Online talk: Daryl Funk

Monday, April 12, 3pm ET (8pm BST, 7am Tue NZST)
Daryl Funk, Douglas College
The class of bicircular matroids has only a finite number of excluded minors

Password: email rose.mccarty ~at~ uwaterloo ~.~ ca if you need the password
Abstract:

We show that the class of bicircular matroids has only a finite number of excluded minors. Key tools used in our proof include representations of matroids by biased graphs and the recently introduced class of quasi-graphic matroids. We show that if $N$ is an excluded minor of rank at least eight, then $N$ is quasi-graphic. Several small excluded minors are quasi-graphic. Using biased-graphic representations, we find that $N$ already contains one of these. We also provide an upper bound, in terms of rank, on the number of elements in an excluded minor, so the result follows.

Online talk: Charles Semple

Monday, March 29, 3pm ET (8pm BST, 8am Tue NZDT)
Charles Semple, University of Canterbury
Matroids with wheel- and whirl-like properties

Abstract:

Tutte showed that wheels and whirls are precisely the $3$-connected matroids in which every element is contained in a $3$-element circuit and a $3$-element cocircuit. As a consequence, wheels and whirls are exactly the $3$-connected matroids in which there is a circular ordering of the ground set such that every two consecutive elements is contained in a $3$-element circuit and a $3$-element cocircuit. More recently, Miller proved that sufficiently large (tipless) spikes are precisely the matroids in which every two elements is contained in a $4$-element circuit and a $4$-element cocircuit. In this talk, we investigate matroids satisfying generalisations of these properties and discuss some recent results. This is joint work with Nick Brettell, Deborah Chun, Tara Fife, James Oxley, Simon Pfeil, Gerry Toft, and Geoff Whittle.

Online talk: Benjamin Moore

Monday, March 15, 3pm ET (7pm GMT, 8am Tue NZDT)
Benjamin Moore, University of Waterloo
A density bound for triangle free 4-critical graphs

Zoom: https://us02web.zoom.us/j/89998025625
Password: email rose.mccarty ~at~ uwaterloo ~.~ ca if you need the password

Abstract:

Carsten Thomassen showed that every girth 5 graph embeddable in the torus or projective plane is 3-colourable. A complementary result of Robin Thomas and Barrett Walls shows that every girth 5 graph embedded in the Klein bottle is 3-colourable.

I’ll show neither the embeddability condition nor the girth 5 condition is needed in the above theorems by showing that every triangle-free 4-critical graph has average degree slightly larger than 10/3.

This is joint work with Evelyne Smith Roberge.

Online talk: Nick Brettell

Monday, March 8, 3pm ET (8pm GMT, 9am Tue NZDT)
Nick Brettell‬, Victoria University of Wellington
The excluded minors for 2-regular matroids, and for Hydra-5-representable matroids