KU Combinatorics Seminar
Spring 2026

• The Combinatorics Seminar meets on Fridays, 11am-12pm, in Snow 306 (including talks on Zoom). There will be cookies!
• Organizers: Jeremy Martin, Reuven Hodges, and Marge Bayer.
• Good general seminar-attending advice, especially for graduate students: The "Three Things" Exercise for getting things out of talks by Ravi Vakil
• Also consider attending the Graduate Online Combinatorics Colloquium.

Friday 1/23
Organizational meeting

Friday 1/30
Linus Setiabrata (MIT)
Double Orthodontia Formulas and Lascoux Positivity

Friday 2/6
Reuven Hodges
Comparability in Bruhat Orders

Abstract: I will talk about how my collaborators and I got a tight asymptotic bound for the number of comparable pairs in weak Bruhat order using tools from linear extensions, Plancherel measure, and RSK.

Friday 2/13
Marge Bayer
Flag Vectors of Polytopes

Friday 2/20
Han Yin
The Record Statistic and Forward Stability of Schubert Products

Friday 2/27
Jeremy Martin
Graph Varieties

Abstract: A graph variety is a space whose points parametrize "pictures" of a graph \(G\): arrangements of points and lines with incidence constraints given by \(G\). These spaces turn out to have lots of structure, both geometrically (the component structure is controlled by rigidity properties of the graph), algebraically (the defining equations are certain generating functions for trees), and topologically (the homology groups are determined by the Tutte polynomial). It has been several years since I studied these things in my PhD dissertation, but I will give an overview of what I know about them and the tools that come into play.

Friday 3/6
Jeremy Martin
Graph Varieties II

Friday 3/13
Andrés Molina
Oriented Matroids

Friday 3/20
No seminar (Spring Break)

Friday 3/27
TBA

Friday 4/3
David Carlip
Title TBA

Friday 4/10
TBA

Friday 4/17
TBA

Friday 4/24
TBA

Friday 5/1
Dan Guyer (University of Washington)
Partitionability for the cd-Index

Abstract: Face enumeration of simplicial complexes is a central topic in geometric combinatorics, and partitionability is a key combinatorial tool to study the associated \(f\)-vector. For objects like polytopes and regular CW spheres, one may wish to not only count the number of faces of each dimension but also keep track of all chains of faces. The flag \(f\)-vector records all possible chains of faces. In its most concise form, the flag \(f\)-vector is expressed via the cd-index, defined by Fine and Bayer--Klapper. In this talk, I will discuss how the concept of partitionability can be extended to study the cd-index. Namely, I will define the class of \(S\)-partitionable posets, demonstrate how they generalize (Eulerian) partitionable simplicial complexes, and illustrate how they admit a recursive combinatorial decomposition that allows one to compute a nonnegative cd-index. This is based on recent joint work with Felipe Caster and José Samper.

Friday 5/8
TBA (Stop Day)

For seminars from previous semesters, please see the KU Combinatorics Group page.

Last updated Wed 3/11/26