jeu. 03 avril 2025 -- Mercedes Rosas (Universidad de Sevilla)

summary: We present a combinatorial proof of the Graham–Pollak formula for the determinant of the distance matrix of a tree, via sign-reversing involutions and the Lindström–Gessel–Viennot Lemma.
This is joint work with Emmanuel Briand, Luis Esquivias-Quintero, Álvaro Gutierrez, and Adrián Lillo.

jeu. 10 avril 2025 -- Ludovic Schwob (LIGM, Combi)

summary: Gog and Magog triangles are simple combinatorial objects which are equienumerated. Howewer, the problem of finding an explicit bijection between these has been an open problem since the 80’s. These are related to other interesting objects such as alternating sign matrices, plane partitions or aztec diamond tillings.
All ...

jeu. 13 mars 2025 -- Clément Chenevière (LISN, GALaC)

summary:

Les treillis cambriens, introduits par N. Reading en 2006, sont une généralisation du treillis de Tamari, à tout choix d'élément de Coxeter, dans tout groupe de Coxeter fini. Le treillis de Tamari correspond au "type A linéaire". Ces ordres partiels admettent plusieurs descriptions, non trivialement équivalentes. Celles-ci donnent ...

ven. 21 février 2025 -- Quentin Chuet (LISN, GALaC)

summary: Given a graph \(G\) of maximum degree \(\Delta\), the proper colouring problem asks for the minimum number of colours that can be assigned to the vertices of \(G\) such that no pair of adjacent vertices are given the same colour; it is easy to show that at most \(\Delta ...

ven. 14 février 2025 -- Pacôme Perrotin (LISN, Galac)

summary: Plane-walking automata were introduced by Salo & Törma to recognise languages of two-dimensional infinite words (subshifts), the counterpart of 4-way finite automata for two-dimensional finite words. We extend the model to allow for nondeterminism and alternation of quantifiers. We prove that the recognised subshifts form a strict subclass of sofic ...