## Seminars and Colloquia by Series

Monday, May 21, 2012 - 15:00 , Location: Skiles 006 , Chris Hillar , UC Berkeley , Organizer: Anton Leykin
We discuss the theory of symmetric Groebner bases, a concept allowing one to prove Noetherianity results for symmetric ideals in polynomial rings with an infinite number of variables. We also explain applications of these objects to other fields such as algebraic statistics, and we discuss some methods for computing with them on a computer. Some of this is joint work with Matthias Aschenbrener and Seth Sullivant.
Tuesday, April 24, 2012 - 14:00 , Location: Skiles 006 , Andrew Dudzik , UC Berkeley , Organizer: Anton Leykin
The construction of the Berkovich space associated to a rigid analytic variety can be understood in a general topological framework as a type of local compactification or uniform completion, and more generally in terms of filters on a lattice.  I will discuss this viewpoint, as well as connections to Huber's theory of adic spaces, and draw parallels with the usual metric completion of $\mathbb{Q}$.
Tuesday, April 17, 2012 - 14:05 , Location: Skiles 006 , , University of Kansas , , Organizer: Josephine Yu
The critical group of a graph G is an abelian group K(G) whose order is the number of spanning forests of G.  As shown by Bacher, de la Harpe and Nagnibeda, the group K(G) has several equivalent presentations in terms of the lattices of integer cuts and flows on G. The motivation for this talk is to generalize this theory from graphs to CW-complexes, building on our earlier work on cellular spanning forests. A feature of the higher-dimensional case is the breaking of symmetry between cuts and flows.  Accordingly, we introduce and study two invariants of X: the critical group K(X) and the cocritical group K^*(X), As in the graph case, these are defined in terms of combinatorial Laplacian operators, but they are no longer isomorphic; rather, the relationship between them is expressed in terms of short exact sequences involving torsion homology. In the special case that X is a graph, torsion vanishes and all group invariants are isomorphic, recovering the theorem of Bacher, de la Harpe and Nagnibeda.  This is joint work with Art Duval (University of Texas, El Paso) and Caroline Klivans (Brown University).
Tuesday, April 10, 2012 - 14:00 , Location: Skiles 006 , Abraham Martin del Campo , Texas A&M , Organizer: Anton Leykin
The Galois group of a problem in enumerative geometry is a subtle invariant that encodes special structures in the set of solutions. This invariant was first introduced by Jordan in 1870. In 1979, Harris showed that the Galois group of such problems coincides with the monodromy group of the total space. These geometric invariants are difficult to determine in general. However, a consequence of Vakil's geometric Littlewood-Richardson rule is a combinatorial criterion to determine if a Schubert problem on a Grassmannian contains at least the alternating group.  Using Vakil's criterion, we showed that for Schubert problems of lines, the Galois group is at least the alternating group.
Tuesday, April 3, 2012 - 14:00 , Location: Skiles 006 , Jonathan Hauenstein , Texas A&M , Organizer: Anton Leykin
In many applications in engineering and physics, one is interested in computing real solutions to systems of equations.  This talk will explore numerical approaches for approximating solutions to systems of polynomial and polynomial-exponential equations.  We will then discuss using certification methods based on Smale's alpha-theory to rigorously determine if the corresponding solutions are real.  Examples from kinematics, electrical engineering, and string theory will be used to demonstrate the ideas.
Tuesday, February 28, 2012 - 14:05 , Location: Skiles 006 , Cynthia Vinzant , University of Michigan , Organizer: Josephine Yu
The central curve of a linear program is an algebraic curve specified by a hyperplane arrangement and a cost vector. This curve is the union of the various central paths for minimizing or maximizing the cost function over any region in this hyperplane arrangement. I will discuss the algebraic properties of this curve and its beautiful global geometry, both of which are controlled by the corresponding matroid and hyperplane arrangement.
Tuesday, February 7, 2012 - 14:00 , Location: Skiles 006 , Douglas A. Leonard , Auburn University , Organizer: Anton Leykin
Let I be an ideal in a polynomial ring R := F[x_n,...,x_1] Let A := R/I be the corresponding quotient ring, and let Q(A) be its eld of fractions. The integral closure C(A, Q(A)) of A in Q(A) is a subring of the latter. But it is often given as a separate quotient ring, a presentation. Surprisingly, different computer algebra systems (Magma, Macaulay2, and Singular) choose to produce very different presentations. Some of these opt for presentations that have seductive forms, but miss the most important, namely a form that allows for determining when elements ofQ(A) are in C(A,Q(A)). This is called membership and is directly related to determining isomorphism.
Tuesday, January 24, 2012 - 14:05 , Location: Skiles 006 , , University of Georgia , Organizer: Josephine Yu
This talk will describe some recent results using exact massformulas to determine all definite quadratic forms of small class number inn>=3 variables, particularly those of class number one.The mass of a quadratic form connects the class number (i.e. number ofclasses in the genus) of a quadratic form with the volume of its adelicstabilizer, and is explicitly computable in terms of special values of zetafunctions.  Comparing this with known results about the sizes ofautomorphism groups, one can make precise statements about the growth ofthe class number, and in principle determine those quadratic forms of smallclass number.We will describe some known results about masses and class numbers (overnumber fields), then present some new computational work over the rationalnumbers, and perhaps over some totally real number fields.
Tuesday, January 10, 2012 - 14:05 , Location: Skiles 006 , Benjamin Weiss , Technion , Organizer:
For two polynomials G(X), H(Y) with rational coefficients, when does G(X) = H(Y) have infinitely many solutions over the rationals? Such G and H have been classified in various special cases by previous mathematicians. A theorem of Faltings (the Mordell conjecture) states that we need only analyze curves with genus at most 1.In my thesis (and more recent work), I classify G(X) = H(Y) defining irreducible genus zero curves. In this talk I'll present the infinite families which arise in this classification, and discuss the techniques used to complete the classification.I will also discuss in some detail the examples of polynomial which occur in the classification. The most interesting infinite family of polynomials are those H(Y) solving a Pell Equation H(Y)^2 - P(Y)Q(Y)^2 = 1. It turns out to be difficult to describe these polynomials more explicitly, and yet we can completely analyze their decompositions, how many such polynomials there are of a fixed degree, which of them are defined over the rationals (as opposed to a larger field), and other properties.
Monday, December 5, 2011 - 16:05 , Location: Skiles 006 , , University of Connecticut , Organizer: Josephine Yu
The ring of invariants for the action of the automorphism group of the projective line on the n-fold product of the projective line is a classical object of study. The generators of this ring were determined by Kempe in the 19th century. However, the ideal of relations has been only understood very recently in work of Howard, Millson, Snowden and Vakil. They prove that the ideal of relations is generated byquadratic equations using a degeneration to a toric variety. I will report on joint work with Benjamin Howard where we further study the  toric varieties arising in this degeneration. As an application we show that the second Veronese subring of the ring of invariants admits a presentation whose ideal admits a quadratic Groebner basis.