# Online talk: Jim Geelen

Mon, August 3, 3pm ET (8pm BST, 7am Tue NZST)
Jim Geelen, University of Waterloo
Towards the excluded-minors for GF(5)-representability

Abstract:
I will discuss a strategy towards finding the excluded-minors for GF(5)-representability of matroids. This includes some joint work with Rutger Campbell and Geoff Whittle.

# Online talk: Zach Walsh

Mon, July 27, 3pm ET (8pm BST, 7am Tue NZST)
Zach Walsh, University of Waterloo

Abstract:
The extremal function of a class of matroids is the function whose value at an integer $n$ is the maximum number of elements of a simple matroid in the class of rank at most $n$. We present a result concerning the role of group-labeled graphs in minor-closed classes of matroids, and then use it to determine the extremal function, for all but finitely many $n$, for the class of complex-representable matroids which exclude a given rank-2 uniform matroid as a minor. This talk will focus on our original motivation, and on the connection between group-labeled graphs and representable matroids.

This is joint work with Jim Geelen and Peter Nelson.

# Online talk: Pascal Gollin

Mon, July 20, 3pm ET (8pm BST, 7am Tue NZST)
Pascal Gollin, Institute for Basic Science
Obstructions for bounded branch-depth in matroids

Abstract:
DeVos, Kwon, and Oum introduced the concept of branch-depth of matroids as a natural analogue of tree-depth of graphs. They conjectured that a matroid of sufficiently large branch-depth contains the uniform matroid $U_{n,2n}$ or the cycle matroid of a large fan graph as a minor. In this talk, I present a proof that matroids of sufficiently large branch-depth either contain the cycle matroid of a large fan graph as a minor or have large branch-width.

This is joint work with Kevin Hendrey, Dillon Mayhew and Sang-il Oum.

# Online Talk: Federico Ardila

Mon, July 13, 3pm ET (8pm BST, 7am Tue NZST)
Federico Ardila, SFSU, U. de Los Andes
Geometry of Matroids