## Past Seminars by Series

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2019
Mon Dec 02

## Topology Seminar

2:30pm - Ford Hall 110
Topology and Arithmetic Statistics
Weiyan Chen, University of Minnesota

Topology studies the shape of spaces. Arithmetic statistics studies the behavior of random algebraic objects such as integers and polynomials. I will talk about a circle of ideas connecting these two seemingly unrelated areas. To illuminate the connection, I will focus on three concrete examples: (1) the Burau representation of the braid groups, (2) analytic number theory for effective 0-cycles on a variety, and (3) cohomology of the space of multivariate irreducible polynomials. These projects are parts of a broader research program, with numerous contributions by topologists, algebraic geometers, and number theorists in the past decade, and lead to many future directions yet to be explored.
(PS. This will be a rehearsal of a job talk accessible to the general audience. Any comments or suggestions are appreciated.)

Mon Nov 25

## Topology Seminar

2:30pm - Ford Hall 110
Topology Seminar
TBATBA
Mon Nov 18

## Topology Seminar

2:30pm - Ford Hall 110
Topology Seminar
TBATBA
Mon Nov 11

## Topology Seminar

2:30pm - Ford Hall 110
Cochain models for the unit group of a differential graded algebra
Tyler Lawson, University of Minnesota

Abstract not available.

Mon Nov 04

## Topology Seminar

2:30pm - Ford Hall 110
Topology Seminar
TBATBA
Mon Oct 28

## Topology Seminar

2:30pm - Ford Hall 110
Compactifying the étale topos
Elden Elmanto, Harvard University

The speaker has long feared the technicalities and intricacies of equivariant stable homotopy theory. Fortunately, beginning with the work of Glasman, major simplification on the foundations of the subject has been made (cf. the work of Ayala-Mazel-Gee-Rozenblyum, Nikolaus-Scholze and the Barwick school). We offer another perspective (that the speaker has a chance of understanding) on equivariant stable homotopy theory, at least for the group C_2, via algebraic geometry. We view it as a way to remedy an infamous annoyance: the 2-étale cohomological dimension of the field of real numbers is infinite. We do this by identifying the genuine C_2-spectra with a category of motives based on Real algebraic geometry ala Scheiderer. This is joint work with Jay Shah.

Mon Oct 21

## Topology Seminar

2:30pm - Ford Hall 110
Descent properties of topological Hochschild homology
Liam Keenan, University of Minnesota

Algebraic K-theory is an extremely rich but notoriously difficult invariant to compute. In order to make calculations tractible, topological Hochschild homology and topological cyclic homology were introduced, along with the Dennis and cyclotomic trace maps. A natural question to consider is whether or not these invariants are sheaves for various topologies arising in algebraic geometry. In fact, it turns out that topological Hochschild homology is a sheaf for the fpqc topology on connective commutative ring spectra. In this talk, I plan to introduce the language necessary and sketch the argument of this result.

Mon Oct 14

## Topology Seminar

2:30pm - Ford Hall 110
Second order terms in arithmetic statistics
Craig Westerland, University of Minnesota

The machinery of the Weil conjectures often allows us to relate the singular cohomology of the complex points of a scheme to the cardinality of its set of points over a finite field. When we apply these methods to a moduli scheme, we obtain an enumeration of the objects the moduli parameterizes. It's rare that we can actually fully compute the cohomology of these moduli spaces, but homological stability results often give a first order approximation to the homology.

In this talk, we'll explain how to obtain second order homological computations for a class of Hurwitz moduli spaces of branched covers; these yield second order terms in enumerating the moduli over finite fields. We may interpret these as second order terms in a function field analogue of the function which counts number fields, ordered by discriminant. Our second order terms match those of Taniguchi-Thorne/Bhargava-Shankar-Tsimerman in the cubic case, and give new predictions in other Galois settings.

This is joint (and ongoing) work with Berglund, Michel, and Tran.

