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Series: Graph Theory Seminar

For a graph G, the Colin de Verdière graph parameter mu(G) is the maximum
corank of any matrix in a certain family of generalized adjacency matrices
of G. Given a non-negative integer t, the family of graphs with mu(G) <= t
is minor-closed and therefore has some nice properties. For example, a
graph G is planar if and only if mu(G) <= 3. Colin de Verdière conjectured
that the chromatic number chi(G) of a graph satisfies chi(G) <= mu(G)+1.
For graphs with mu(G) <= 3 this is the Four Color Theorem. We conjecture
that if G has at least t vertices and mu(G) <= t, then |E(G)| <= t|V(G)| -
(t+1 choose 2). For planar graphs this says |E(G)| <= 3|V(G)|-6. If this
conjecture is true, then chi(G) <= 2mu(G). We prove the conjectured edge
upper bound for certain classes of graphs: graphs with mu(G) small, graphs
with mu(G) close to |V(G)|, chordal graphs, and the complements of chordal
graphs.

Series: Stochastics Seminar

Estimation of the covariance matrix has attracted significant attention
of the statistical research community over the years, partially due to
important applications such as Principal Component Analysis. However,
frequently used empirical covariance estimator (and its modifications)
is very sensitive to outliers, or ``atypical’’ points in the sample.
As P. Huber wrote in 1964, “...This raises a question which could have
been asked already by Gauss, but which was, as far as I know, only
raised a few years ago (notably by Tukey): what happens if the true
distribution deviates slightly from the assumed normal one? As is now
well known, the sample mean then may have a catastrophically bad
performance…”
Motivated by Tukey's question, we develop a new estimator of the
(element-wise) mean of a random matrix, which includes covariance
estimation problem as a special case. Assuming that the entries of a
matrix possess only finite second moment, this new estimator admits
sub-Gaussian or sub-exponential concentration around the unknown mean in
the operator norm. We will present extensions of our approach to
matrix-valued U-statistics, as well as applications such as the matrix
completion problem.
Part of the talk will be based on a joint work with Xiaohan Wei.

Series: IMPACT Distinguished Lecture

The
use of evolutionary game theory biology dates to work of
Maynard-Smith who used it to explain why most fights between animals
were of the limited war type. Nowak and collaborators have shown that
a spatial distribution of players can explain the existence of
altruism, which would die out in a homogeneously mixing population.
For the last twenty years, evolutionary games have been used to model
cancer. In applications to ecology and cancer, the system is not
homogeneously mixing so it is important to understand how space
changes the outcome of these games. Over the last several years we
have developed a theory for understanding the behavior of
evolutionary games in the weak selection limit. We will illustrate
this theory by discussing a number of examples. The most recent work
was done in collaboration with a high school student so the talk
should be accessible to a broad audience.

Wednesday, March 15, 2017 - 14:05 ,
Location: Skiles 006 ,
Shane Scott ,
Georgia Tech ,
Organizer: Justin Lanier

Much of what is known about automorphisms of free groups is given by analogy to results on mapping class groups. One desirable result is the celebrated Nielson-Thurston classification of the mapping class group into reducible, periodic, or pseudo Anosov homeomorphisms. We will discuss attempts at analogous results for free group automorphisms.

Series: Analysis Seminar

In this talk we will discuss several ways to construct new convex bodies out of old ones. We will start by defining various methods of "averaging" convex bodies, both old and new. We will explain the relationships between the various definitions and their connections to basic conjectures in convex geometry. We will then discuss the power operation, and explain for example why every convex body has a square root, but not every convex body has a square. If time permits, we will briefly discuss more complicated constructions such as logarithms. The talk is based on joint work with Vitali Milman.

Series: Research Horizons Seminar

Every graph G has canonically associated to a finite abelian group called the Jacobian group. The cardinality of this group is the number of spanning trees in G. If G is planar, the Jacobian group admits a natural simply transitive action on the set of spanning trees. More generally, for any graph G one can define a whole family of (non-canonical) simply transitive group actions. The analysis of such group actions involves ideas from tropical geometry. Part of this talk is based on joint work with Yao Wang, and part is based on joint work with Spencer Backman and Chi Ho Yuen.

Wednesday, March 15, 2017 - 11:05 ,
Location: Skiles 006 ,
Max Alekseyev ,
George Washington University ,
maxal@gwu.edu ,
Organizer: Torin Greenwood

Genome median and genome halving are combinatorial optimization problems that aim at reconstruction of ancestral genomes by minimizing the number of possible evolutionary events between the reconstructed genomes and the genomes of extant species. While these problems have been widely studied in past decades, their known algorithmic solutions are either not efficient or produce biologically inadequate results. These shortcomings have been recently addressed by restricting the problems solution space. We show that the restricted variants of genome median and halving problems are, in fact, closely related and have a neat topological interpretation in terms of embedded graphs and polygon gluings. Hence we establish a somewhat unexpected link between comparative genomics and topology, and further demonstrate its advantages for solving genome median and halving problems in some particular cases. As a by-product, we also determine the cardinality of the genome halving solution space.

Series: Math Physics Seminar

We study regularity properties of weak solutions of the homogeneous Boltzmann equation. While under the so called Grad cutoff assumption the homogeneous Boltzmann equation is known to propagate smoothness and singularities, it has long been suspected that the non-cutoff Boltzmann operator has similar coercivity properties as a fractional Laplace operator. This has led to the hope that the homogenous Boltzmann equation enjoys similar smoothing properties as the heat equation with a fractional Laplacian. We prove that any weak solution of the fully nonlinear non-cutoff homogenous Boltzmann equation (for Maxwellian molecules) with initial datum $f_0$ with ﬁnite mass, energy and entropy, that is, $f_0 \in L^1_2(\R^d) \cap L \log L(\R^d)$, immediately becomes Gevrey regular for strictly positive times, i.e. it gains infinitely many derivatives and even (partial) analyticity.This is achieved by an inductive procedure based on very precise estimates of nonlinear, nonlocal commutators of the Boltzmann operator with suitable test functions involving exponentially growing Fourier multipliers.(Joint work with Jean-Marie Barbaroux, Dirk Hundertmark, and Semjon Vugalter)

Series: Geometry Topology Seminar

This is joint work with Jeff Meier. The Gluck twist operation removes an S^2XB^2 neighborhood of a knotted S^2 in S^4 and glues it back with a twist, producing a homotopy S^4 (i.e. potential counterexamples to the smooth Poincare conjecture, although for many classes of 2-knots theresults are in fact known to be smooth S^4's). By representing knotted S^2's in S^4 as doubly pointed Heegaard triples and understanding relative trisection diagrams of S^2XB^2 carefully, I'll show how to produce trisection diagrams (a.k.a. Heegaard triples) for these homotopy S^4's.(And for those not up on trisections I'll review the foundations.) The resulting recipe is surprisingly simple, but the fun, as always, is in the process.

Series: Algebra Seminar

For a given generic form, the problem of finding the nearest rank-one form with respect to the Bombieri norm is well-studied and completely solved for binary forms. Nonetheless, higher-rank approximation is quite mysterious except in the quadratic case. In this talk we will discuss such problems in the binary case.