# Colloquium

The colloquium typically meets Mondays at 4:00 PM in Room 617 on the sixth floor of Wachman Hall.

The colloquium is preceded by tea starting at 3:30 in the Faculty Lounge, adjacent to Room 617. Click on title for abstract.

• Monday January 23, 2017 at 16:00, Wachman 617
Asymptotic representation theory over Z

Tom Church, Stanford University and IAS

Representation theory over Z is famously intractable, but "representation stability" provides a way to get around these difficulties, at least asymptotically, by enlarging our groups until they behave more like commutative rings. Moreover, it turns out that important questions in topology / number theory / representation theory / ... correspond to asking whether familiar algebraic properties hold for these "rings". I'll explain how these connections work; describe what we know and don't know; and give a wide sampling of concrete applications in different fields. No knowledge of representation theory will be required -- indeed, that's sort of the whole point!

• Monday February 20, 2017 at 16:00, Wachman 617
2-Segal spaces and the Waldhausen S-construction

Julia Bergner, University of Virginia

The notion of a 2-Segal space was recently defined by Dyckerhoff and Kapranov, and independently by Galvez-Carrillo, Kock, and Tonks under the name of decomposition space. Unlike Segal spaces, which encode the structure of a category up to homotopy, 2-Segal spaces encode a more general structure in which composition need not exist or be unique, but is still associative. Both sets of authors above proved that the output of the Waldhausen $S_\bullet$-construction is a 2-Segal space. In joint work with Osorno, Ozornova, Rovelli, and Scheimbauer, we look at a discrete version of this construction whose output is a 2-Segal set. We show that, via this construction, the category of 2-Segal sets is equivalent to the category of augmented stable double categories. In this talk, I'll introduce 2-Segal sets and spaces, discuss this result and a conjectured homotopical generalization, and, time permitting, look at some other interesting features of 2-Segal spaces.

• Monday March 6, 2017 at 16:00, Wachman 617
Celebrating the mathematics accomplishments of Cristian Gutierrez and Martin Lorenz

This special event celebrates the professional accomplishments of two distinguished colleagues, Cristian Gutierrez and Martin Lorenz.

• Monday March 20, 2017 at 16:00, Wachman 617
Modelling collective cell movement

Philip Maini, Oxford University

Collective cell movement is a phenomenon that occurs in normal development, wound healing and disease (such as cancer). In many cases, the ability of cell populations to move large distances coherently arises due to a structure of "leaders" and "followers" within the population. I will present two such examples: (i) angiogenesis -- this the process by which new blood vessels form in response to injury, or in response to a cancerous tumour's demand for more nutrient. We systematically derive a discrete cell-based model for the "snail-trail" phenomenon of blood vessel growth and show that this leads to a novel partial differential equation model. We compare and constrast this model with those in the literature. (ii) neural crest cell invasion - this is the process by which cells move to target locations within the embryo to begin construction of body parts. Through an interdisciplinary research project we show how a hybrid discrete-cell-based mathematical model, and an experimental model, combine to allow us to gain new insights into this phenomenon.

• Monday April 10, 2017 at 16:00, Wachman 617
Paths in a random medium

Timo Seppalainen, University of Wisconsin Madison

This talk begins with a reminder of the law of large numbers and the central limit theorem for classic random walk and then proceeds to models of random paths currently studied in probability and statistical mechanics. In particular, we discuss directed percolation and directed polymer models. Subadditive ergodic theory gives deterministic large scale limiting shapes for these models, but properties of these limits have remained a challenge for decades. We describe some new variational formulas that characterize these limits and connections with other features of the models such as fluctuation exponents. Percolation and polymer models are expected to obey Kardar-Parisi-Zhang (KPZ) universality, as opposed to the Gaussian universality of random walk.

• Monday April 17, 2017 at 16:00, Wachman 617
The curve complex of a surface

Joan Birman, Columbia University

In the early 1980’s William Harvey, a mathematician working on Teichmuller spaces, introduced a finite dimensional simplicial complex C(S) on which the mapping class group M(S) of a surface S acts. His hope was that C(S) would play a role for M(S) analogous to the role of buildings in the work of Tits on linear groups. The group M(S) is of broad interest in mathematics because of its role in topology, analysis, geometric group theory, algebraic geometry.... The simplicial complex C(S) is connected, has infinite diameter, and an index 2 extension M*(S) turns out to be its automorphism group. (In M(S) maps preserve orientation, in M*(S) one allows orientation-reversing maps too.) We will discuss recent efforts, including a computer program, to use elementary tools to understand the local topology of C(S), focussing on its 1-skeleton, a metric graph in which every edge has length 1.

• Monday April 24, 2017 at 16:00, Wachman Hall 617
Singular Values of Large Matrices

Gilbert Strang, MIT

The "fundamental theorem of linear algebra" tells us about orthogonal bases for the row space and column space of any matrix. More than that, it identifies the most important part of the matrix -- which is a central goal for a matrix of data. Since data matrices are normally rectangular, singular values must replace eigenvalues. This talk will be partly about the underlying theory and partly about some of its applications to understanding what the matrix tells us. For several one-zero matrices we have open questions about the rank. Alex Townsend has identified an important class of large matrices that have rapidly decaying singular values --- allowing superfast algorithms.

• Monday September 11, 2017 at 16:00, Wachman 617
Linearly reinforced random walks

Nick Crawford, Technion

I'll discuss some probabilistic models of random walks which interact with their environments. These models are interesting for various reasons. They are relatively natural, yet extremely challenging to control mathematically. Indeed, the simplest variant, Linearly Reinforced Random Walk (LRRW), was introduced by P. Diaconis in the mid 1980’s but general recurrence and transience results only appeared in the last 5 years. Notably, variants (LRRW) have popped up in such diverse subjects as the modeling of path optimization by ant colonies and (the supersymmetric approach to) Anderson localization, first uncovered empirically through the work of M. Disertori, T. Spencer, and M. Zirnbauer and made precise by C. Sabot and P. Tarres.

In this talk I will provide an introduction to LRRW, and a few other walks with reinforcement for the purposes of comparison. Then I will explain the probabilistic concept of exchangeability, which provides LRRW with features that can be exploited. Finally I will survey the recent results on recurrence and transience, obtained by two groups of authors — one group being the 5 authors mentioned above and the other group being O. Angel, myself and G. Kozma.

• Monday September 25, 2017 at 16:00, Wachman 617
Left-orderability and 3-manifold groups

Cameron Gordon, University of Texas at Austin

The fundamental group is a more or less complete invariant of a 3-dimensional manifold. We will discuss how the purely algebraic question of whether or not this group has a left-invariant total order appears to be related to two other, seemingly quite different, properties of the manifold, one geometric and the other essentially analytic.

• Monday October 9, 2017 at 16:00, Wachman 617
TBA

Jason Manning, Cornell University

• Monday October 23, 2017 at 13:30,
TBA

Frank Farris, Santa Clara University

• Monday October 30, 2017 at 16:00, Wachman 617
TBA

Florian Pop, University of Pennsylvania

• Monday November 13, 2017 at 16:00, Wachman 617
TBA

Alex Townsend, Cornell University