Mon Sep 30

## Topology Seminar

2:30pm - Ford Hall 110
Mysterious Duality
Sasha Voronov, University of Minnesota

Mysterious Duality was discovered by Iqbal, Neitzke, and Vafa in 2001. They noticed that toroidal compactifications of M-theory lead to the same series of combinatorial objects as del Pezzo surfaces do, along with numerous mysterious coincidences: both toroidal compactifications and del Pezzo surfaces give rise to the exceptional series E_k; the U-duality group corresponds to the Weyl group W(E_k), arising also as a group of automorphisms of the del Pezzo surface; a collection of various M- and D-branes corresponds to a set of divisors; the brane tension is related to the area of the corresponding divisor, etc. The mystery is that it is not at all clear where this duality comes from. In the talk, I will present another series of mathematical objects: certain versions of multiple loop spaces of the sphere S^4, which are, on the one hand, directly connected to M-theory and its combinatorics, and, on the hand, possess the same combinatorics as the del Pezzo surfaces. This is a report on an ongoing work with Hisham Sati.

Mon Sep 23

## Topology Seminar

2:30pm - Ford Hall 110
Homology class of Deligne-Lusztig varieties
Dongkwan Kim, University of Minnesota

Since first defined by Deligne and Lusztig, a Deligne-Lusztig variety has become unavoidable when studying the representation theory of finite groups of Lie type. This is a certain subvariety of the flag variety of the corresponding reductive group, and its cohomology groups are naturally endowed with the action such finite groups, which in turn gives a decomposition of irreducible representations called Lusztig series. In this talk, I will briefly discuss the background of Deligne-Lusztig theory and provide a formula to calculate the homology class of Deligne-Lusztig varieties in the Chow group of the flag variety. If time permits, I will also discuss their analogues.

Mon Sep 16

## Topology Seminar

2:30pm - Ford Hall 110
Transfer in the homology and cohomology of categories
Peter Webb, University of Minnesota

The cohomology of a category has many properties that extend those that are familiar when the category is a group. Second cohomology classifies equivalence classes of category extensions, first cohomology parametrizes conjugacy classes of splittings, first homology is the abelianization of the fundamental group, and second homology has a theory that extends that of the Schur multiplier. Defining restriction and corestriction maps on the homology of categories is problematic: most attempts to do this require induction and restriction functors to be adjoint on both sides, and this typically does not happen with categories. We describe an approach to defining these maps that includes all the situations where they can be defined in group cohomology, at least when the coefficient ring is a field. The approach uses bisets for categories, the construction by Bouc and Keller of a map on Hochschild homology associated to a bimodule, and the realization by Xu of category cohomology as a summand of Hochschild cohomology.

Mon Sep 09

## Topology Seminar

2:30pm - Ford Hall 110
Cohomology of the space of complex irreducible polynomials in several variables
Weiyan Chen, University of Minnesota

We will show that the space of complex irreducible polynomials of degree d in n variables satisfies two forms of homological stability: first, its cohomology stabilizes as d increases, and second, its compactly supported cohomology stabilizes as n increases. Our topological results are inspired by counting results over finite fields due to Carlitz and Hyde.

2018
Mon Dec 10

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: The geometric average size of Selmer groups over function fields
Aaron Landesman, Stanford

We use topological methods to investigate the average size of n-Selmer groups of elliptic curves over F_q(t).
Loosely speaking, the n-Selmer group of an elliptic curve measures objects which look like the n-torsion of the elliptic curve.
We relate the question of computing the average size of the n-Selmer group to demonstrating
homological stability for a sequence of moduli spaces
of these n-Selmer elements.
Via monodromy arguments, we show the number of components of these moduli spaces stabilizes, which determines the
average size after taking a large q limit.

Fri Dec 07

## Topology Seminar

3:30pm - 301 Vincent Hall
Topology Seminar: Power operations in normed motivic spectra
Tom Bachmann, MIT

In joint work with M. Hoyois, we established (the beginnings of) a theory of "normed motivic spectra". These are motivic spectra with some extra structure, enhancing the standard notion of a motivic E_oo-ring spectrum (this is similar to the notion of G-commutative ring spectra in equivariant stable homotopy theory). It was clear from the beginning that the homotopy groups of such normed spectra afford interesting *power operations*. In ongoing joint work with E. Elmanto and J. Heller, we attempt to establish a theory of these operations and exploit them calculationally. I will report on this, and more specifically on our proof of a weak motivic analog of the following classical result of Würgler: any (homotopy) ring spectrum with 2=0 is generalized Eilenberg-MacLane.

Mon Nov 26

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: Cohomology of arithmetic groups and characteristic classes of manifold bundles
Bena Tshishiku, Harvard University

A basic problem in the study of fiber bundles is to compute the ring H*(BDiff(M)) of characteristic classes of bundles with fiber a smooth manifold M. When M is a surface, this problem has ties to algebraic topology, geometric group theory, and algebraic geometry. We have a good understanding of the cohomology in the "stable range", but this accounts for a small percentage of the total cohomology, and little is known beyond that. In this talk I describe some new characteristic classes (in the case dim M >>0) that come from the unstable cohomology of arithmetic groups.

