability and reachability problems in an interesting subclass of Timed Petri Nets, which naturally lends itself to parametrised safety checking of concurrent, real-time systems. This contrasts with known super-Ackermannian completeness, and undecidability results for unrestricted Timed Petri nets.

This is joint work with Lorenzo Clemente and Piotr Hofman, and was presented at CONCUR'19. Full details are available at https://arxiv.org/abs/1907.01240.

I will talk about two fun constructions for reachability analysis of one-clock timed automata, which lead to concise logical characterizations in existential Linear Arithmetic.

The first one describes “punctual” reachability relations: reachability in exact time t. It uses a coarse interval abstraction and counting of resets via Parikh-Automata. The other is a “sweep line” construction to compute optimal time to reach in reachability games played on one-clock TA.

Together, these can be used to derive a (tight) NP complexity upper bound for the coverability and reachability problems in an interesting subclass of Timed Petri Nets, which naturally lends itself to parametrised safety checking of concurrent, real-time systems. This contrasts with known super-Ackermannian completeness, and undecidability results for unrestricted Timed Petri nets.

This is joint work with Lorenzo Clemente and Piotr Hofman, and was presented at CONCUR'19. Full details are available at https://arxiv.org/abs/1907.01240.

We consider the model-checking problem for freeze LTL on one-counter automata (OCAs). Freeze LTL extends LTL with the freeze quantifier, which allows one to store different counter values of a run in registers so that they can be compared with one another. As the model-checking problem is undecidable in general, we focus on the flat fragment of freeze LTL, in which the usage of the freeze quantifier is restricted. Recently, Lechner et al. showed that model checking for flat freeze LTL on OCAs with binary encoding of counter updates is decidable and in 2NEXPTIME. In this paper, we prove that the problem is, in fact, NEXPTIME-complete no matter whether counter updates are encoded in unary or binary. Like Lechner et al., we rely on a reduction to the reachability problem in OCAs with parameterized tests (OCAPs). The new aspect is that we simulate OCAPs by alternating two-way automata over words. This implies an exponential upper bound on the parameter values that we exploit towards an NP algorithm for reachability in OCAPs with unary updates. We obtain our main result as a corollary.