/ Eric Ramos: Categories of graphs with contractions

Eric Ramos: Categories of graphs with contractions

November 12, 2019
4:30 pm - 5:30 pm

Abstract: For a (connected, finite) graph G, we define its genus to be the quantity g := E – V + 1, where E is the number of edges of G and V is the number of vertices. While it is not the case that graph homomorphisms preserve this invariant, it is the case that contractions between graphs do. In this talk we will consider the category of all genus g graphs and contractions. More specifically, we consider integral representations of the opposite category, i.e. functors from the opposite category to Abelian groups. Using the combinatorics of graph minors, we will show that representations of this kind satisfy a Noetherian property. As applications of this technical result, we show that configuration spaces of graphs must satisfy certain strong finiteness conditions. We also deduce connections between the cohomology of this category and the integral cohomology of outer automorphism groups of free groups.This is joint work with Nick Proudfoot.