Mon Nov 19

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: The spectrum of units of a height 2 theory
Jeremy Hahn, Harvard University

The space BSU admits two infinite loop space structures, one arising from addition of vector bundles and the other from tensor product. A surprising fact, due to Adams and Priddy, is that these two infinite loop spaces become equivalent after p-completion. I will explain how the Adams-Priddy theorem allows for an identification of sl_1(ku_p), the spectrum of units of p-complete complex K-theory. I will then describe work, joint with Andrew Senger, that attempts to similarly understand the spectrum of units of the 2-completion of tmf_1(3).

Mon Nov 12

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: Localizing the E_2 page of the Adams spectral sequence
Eva Belmont, Northwestern

The Adams spectral sequence is one of the central tools for calculating the stable homotopy groups of spheres, one of the motivating problems in stable homotopy theory. In this talk, I will discuss an approach for computing the Adams E_2 page for the sphere at p = 3 in an infinite region, by computing its localization by the non-nilpotent element b_{10}. This approach relies on computing an analogue of the Adams spectral sequence in Palmieri's stable category of comodules, which can be regarded as an algebraic analogue of stable homotopy theory. This computation fits in the framework of chromatic homotopy theory in the stable category of comodules.

Mon Nov 05

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: An isovariant Elmendorf's theorem
Sarah Yeakel, University of Maryland

An isovariant map is an equivariant map which preserves isotropy groups. Isovariant maps show up in equivariant surgery theory and in other settings when homotopy theory is applied to geometry. For a finite group G, we consider the category of G-spaces with morphisms given by isovariant maps. We will discuss a cofibrantly generated model structure on this category, along with isovariant versions of Elmendorf's theorem and a theorem of Klein and Williams about homotoping a map off a submanifold.

Mon Oct 29

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: Symmetric monoidal categories and \Gamma-categories
Amit Sharma, Kent State University

A \Gamma-category is a functor from the category of finite based sets and basepoint preserving functions \Gamma^op to Cat. We construct a model category structure on the category of \Gamma-categories, which is symmetric monoidal closed to the Day convolution product. The fibrant objects in this model category structure are those
\Gamma-categories which are often called special \Gamma-categories. The main objective of this research is to establish a Quillen equivalence between a natural model category structure on the category of (small) permutative categories and strict symmetric monoidal functors Perm and our model category structure on \GammaCat. The weak equivalences of the natural model category structure are equivalences of underlying categories. In the paper [1], Segal defined a functor from the category of (small) symmetric monoidal categories into \GammaCat which can be described as a nerve functor for symmetric monoidal categories. The right adjoint \bar{K} of our Quillen equivalence is a thickening of Segal's nerve functor. We construct a permutative category L called Leinster's category, having the universal property that each \Gamma-category extends uniquely to a symmetric monoidal functor along an inclusion functor \Gamma^op > L. The left adjoint L of our Quillen equivalence is a composite functor composed of the symmetric monoidal extension functor indicated above followed by a homtopy colimit functor. In the paper [2], Mandell had shown that Segal's nerve functor (followed by the ordinary nerve functor) induces an equivalence between a homotopy category of Perm, obtained by inverting those strict symmetric monoidal functors which induce a weak homotopy equivalence of simplicial sets upon applying the nerve functor, and a homotopy category of \Gamma-spaces \GammaS obtained by inverting pre-stable equivalences which are those maps of \Gamma-categories which induce a degreewise weak homotopy equivalence of simplicial sets upon applying an E_\infty-completion functor. The objective of Mandell's work is to understand the relation between connective spectra and \Gamma-spaces obtained by applying the Segal's nerve functor to symmetric monoidal categories whereas our objective is to construct a model category of symmetric monoidal categories which is
symmetric monoidal closed.

References:
[1] G. Segal, Categories and cohomology theories, Topology 13 (1974) 293-312.
[2] M. A. Mandell, An Inverse K-theory functor, Doc. Math 15 (2010) 765-791.

Mon Oct 22

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: Finitely generated sequences of linear subspace arrangements

Hyperplane arrangements are a classical meeting point of topology, combinatorics and representation theory. Generalizing to arrangements of linear subspaces of arbitrary codimension, the theory becomes much more complicated. However, a crucial observation is that many natural sequences of arrangements seem to be defined using a finite amount of data.

