## Seminars and Colloquia by Series

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.
Monday, October 5, 2015 - 15:05 , Location: Skiles 006 , , Texas A&M University , Organizer: Josephine Yu
Deciding nonnegativity of real polynomials is a key question in real algebraic geometry with crucial importance in polynomial optimization. Since this problem is NP-hard, one is interested in finding sufficient conditions (certificates) for nonnegativity, which are easier to check. The standard certificates are sumsof squares (SOS), which trace back to Hilbert (see Hilbert’s 17th problem).In this talk we completely characterize sections of the cones of nonnegativepolynomials and sums of squares with polynomials supported on circuits, a genuine class of sparse polynomials. In particular, nonnegativity is characterized by an invariant, which can be immediately derived from the initial polynomial. Based on these results, we obtain a completely new class of nonnegativity certificates independent from SOS certificates. Furthermore, nonnegativity of such circuit polynomials f coincides with solidness of the amoeba of f , i.e., the Log-absolute-value image of the algebraic variety V(f) in C^n of f. These results establish a first direct connection between amoeba theory and nonnegativity of polynomials.These results generalize earlier works by Fidalgo, Ghasemi, Kovacec, Marshall and Reznick. The talk is based on joint work with Sadik Iliman.
Monday, September 28, 2015 - 15:05 , Location: Skiles 006 , Anders Jensen , Aarhus University, Denmark , Organizer: Josephine Yu
In numerical algebraic geometry the key idea is to solve systems of polynomial equations via homotopy continuation. By this is meant, that the solutions of a system are tracked as the coefficients change continuously toward the system of interest. We study the tropicalisation of this process. Namely, we combinatorially keep track of the solutions of a tropical polynomial system as its coefficients change. Tropicalising the entire regeneration process of numerical algebraic geometry, we obtain a combinatorial algorithm for finding all tropical solutions. In particular, we obtain the mixed cells of the system in a mixed volume computation. Experiments suggest that the method is not only competitive but also asymptotically performs better than conventional methods for mixed cell enumeration. The method shares many of the properties of a recent tropical method proposed by Malajovich. However, using symbolic perturbations, reverse search and exact arithmetic our method becomes reliable, memory-less and well-suited for parallelisation.
Monday, September 21, 2015 - 15:05 , Location: Skiles 006 , Rainer Sinn , Georgia Tech , Organizer: Josephine Yu
Duality is an important feature in convexity and in projective algebraic geometry. We will discuss the interplay of these two dualities for the cone of sums of squares of ternary forms and its dual cone, the Hankel spectrahedron.