Previous talks, of the year and of the previous ones: see the slidebar Talks at the right hand side.
Coming talks: (scheduled on Mondays from 9:30 to 10:30 (or during the slot of the GT if there is no GT) for the Petite École de Combinatoire and from 10:45 to 11:45 for the workgroup, in room 076 by default)
Contact the secretary (Nicolas Bonichon) if you want to propose a talk or to receive announcements.
Connexion (Zoom) : https://u-bordeaux-fr.zoom.us/j/89261618564?pwd=RlNWdldvN1dYMDdCQ2pYSVBUdE9xdz09

 password : via la mailing list


2021-2022

Lundi 6 décembre : Torsten Mütze

Titre : Efficient generation of elimination trees and Hamilton paths on graph associahedra

Résumé : An elimination tree for a connected graph G is a rooted tree on the vertices of G obtained by choosing a root x and recursing on the connected components of G−x to produce the subtrees of x. Elimination trees appear in many guises in computer science and discrete mathematics, and they are closely related to centered colorings and tree-depth. They also encode many interesting combinatorial objects, such as bitstrings, permutations and binary trees. We apply the recent Hartung-Hoang-Mütze-Williams combinatorial generation framework to elimination trees, and prove that all elimination trees for a chordal graph G can be generated by tree rotations using a simple greedy algorithm (see www.combos.org/elim). This yields a short proof for the existence of Hamilton paths on graph associahedra of chordal graphs. Graph associahedra are a general class of high-dimensional polytopes introduced by Carr, Devadoss, and Postnikov, whose vertices correspond to elimination trees and whose edges correspond to tree rotations. As special cases of our results, we recover several classical Gray codes for bitstrings, permutations and binary trees, and we obtain a new Gray code for partial permutations. Our algorithm for generating all elimination trees for a chordal graph G can be implemented in time O(m+n) per generated elimination tree, where m and n are the number of edges and vertices of G, respectively. If G is a tree, we improve this to a loopless algorithm running in time O(1) per generated elimination tree. We also prove that our algorithm produces a Hamilton cycle on the graph associahedron of G, rather than just Hamilton path, if the graph G is chordal and 2-connected. Moreover, our algorithm characterizes chordality, i.e., it computes a Hamilton path on the graph associahedron of G if and only if G is chordal.

This is joint work with Jean Cardinal (ULB) and Arturo Merino (TU Berlin)

Lundi 13 décembre : non disponible

Lundi 20 décembre : à saisir !

Lundi 10 janvier : Weinji Fang

Lundi 17 janvier : Julien Courtiel

11 octobre : journée cartes (40 ans de la bijection)

Connexion (BBB) : https://webconf.u-bordeaux.fr/b/nic-etz-jed

Titre :

Résumé :

Lundi 21 octobre :

Xavier Viennot

Titre : Une bijection insolite pour les arbres binaires

Résumé : We present a bijection, probably new, between binary trees and pairs of vectors (w,h). The first component is the so-called canopy of the binary tree, introduced by Loday and Ronco in relation with some Hopf algebras of trees and permutations. This bijection can be defined directly, but is in fact a by-product of a more general methodology called « demultiplication of equations in a quadratic algebra ». This methodology can also produce the famous Robinson-Schensted correspondence between permutations and pairs of Young tableaux. We will introduce various concepts such as normal ordering, Q-tableaux, reverse Q-tableaux, reverse quadratic algebra, planarization of the quadratic equations, demultiplication of equations, from which we will deduce the two above bijections. If time permits we will present other bijections, classical and new, obtained by this same methodology.

reference: Chapters 2 and 4, part III of "ABjC", http://www.viennot.org/abjc.html

Cédric Chauve

Titre :Exponentially few RNA structures are designable

Résumé :The problem of RNA design attempts to construct RNA sequences that performs a predefined biological function, identified by several additional constraints. One of the foremost objective of RNA design is that the designed RNA sequence should adopt a predefined target secondary structure preferentially to any alternative structure, according to a given metrics and folding model. It was observed in several works that some secondary structures are undesignable, i.e. no RNA sequence can fold into the target structure while satisfying some criterion measuring how preferential this folding is compared to alternative conformations. We show that the proportion of designable secondary structures decreases exponentially with the size of the target secondary structure, for various popular combinations of energy models and design objectives. This exponential decay is, at least in part, due to the existence of undesignable motifs, which can be generically constructed, and jointly analyzed to yield asymptotic upper-bounds on the number of designable structures.

Work in collaboration with Hua-Ting Yao , Mireille Regnier, Yann Ponty

02/09/2019 en 178 car colloque/workshop

28/10/2019 en 178 car 076 pour le colloque BGW

---

Muriel Livernet (Université Paris Diderot) https://webusers.imj-prg.fr/~muriel.livernet/ AÀP

Titre : Questions combinatoires ouvertes en topologie algébrique au travers des opérades

---

Pascal Weil

---

invité de Neuchatel d'Adrian Tanasa et Pascal Weil

Raoul Santachiara (Laboratoire de Physique Théorique et Modèles Statistiques, Université Paris-Sud (Paris-Saclay)) http://lptms.u-psud.fr/raoul_santachiara/ : à confirmer

Vincent Rivasseau (Université Paris-Sud (Paris-Saclay)) https://en.wikipedia.org/wiki/Vincent_Rivasseau

à confirmer (demande AAP 2018)

Iain Moffatt (Royal Holloway (université de Londres), Royaume-Uni) http://www.personal.rhul.ac.uk/uxah/001/

à confirmer (demande AAP 2018)

Laurent Vuillon (LAboratoire de MAthématiques (LAMA), Université Savoie Mont Blanc) http://www.lama.univ-savoie.fr/~vuillon/

Titre : version combinatoire des mots de la conjecture de Frobenius sur les nombres de Markoff [travaux récents avec C. Reutenauer]

Résumé :

Historique du Groupe de Travail et de Petite École de Combinatoire

Jours fériés (susceptibles de concerner un vendredi) : 1er novembre, 11 novembre, 25 décembre, 1er janvier, 1er mai, 8 mai, jeudi de l'Ascension (vers la mi-mai)

Colloque annuel ALEA (vers fin mars) ! 78th Séminaire Lotharingien de combinatoire, Ottrott March 26 - 29 2017, ThibonFest

Friday, jj(th) mmmm: ...

Title: ...

Abstract: ...

Envoyez un courriel au secrétaire (Olivier Guibert) du Groupe de Travail si vous avez un exposé à présenter.

journées ALÉA du ... au .../.../2015