Schedule, Notes/Handouts & Videos

We introduce a new model of a realization space of a polytope that arises as the positive part of a real variety. The variety is determined by the slack ideal of the polytope, a saturated determinantal ideal of a sparse generic matrix that encodes the combinatorics of the polytope. The slack ideal offers a uniform computational framework for several classical questions about polytopes such as rational realizability, projectively uniqueness, non-prescribability of faces, and realizability of combinatorial polytopes. The simplest slack ideals are toric. We identify the toric ideals that arise from projectively unique polytopes. New and classical examples illuminate the relationships between projective uniqueness and toric slack ideals.

When does a candidate have the approval of a majority? How does the geometry of the political spectrum influence the outcome? When mathematical objects have a social interpretation, the associated results have social applications. We will show how generalizations of Helly's Theorem can be used to understand voting in "agreeable" societies. This talk also features research with undergraduates.

Matthew Dyer’s proof that reflection orders give rise to EL-shellings for Bruhat order relied on the fact that the number of so-called ascending chains in a closed interval in Bruhat order is the leading coefficient in a polynomial closely related to the Kazhdan– Lusztig polynomial. We give a new, poset-theoretic proof that reflection orders yield EL-shellings for Bruhat order based on a characterization of cover relations in Bruhat order which may not be so well known.  This perspective also enables us to prove a conjecture of Thomas Lam that face posets of stratified spaces of response matrices of electrical networks are dual EL-shellable and are CW posets.

In this talk, I will discuss domino tilings of three dimensional manifolds. In particular, I will focus on the connected components of the space of tilings of such regions under local moves. Using topological techniques we introduce two parameters of tilings: the flux and the twist. Our main result characterizes when two tilings are connected by local moves in terms of these two parameters. (I will not assume any familiarity with the theory of tilings for the talk.)