jeu. 12 juin 2025 -- Giannos Stamoulis (CNRS, IRIF)

Roditty and Vassilevska-Williams [STOC 2013] showed that computing the diameter, i.e., farthest distance between any two vertices, of an (unweighted, undirected) graph cannot be done in subquadratic time unless the Strong Exponential Time Hypothesis fails. In a breakthrough work, Cabello [TALG 2019] showed a subquadratic algorithm in planar graphs ...

mer. 30 avril 2025 -- Benjamin Dequêne (Université de Leeds)

Un méandre (de longueur n) est une paire de correspondences non-croisées sur {1,...,n}. On peut le représenter sur le plan réel comme un paire d'ensemble de demi-cercles, qui ne s'intersentent pas deux à deux, reliant n points sur une droite. Cette représentation géométrique permet d'introduire différentes ...

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. 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.