## Past Seminars by Series

Mon Dec 02 |
## Topology Seminar2:30pm - Ford Hall 110Topology 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. |

Mon Nov 25 |
## Topology Seminar2:30pm - Ford Hall 110Topology Seminar TBATBA |

Mon Nov 18 |
## Topology Seminar2:30pm - Ford Hall 110Topology Seminar TBATBA |

Mon Nov 11 |
## Topology Seminar2:30pm - Ford Hall 110Cochain models for the unit group of a differential graded algebra Tyler Lawson, University of Minnesota Abstract not available. |

Mon Nov 04 |
## Topology Seminar2:30pm - Ford Hall 110Topology Seminar TBATBA |

Mon Oct 28 |
## Topology Seminar2:30pm - Ford Hall 110Compactifying 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 Seminar2:30pm - Ford Hall 110Descent 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 Seminar2:30pm - Ford Hall 110Second 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 Seminar2:30pm - Ford Hall 110Mysterious 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 Seminar2:30pm - Ford Hall 110Homology 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 Seminar2:30pm - Ford Hall 110Transfer 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 Seminar2:30pm - Ford Hall 110Cohomology 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. |

Mon Dec 10 |
## Topology Seminar3:30pm - Vincent Hall 301Topology 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). |

Fri Dec 07 |
## Topology Seminar3:30pm - 301 Vincent HallTopology 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar3:30pm - Vincent Hall 301Topology 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 References: |

Mon Oct 22 |
## Topology Seminar3:30pm - Vincent Hall 301Topology Seminar: Finitely generated sequences of linear subspace arrangements Nir Gadish, University of Chicago 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar3:30pm - Vincent Hall 301Topology 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 Seminar2:30pm - Vincent Hall 2Edge 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 Seminar2:30pm - Vincent Hall 6The Orlik-Terao algebra and the cohomology of configuration spaces Nick Proudfoot, University of Oregon The Orlik-Terao algebra is the subalgebra of all rational |

Tue Apr 03 |
## Topology Seminar2:30pm - Vincent Hall 2HF_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 Seminar2:30pm - Vincent Hall 2Topological 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 Seminar2:30pm - Vincent Hall 16Topological 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 Seminar2:30pm - Vincent Hall 2Topology Seminar Weiyan Chen, University of Minnesota, Twin Cities TBA |

Tue Jan 23 |
## Topology Seminar2:30pm - Vincent Hall 2Quantum 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. |

Fri Dec 15 |
## Topology Seminar10:30am - Vincent Hall 207Motivic 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, FinashinKharlamov and OkonekTeleman 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 Seminar7:25pm - Vincent Hall 301Evenness 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 Seminar2:30pm - Vincent Hall 207The 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. |