In this talk I will describe a notion of 'finitely generation' for collections of arrangements, unifying the treatment of known examples. Such collections turn out to exhibit strong forms of stability, both in their combinatorics and in their cohomology representation. This structure makes the appearance of representation stability transparent and opens the door to generalizations.

Mon Oct 15

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: Rigidity of warped cones and coarse geometry of expanders
Wouter van Limbeek, UIC

Finitely generated subgroups of compact Lie groups give rise to expander graphs via a warped cone construction. We study the dependence of the coarse geometry of such expander graphs on the original subgroup and establish a dynamical analogue of quasi-isometric rigidity theorems in geometric group theory: Namely, the coarse geometry of the warped cone determines the subgroup up to commensurability, unless the group has abelian factors. This is joint work with David Fisher and Thang Nguyen.

Mon Oct 08

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: A chromatic approach to tmf cooperations
Paul van Koughnett, Purdue

The topological modular forms spectrum, tmf, is a cohomology theory constructed from elliptic curves that captures information about stable homotopy at chromatic heights less than or equal to 2. We present a description of the height 1 part of the algebra of tmf cooperations, and interpret it in terms of modular forms. This is joint work with Dominic Culver.

Mon Oct 01

## Topology Seminar

3:30pm - Vincent Hall 301
Topology Seminar: Grothendieck-Teichmuller group and braided Hopf algebras
Craig Westerland, University of Minnesota

The primitives in a classical Hopf algebra form a Lie algebra (in fact, a Lie subalgebra of the Hopf algebra).  For a braided Hopf algebra, this is no longer the case.  Consequently, all of the structure theorems for Hopf algebras (e.g., the Milnor-Moore and Poincaré-Birkhoff-Witt theorems) break down in this setting. This is a report on ongoing work in which we construct an operad (a braided form of the Lie operad) which governs the algebraic structure of the primitives in a braided Hopf algebra.  We can interpret this operad in terms of the homology of covering spaces of the 2-dimensional little disks operad.  This gives rise to an action of Drinfeld's Grothendieck-Teichmuller group on this operad which may be related to Drinfeld's original definition of this group.

Tue May 01

## Topology Seminar

2:30pm - Vincent Hall 2
Edge stabilization in the homology of graph braid groups
Ben Knudsen, Harvard University

We discuss a novel type of stabilization map on the configuration spaces of a graph, which increases the number of particles occupying an edge. Through these maps, the homology of the configuration spaces forms a module over the polynomial ring generated by the edges of the graph, and we show that this module is finitely generated, implying eventual polynomial growth of Betti numbers over any field. Moreover, the action lifts to an action at the level of singular chains, which contains strictly more information; indeed, we show that this differential graded module is almost never formal over the ring of edges. These results, along with numerous calculations, arise from consideration of an explicit chain complex, which is a structured enhancement of a cellular model first considered by Swiatkowski. We arrive at this model through a local-to-global approach combining ideas from factorization homology and discrete Morse theory. This is joint work with Byung Hee An and Gabriel Drummond-Cole.

Mon Apr 23

## Topology Seminar

2:30pm - Vincent Hall 6
The Orlik-Terao algebra and the cohomology of configuration spaces
Nick Proudfoot, University of Oregon

The Orlik-Terao algebra is the subalgebra of all rational
functions in n variables generated by 1/(x_i - x_j). I will explain
how to use topological techniques to understand this algebra as a
graded representation of the symmetric group. I will also describe
two different connections (one proven and one conjectural) between
this algebra and the cohomology of configuration spaces.

Tue Apr 03

## Topology Seminar

2:30pm - Vincent Hall 2
HF_p is an equivariant Thom spectrum
Dylan Wilson, University of Chicago

Mahowald (and later many others) proved that HF_2 is a Thom spectrum for a bundle over the double loop space on the 3-sphere. This has had lots of applications, including (more recently) some nilpotence results for E_2-algebras and a calculation of THH of F_2. In this talk, we explain how to extend the result to the case of equivariant spectra over a cyclic group of prime power order. The case of HF_2 over C_2 is joint with Mark Behrens, and the general case is joint with Jeremy Hahn. If there's time, we'll discuss how this fits into a program of Hill-Hopkins-Ravenel for solving the 3-primary Kervaire invariant problem.

