Combinatorics

The main focus of this activity is the interrelation between algebraic structure and algorithms. We plan to work on the following subjects:

More precisely, the research project takes place in effective algebraic combinatorics, at the interface of enumerative combinatorics and analysis of algorithms on one hand and symbolic and algebraic computation on the other hand. The objective is twofold: firstly, thanks to vast generalization of the notion of generating series, we hope to give a theoretical framework allowing to study the fine behavior of various algorithms. Reciprocally, the study of those very same algorithms gives a new mean to discover algebraic identities. Those identities have many applications in mathematics, in particular in representation theory but also in physics (mainly statistical physics).

The research relies deeply on computer experimentation and contains as a consequence an important software development part within the Sage-Combinat software project. However, the required level of sophistication, flexibility, and breath of computational tools is reaching a point where large scale collaborative development is critical. The design and collaborative development of such a software is raising research-grade computer science challenges around the modelling of mathematics, the management of large hierarchy of (object oriented) classes, etc.

Those very specific questions also raise more general combinatorial questions. We therefore plan to work on enumerative combinatorics and cellular automaton, in particular on trees. This activity is conducted with close collaborators in France, Germany, North America, and India.

Skipless chain decompositions and improved poset saturation bounds

-- Paul Bastide (LaBRI (Bordeaux))

summary: We show that given m disjoint chains in the Boolean lattice, we can create m disjoint skipless chains that cover the same elements (where we call a chain skipless if any two consecutive elements differ in size by exactly one). By using this result we are able to answer ...


Asymptotic behaviour of cyclic automata

-- Benjamin Hellouin de Menibus (LISN, Galac)

summary: Cyclic dominance describes models where different states (species, strategies...) are in some cyclic prey-predator relationship: for example, rock-paper-scissors. This occurs in many contexts such as ecological systems, evolutionary games on graphs, etc. Many models exhibit heteroclinic cycles where one state dominates almost the whole space before being replaced by ...

La boîte aux lettres avait des dents : propriétés des pièges à facteurs dans le cas bi-infini

-- Pierre Béaur (LISN, Galac)

summary: En 2017, en algorithmique de texte, Prezza a introduit la notion de piège à facteurs : pour un mot fini w, un piège à facteurs pour w est un ensemble E de positions de w telles que pour tout facteur f de w, il existe une position de E qui ...

Polynômes de Jack et constellations b-déformées

-- Victor Nador (LaBRI, Bordeaux)

La série génératrice des cartes orientables pondérées (et sa généralisation aux constellations) peut s’exprimer simplement à l’aide des fonctions de Schur. La série des cartes non-orientées (c’est à dire orientable ou non) admet une expression similaire où les fonctions de Schur sont remplacées par les polynomes zonaux ...

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Translations: fr