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.

Colorful complete bipartite subgraphs in generalized Kneser graphs

-- Frédéric Meunier (Ecole Nationale des Ponts et Chaussées, CERMICS)

Any proper coloring of a Kneser graph with a minimum number of colors contains an almost-complete bipartite subgraph with all colors on each side. (An almost-complete bipartite graph is a complete bipartite graph minus a perfect matching.) This is a theorem due to Chen (2012), which solved a conjecture about ...

Théorèmede rigidité et fonctions de Parking

-- Bérénice Delcroix-Oger (IRIF, Paris 7 Diderot)

Une question classique, mais difficile, de combinatoire algébrique est de savoir si une algèbre d'un type donné est libre sur l'ensemble de ses générateurs. Après avoir introduit tous les prérequis, j'expliquerai en quoi les théorèmes de rigidité pour les opérades, introduits en 2008 par Loday et récemment ...

Binary pattern of length greater than 14 are abelian-2-avoidable

-- Matthieu Rosenfeld (GALAC, LRI)

Summary: Two words u and v are abelian equivalent if they are permutation of each other ("aabc" and "baca" are abelian equivalent). Let w be a word and P= P1...Pn (where the Pi are the letters of P) a pattern (a word over another alphabet), we say that w ...


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Translations: fr