Tue Mar 27

## Topology Seminar

2:30pm - Vincent Hall 2
Topological Hochschild Homology and Characteristics
Jonathan Campbell, Vanderbilt University

In this talk I'll review the definition of duality in categories and bicategories and how certain functors called "shadows", due to Kate Ponto, can be used to extract Euler characteristic-type invariants from this data. It turns out that topological Hochschild homology (which I'll define) is an example of such a shadow, and this can be used to relate classical invariants from fixed point theory (e.g. the Reidemeister trace) with the image of the cyclotomic trace in K-theory. Time permitting, I'll sketch how anything that one calls a "characteristic" should fit into this story. This is joint work with Kate Ponto.

Tue Feb 06

## Topology Seminar

2:30pm - Vincent Hall 16
Topological vistas in neuroscience (special colloquium)
Kathryn Hess, École polytechnique fédérale de Lausanne

I will describe results obtained in collaboration with the Blue Brain Project on the topological analysis of the structure and function of digitally reconstructed microcircuits of neurons in the rat cortex and outline our on-going work on topology and synaptic plasticity. The talk will include an overview of the Blue Brain Project and a brief introduction to the topological tools that we use. If time allows, I will also briefly sketch other collaborations with neuroscientists in which my group is involved.

Tue Jan 30

## Topology Seminar

2:30pm - Vincent Hall 2
Topology Seminar
Weiyan Chen, University of Minnesota, Twin Cities

TBA

Tue Jan 23

## Topology Seminar

2:30pm - Vincent Hall 2
Quantum Deformation Theory
Sasha Voronov, University of Minnesota, Twin Cities

Classical deformation theory is based on the Classical Master Equation (CME), a.k.a. the Maurer-Cartan Equation: dS + 1/2 [S,S] = 0. Physicists have been using a quantized CME, called the Quantum Master Equation (QME), a.k.a. the Batalin-Vilkovisky (BV) Master Equation: dS + h \Delta S + 1/2 {S,S} = 0. The CME is defined in a differential graded (dg) Lie algebra, whereas the QME is defined in a space V[[h]] of formal power series or V((h)) of formal Laurent series with values in a dg BV algebra V. One can anticipate a generalization of classical deformation theory arising from the QME, quantum deformation theory. Quantum deformation functor and its representability will be discussed in the talk.

2017
Fri Dec 15

## Topology Seminar

10:30am - Vincent Hall 207
Motivic Euler numbers and an arithmetic count of the lines on a cubic surface
Kirsten Wickelgren, Georgia Tech

A celebrated 19th century result of Cayley and Salmon is that a smooth cubic surface over the complex numbers contains exactly 27 lines. Over the real numbers, it is a lovely observation of Benedetti_Silhol, FinashinKharlamov and OkonekTeleman that while the number of real lines depends on the surface, a certain signed count of lines is always 3. We extend this count to an arbitrary field k using an Euler number in A1-homotopy theory. The resulting count is valued in the Grothendieck-Witt group of non-degenerate symmetric bilinear forms. This is joint work with Jesse Kass.

Mon Dec 11

## Topology Seminar

7:25pm - Vincent Hall 301
Evenness in equivariant topology
Mike Hill, UCLA

Motivated by work of Wilson, I will describe a version of evenness'' in equivariant homotopy theory. There are dual notions of even cells and even homotopy groups, and one can also ask for things which have both properties. I'll describe some of the ongoing work in this area, all of which is joint with Hopkins.

Fri Dec 08

## Topology Seminar

2:30pm - Vincent Hall 207
The Theory of Resolvent Degree - After Hamilton, Hilbert, Segre and Brauer
Jesse Wolfson, UC Irvine

Resolvent degree is an invariant of a branched cover which quantifies how "hard" is it to specify a point in the cover given a point under it in the base. Historically, this was applied to the branched cover P^n/S_{n-1} --> P^n/S_n, from the moduli of degree n polynomials with a specified root to the moduli of degree n polynomials. Classical enumerative problems and congruence subgroups provide two additional sources of branched covers to which this invariant applies. In ongoing joint work with Benson Farb, we develop the theory of resolvent degree as an extension of Buhler and Reichstein's theory of essential dimension. We apply this theory to systematize an array of classical results and to relate the complexity of seemingly different problems such as finding roots of polynomials, lines on cubic surfaces, and level structures on intermediate Jacobians.