A place cell is a neuron corresponding to a subset of Euclidean space known as a place field, that will fire if and only if the individual to which the neuron belongs is within that place field. The firing patterns of a collection of n place fields can be represented by a neural code C on n neurons, which is a subset of 2n . Determining whether C is convex, meaning that there is an arrangement of convex place fields for which C is the code, remains an open problem. A sufficient condition for convexity is being max intersection complete: any intersection of maximal codewords is also a codeword. Currently, the only way to determine this property is to evaluate all such intersections. We present a new method to determine max intersection completeness by introducing a simplicial complex for a code C called the factor complex ∆O(C) of C. We show how to construct ∆O(C) using Stanley-Reisner theory, describe how ∆O(C) encodes information about C, and give an algorithm to check whether C is max intersection complete using the factor complex of a closely related code.

The coordinate ring $S = \mathbb{C}[x_{i,j}]$ of space of $m \times n$ matrices carries an action of the group $\mathrm{GL}_m \times \mathrm{GL}_n$ via row and column operations on the matrix entries. If we consider any $\mathrm{GL}_m \times \mathrm{GL}_n$-invariant ideal $I$ in $S$, the syzygy modules $\mathrm{Tor}_i(I,\mathbb{C})$ will carry a natural action of $\mathrm{GL}_m \times \mathrm{GL}_n$. Via BGG correspondence, they also carry an action of $\bigwedge^{\bullet} (\mathbb{C}^m \otimes \mathbb{C}^n)$. It is a recent result by Raicu and Weyman that we can combine these actions together and make them modules over the general linear Lie superalgebra $\mathfrak{gl}(m|n)$. We will explain how this works and how it enables us to compute all Betti numbers of any $\mathrm{GL}_m \times \mathrm{GL}_n$-invariant ideal $I$. The latter part will involve combinatorics of Dyck paths

Intuitively, "dense" graphs contain any "desired substructure". In this talk, we will use a unified tool to prove few conjectures and open questions proposed since 1980s with this flavor, where the "desired substructure" is a set of cycles whose lengths satisfy certain conditions. They include two conjectures of Thomassen about minimum degree, a conjecture of Dean about connectivity, a conjecture of Sudakov and Verstraete about chromatic number, and an optimal answer of a question of Bondy and Vince about minimum degree. Joint work with Jun Gao, Qingyi Huo and Jie Ma.

A composition of Schubert problems is a construction that takes two Schubert problems on possibly different Grassmannians and gives a Schubert problem on a larger Grassmannian whose number of solutions is the product of the numbers of solutions of the original problems. This generalizes a construction that was discovered while classifying Schubert problems with imprimitive Galois groups.
I will explain this construction and the product formula, which has both an algebraic and a bijective proof. I will also discuss how this construction is related to Galois groups of Schubert problems. This is joint work with Li Ying and Robert Williams.

We prove several results about chordal graphs and weighted chordal graphs by focusing on exposed edges. These are edges that are properly contained in a single maximal complete subgraph. This leads to a characterization of chordal graphs via deletions of a sequence of exposed edges from a complete graph. Most interesting is that in this context the connected components of the edge-induced subgraph of exposed edges are 2-edge connected. We use this latter fact in the weighted case to give a modified version of Kruskal’s second algorithm for finding a minimum spanning tree in a weighted chordal graph. This modified algorithm benefits from being local in an important sense. In recent work with Culbertson, Dochtermann and Guralnik these results have been generalized, leading to a new result on Simon's conjecture concerning the extendable shellability of certain complexes.