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Series: Algebra Seminar

A matroid is a structure that captures the notion of "independence". For example, given a set of vectors in a vector space, one can define a matroid. Graphs also naturally give rise to matroids. I will talk about various simplicial complexes associated to matroids. These include the "matroid complex", the "broken circuit complex", and the "order complex" of the associated geometric lattice. They carry some of the most important invariants of matroids and graphs. I will also show how the Bergman fan and its refinement (which arise in tropical geometry) relate to the classical theory. If time permits, I will give an outline of a recent breakthrough result of Huh and Katz on log-concavity of characteristic (chromatic) polynomials of matroids. No prior knowledge of the subject will be assumed. Most of the talk should be accessible to advanced undergraduate students.

Series: Algebra Seminar

The talk will discuss the notion of Hilbert-Kunz multiplicity,
presenting its general theory and listing some of the outstanding open
problems together with recent progress on them.

Series: Algebra Seminar

In celestial mechanics a configuration of n point masses is called central if it collapses by scaling to the center of mass when released with initial velocities equal to zero. We strengthen a generic finiteness result due to Moeckel by showing that the number of spatial central configurations in the Newtonian five-body problem with positive masses is finite, except for some explicitly given special choices of mass values. The proof will be computational using tropical geometry, Gröbner bases and sum-of-squares decompositions.This is joint work with Marshall Hampton.

Series: Algebra Seminar

[Note unusual day and time!]

In the last decades, path following methods have become a very popularstrategy to solve systems of polynomial equations. Many of the advances are due tothe correct understanding of the geometrical properties of an algebraic object, the so-called solution variety for polynomial system solving. I summarize here some of the mostrecent advances in the understanding of this object, focusing also on the certification andcomplexity of the numerical procedures involved in path following methods.

Series: Algebra Seminar

An ideal of a local polynomial ring can be described by calculating astandard basis with respect to a local monomial ordering. However if we areonly allowed approximate numerical computations, this process is notnumerically stable. On the other hand we can describe the ideal numericallyby finding the space of dual functionals that annihilate it. There areseveral known algorithms for finding the truncated dual up to any specifieddegree, which is useful for zero-dimensional ideals. I present a stoppingcriterion for positive-dimensional cases based on homogenization thatguarantees all generators of the initial monomial ideal are found. This hasapplications for calculating Hilbert functions.

Series: Algebra Seminar

The rational solutions to the equation describing an elliptic curve form a finitely generated abelian group, also known as the Mordell-Weil group. Detemining the rank of this group is one of the great unsolved problems in mathematics. The Shafarevich-Tate group of an elliptic curve is an important invariant whose conjectural finiteness can often be used to determine the generators of the Mordell-Weil group. In this talk, we first introduce the definition of the Shafarevich-Tate group. We then discuss the theory of visibility, initiated by Mazur, by means of which non-trivial elements of the Shafarevich-Tate group of an elliptic curve an be 'visualized' as rational points on an ambient curve. Finally, we explain how this theory can be used to give theoretical evidence for the celebrated Birch and Swinnerton-Dyer Conjecture.

Series: Algebra Seminar

Come and see!

Series: Algebra Seminar

For q a power of a prime, consider the ring \mathbb{F}_q[T].
Due to the many similarities between \mathbb{F}_q[T] and the
ring of integers \mathbb{Z}, we can define for
\mathbb{F}_q[T] objects that are analogous to elliptic curves,
modular forms, and modular curves. In particular, for
\mathfrak{p} a prime ideal in \mathbb{F}_q[T], we can define
the Drinfeld modular curve X_0(\mathfrak{p}), and study the
reduction modulo \mathfrak{p} of its Weierstrass points, as is
done in the classical case by Rohrlich, and Ahlgren and Ono. In
this talk we will present some partial results in this
direction, defining all necessary objects as we go. The first 20
minutes should be accessible to graduate students interested in
number theory.

Series: Algebra Seminar

We will discuss several instances of sequences of complex manifolds
X_n whose Betti numbers b_i(X_n) converge, when properly scaled,
to a
limiting distribution. The varieties considered have Betti
numbers
which are described in a combinatorial way making their study
possible. Interesting examples include varieties X for which
b_i(X)
is the i-th coefficient of the reliability polynomial of an
associated graph.

Series: Algebra Seminar

Which commutative groups can occur as the ideal class group (or
"Picard group") of some Dedekind domain? A number theorist naturally
thinks of the case of integer rings of number fields, in which the
class group must be finite and the question of which finite groups
occur is one of the deepest in algebraic number theory. An algebraic
geometer naturally thinks of affine algebraic curves, and in
particular, that the Picard group of the standard affine ring of an
elliptic curve E over C is isomorphic to the group of rational points
E(C), an uncountably infinite (Lie) group. An arithmetic geometer
will be more interested in Mordell-Weil groups, i.e., E(k) when k is a
number field -- again, this is one of the most notorious problems in
the field. But she will at least be open to the consideration of E(k)
as k varies over all fields.
In 1966, L.E. Claborn (a commutative algebraist) solved the "Inverse
Picard Problem": up to isomorphism, every
commutative group is the Picard group of some Dedekind domain. In the
1970's, Michael Rosen (an arithmetic geometer) used elliptic curves to
show that any countable commutative group can serve as the class group
of a Dedekind domain. In 2008 I learned about Rosen's work and showed
the following theorem: for every commutative group G there is a field
k, an elliptic curve E/k and a Dedekind domain R which is an overring
of the standard affine ring k[E] of E -- i.e., a domain in between
k[E] and its fraction field k(E) -- with ideal class group isomorphic
to G. But being an arithmetic geometer, I cannot help but ask about
what happens if one is not allowed to pass to an overring: which
commutative groups are of the form E(k) for some field k and some
elliptic curve E/k? ("Inverse Mordell-Weil Problem")
In this talk I will give my solution to the "Inverse Picard Problem"
using elliptic curves and give a conjectural answer to the "Inverse
Mordell-Weil Problem". Even more than that, I can (and will, time
permitting) sketch a proof of my conjecture, but the proof will
necessarily gloss over a plausible technicality about Mordell-Weil
groups of "arithmetically generic" elliptic curves -- i.e., I do not
in fact know how to do it. But the technicality will, I think, be of
interest to some of the audience members, and of course I am (not so)
secretly hoping that someone there will be able to help me overcome
it.