# Online talk: Alexey Pokrovskiy (non-standard day & time!)

We’re really excited to have Alexey Pokrovskiy speak on his fantastic recent result on Rota’s Basis Conjecture this upcoming week. This talk has been moved to be a part of the Tutte Colloquium at Waterloo and so will be held at a different day and time than usual. Here are the full details.

##### Friday, September 25,1pm ET (6pm BST, 5am Tue NZST)Alexey Pokrovskiy, Birkbeck, University of LondonRota’s Basis Conjecture holds asymptoticallyZoom [Type the first ten characters of the usual password. Email rose.mccarty ~at~ uwaterloo ~.~ ca if you need the password]

Abstract:
Rota’s Basis Conjecture is a well known problem, that states that for any collection of $n$ bases in a rank $n$ matroid, it is possible to decompose all the elements into $n$ disjoint rainbow bases. Here an asymptotic version of this is will be discussed – that it is possible to find $n – o(n)$ disjoint rainbow independent sets of size $n – o(n)$.

# Online talk: Oliver Lorscheid

Mon, September 14, 3pm ET (8pm BST, 7am Tue NZST)
Oliver Lorscheid, Instituto Nacional de Matemática Pura e Aplicada
Foundations of Matroids without Large Uniform Minors, Part 2
Zoom
[email rose.mccarty ~at~ uwaterloo ~.~ ca if you need the password]

Abstract:
In this talk, we take a look under the hood of last week’s talk by Matt Baker: we inspect the foundation of a matroid.

The first desired properties follow readily from its definition: the foundation represents the rescaling classes of the matroid and shows a functorial behaviour with respect to minors and dualization. It requires however deep results by Tutte, Dress-Wenzel and Gelfand-Rybnikov-Stone to gain an understanding of the foundation in terms of generators and relations.

For small matroids, this allows us to determine the foundation explicitly. This, in turn, lets us derive the structure theorem for foundations of matroids without large uniform minors, which is the central result behind the applications from last week.

In a concluding part of the talk, we turn to potential future directions and open problems.

# Online talk: Matt Baker

Mon, September 7, 3pm ET (8pm BST, 7am Tue NZST)
Matt Baker, Georgia Tech
Foundations of Matroids without Large Uniform Minors, Part 1

Abstract:
Matroid theorists are of course very interested in questions concerning representability of matroids over fields. More generally, one can ask about representability over partial fields in the sense of Semple and Whittle. Pendavingh and van Zwam introduced the universal partial field of a matroid $M$, which governs the representations of $M$ over all partial fields. Unfortunately, almost all matroids are not representable over any partial field, and in this case, the universal partial field gives no information.

Oliver Lorscheid and I have introduced a generalization of the universal partial field which we call the foundation of a matroid. The foundation of $M$ is a type of algebraic object which we call a pasture; pastures include both hyperfields and partial fields. Pastures form a natural class of field-like objects within Lorscheid’s theory of ordered blueprints, and they have desirable categorical properties (e.g., existence of products and coproducts) that make them a natural context in which to study algebraic invariants of matroids. The foundation of a matroid $M$ represents the functor taking a pasture $F$ to the set of rescaling equivalence classes of $F$-representations of $M$; in particular, $M$ is representable over a pasture $F$ if and only if there is a homomorphism from the foundation of $M$ to $F$.

As a particular application of this point of view, I will explain the classification which Lorscheid and I have recently obtained of all possible foundations for matroids having no $U(2,5)$ or $U(3,5)$ minors. The proof of this classification theorem relies crucially on Tutte’s Homotopy Theorem and the theory of cross-ratios for matroids. Among other things, our classification provides a short conceptual proof of the 1997 theorem of Lee and Scobee which says that a matroid is both ternary and orientable if and only if it is dyadic.

This is part 1 of a series of two talks. The second talk will be given the following week by Oliver Lorscheid.

# Online talk: Tony Huynh

Mon, August 31, 3pm ET (8pm BST, 7am Tue NZST)
Tony Huynh, Monash University
Subgraph densities in a surface
We consider the following problem at the intersection of extremal and structural graph theory. Given a fixed graph $H$ and surface $\Sigma$, what is the maximum number of copies of $H$ in an $n$-vertex graph that embeds in $\Sigma$? Here a copy means a subgraph isomorphic to $H$. Exact answers are known for some $H$ when $\Sigma$ is the sphere. Our main result answers the question for all $H$ and $\Sigma$ (up to a constant factor). We show that the answer is $\Theta(n^{f(H)})$ where $f(H)$ is a graph invariant called the flap number of $H$, which is independent of the surface $\Sigma$.