## Seminars and Colloquia by Series

Monday, February 15, 2016 - 15:05 , Location: Skiles 006 , , University of Georgia , Organizer: Josephine Yu
I'll discuss joint work with my brother Jeff Giansiracusa in which we introduce an exterior algebra and wedge product in the idempotent setting that play for tropical linear spaces (i.e., valuated matroids) a very similar role as the usual ones do for vector spaces.  In particular, by working over the Boolean semifield this gives a new perspective on matroids.
Monday, February 8, 2016 - 15:05 , Location: Skiles 006 , Felipe Pérez , Georgia State University , Organizer: Josephine Yu
For the last four decades, mathematicians have used the Frobenius map to investigate phenomena in several fields of mathematics including Algebraic Geometry. The goal of this talk is twofold, first to give a brief introduction to the study of singularities in positive characteristic (aided by the Frobenius map). Second to define an explain the constancy regions for mixed test ideals in the case of a regular ambient;  an invariant associated to a family of functions that shows a Fractal behavior.
Monday, February 1, 2016 - 15:05 , Location: Skiles 006 , , UT Knoxville , Organizer: Josephine Yu
The dual complex of a semistable degeneration records the combinatorics of the intersections in the special fiber. More generally, one can associate a polyhedral dual complex to any toroidal degeneration. It is natural to ask for connections between the geometry of an algebraic variety and the combinatorial properties of its dual complex. In this talk, I will explain one such result: The dual complex of an n-dimensional uniruled variety has the homotopy type of an (n-1)-dimensional simplicial complex. The key technical tool is a specialization map to dual complexes and a balancing condition for these specialization.
Monday, January 25, 2016 - 15:05 , Location: Skiles 006 , Bo Lin , UC Berkeley , Organizer: Josephine Yu
A Linear system on metric graphs is a set of effective divisors.  It has the structure of a cell complex. We introduce the anchor divisors in it - they serve as the landmarks for us to compute the f-vector of the complex and find all cells in the complex. A linear system can also be identified as a tropical convex hull of rational functions. We can also compute the extremal generators of the tropical convex hull using the landmarks. We apply these methods to some examples - $K_{4}$ and $K_{3,3}$..
Monday, January 11, 2016 - 15:05 , Location: Skiles 006 , Louis Rowen , Bar-Ilan University , Organizer: Matt Baker
Tropicalization involves passing to an ordered group M, usually taken to be (R, +) or (Q, +), and viewed as a semifield. Although there is a rich theory arising from this viewpoint, idempotent semirings possess a restricted algebraic structure theory, and also do not reflect important valuation-theoretic properties, thereby forcing researchers to rely often on combinatoric techniques. Our research in tropical algebra has focused on coping with the fact that the max-plus’ algebra lacks negation, which is used throughout the classical structure theory of modules. At the outset one is confronted with the obstacle that different cosets need not be disjoint, which plays havoc with the traditional structure theory. Building on an idea of Gaubert and his group (including work of Akian and Guterman), we introduce a general way of artificially providing negation, in a manner similar to the construction of Z from N but with one crucial difference necessitated by the fact that the max-plus algebra is not additively cancelative! This leads to the possibility of defining many auxiliary tropical structures, such as Lie algebras and exterior algebras, and also providing a key ingredient for a module theory that could enable one to use standard tools such as homology.
Monday, November 30, 2015 - 15:05 , Location: Skiles 006 , , University of Kentucky , , Organizer:
Given a graph G on p vertices we consider the cone of concentration matrices associated to G; that is, the cone of all (p x p) positive semidefinite matrices with zeros in entries corresponding to the nonedges of G.  Due to its applications in PSD-completion problems and maximum-likelihood estimation, the geometry of this cone is of general interest.  A natural pursuit in this geometric investigation is to characterize the possible ranks of the extremal rays of this cone.  We will investigate this problem combinatorially using the cut polytope of G and its semidefinite relaxation, known as the elliptope of G.  For the graphs without K_5 minors, we will see that the facet-normals of the cut polytope identify a distinguished set of extremal rays for which we can recover the ranks.  In the case that these graphs are also series-parallel we will see that all extremal ranks are given in this fashion.  Time permitting, we will investigate the potential for generalizing these results.  This talk is based on joint work with Caroline Uhler and Ruriko Yoshida.
Monday, November 16, 2015 - 15:00 , Location: Skiles 006 , Cameron Farnsworth , Texas A&M , , Organizer: Greg Blekherman
Let det_n be the homogeneous polynomial obtained by taking the determinant of an n x n matrix of indeterminates.  In this presentation linear maps called Young flattenings will be defined and will be used to show new lower bounds on the symmetric border rank of det_n.
Monday, November 9, 2015 - 15:05 , Location: Skiles 005 or 006 , Caroline Uhler , MIT , Organizer: Anton Leykin
Exponential varieties arise from exponential families in statistics. These real algebraic varieties have strong positivity and convexity properties, generalizing those of toric varieties and their moment maps. Another special class, including Gaussian graphical models, are varieties of inverses of symmetric matrices satisfying linear constraints. We develop a general theory of exponential varieties, with focus on those defined by hyperbolic polynomials. Joint work with Mateusz Michałek, Bernd Sturmfels, and Piotr Zwiernik.
Monday, November 2, 2015 - 15:05 , Location: Skiles 006 , Doug Ulmer , Georgia Tech , Organizer: Josephine Yu
Given a non-isotrivial elliptic curve E over K=Fq(t), there is always a finite extension L of K which is itself a rational function field such that E(L) has large rank.  The situation is completely different over complex function fields:  For "most" E over K=C(t), the rank E(L) is zero for any rational function field L=C(u).  The yoga that suggests this theorem leads to other remarkable statements about rational curves on surfaces generalizing a conjecture of Lang.
Monday, October 26, 2015 - 15:05 , Location: Skiles 005 , , The Ohio State University , Organizer: Josephine Yu
Tropical geometry is sensitive to embeddings of algebraic varieties inside toric varieties. In this talk, I will advertise tropical modifications as a tool to locally repair bad embeddings of plane curves, allowing the re-embedded tropical curve to better reflect the geometry of the input one.  Our motivating examples will be plane elliptic cubics and genus two hyperelliptic curves.  Based on joint work with Hannah Markwig (arXiv:1409.7430) and ongoing work in progress with Hannah Markwig and Ralph Morrison.