# University of Regina Topology Seminar

## Winter 2023

### Topic: Topological quantum field theories and the cobordism hypothesis

Time: Tuesdays 1:00 - 2:15 PM, unless otherwise noted.
Location: CL 251

Most talks are held in person. Some talks are on Zoom:
https://uregina-ca.zoom.us/j/99127226830?pwd=bnFQR1R3UUdyWUxqSS9JMExMRlZwZz09

In this learning seminar, we will study topological quantum field theories (TQFTs), cobordism categories, and the cobordism hypothesis.

The lecture series will be interspersed with guest research talks, which are posted on our Youtube channel.

Date Speaker Title and abstract References
January 17   No seminar.
January 24 Martin Frankland Introduction to the cobordism hypothesis

Topological quantum field theories (TQFTs) are one of several mathematical approaches to quantum field theory in physics. They were introduced by Atiyah in the 1980s. The cobordism hypothesis, proposed by Baez and Dolan in the 1990s, is a statement about a classification of TQFTs. It was proved by Hopkins and Lurie, with contributions from others.

I will present an overview of the cobordism hypothesis and related notions: cobordisms, TQFTs, low-dimensional examples, higher categories of cobordisms, and extended TQFTs. After the overview, I will introduce Atiyah's axioms for a TQFT.

A pleasantry
January 31
On Zoom
Igor Sikora (Bilkent University) $RO(C_2)$-graded coefficients of $C_2$-Eilenberg-MacLane spectra

In non-equivariant topology, the ordinary homology of a point is described by the dimension axiom and is quite simple - namely, it is concentrated in degree zero. The situation in $G$-equivariant topology is different. This is because Bredon homology - the equivariant counterpart of ordinary homology - is naturally graded over $RO(G)$, the ring of $G$-representations. Whereas the equivariant dimension axiom describes the part of the Bredon homology of a point graded over trivial representations, it does not put any requirements on the rest of the grading - in which the homology may be quite complicated.

The $RO(G)$-graded Bredon homology theories are represented by $G$-Eilenberg-MacLane spectra, and thus the Bredon homology of a point is the same as coefficients of these spectra. During the talk, I will present the method of computing the $RO(C_2)$-graded coefficients of $C_2$-Eilenberg-MacLane spectra based on the Tate square. As demonstrated by Greenlees, the Tate square gives an algorithmic approach to computing the coefficients of equivariant spectra. In the talk, we will discuss how to use this method to obtain the $RO(C_2)$-graded coefficients of a $C_2$-Eilenberg-MacLane spectrum as a $RO(C_2)$-graded abelian group. We will also present the multiplicative structure of the $C_2$-Eilenberg-MacLane spectrum associated to the Burnside Mackey functor. Time permitting, we will further discuss how to use this knowledge to derive a multiplicative structure for the coefficients for any ring Mackey functor.

Recording
February 7 Carlos Gabriel Valenzuela On the classification of 1-dimensional TQFTs

In general Topological Quantum Field Theories are very diverse, but we may focus on low dimensional cases. As a starting point, we'll give a complete classification of 1-dimensional TQFTs.

To this end, I'll present a review of dualization in a symmetric monoidal category, discuss a few examples focusing on the category of vector spaces, and finally present a proof for the equivalence of 1-dimensional TQFTs and finite dimensional vector spaces.

February 14 Arnaud Ngopnang Ngompe Classification of 2-dimensional TQFTs

In this talk, we give a description of finite-dimensional commutative Frobenius algebras and we recall the description of 2-dimensional TQFTs. We show that these two structures are equivalent to each other.

February 21   Reading Week. No seminar.
February 28 Allen Herman Frobenius algebras

In Arnaud's talk, we saw that the 2-dimensional TQFT category is equivalent to the category of commutative Frobenius algebras with isomorphisms. In this sequel to Arnaud's talk, we take a closer look at Frobenius algebras. We will explore their algebraic and coalgebraic properties, look at important sources of examples, and see how the natural coalgebraic structure of a Frobenius algebra compares to that of bialgebras and Hopf algebras.

March 7 Matthew Alexander TQFTs from physical systems

When the same topic appears in both mathematics and physics, there are often stark differences between how that topic is conceptualized and studied in each discipline. In this talk we will examine what it means to a physicist for a field theory to be 'topological' and how to translate that information into the more familiar language of a functor on a cobordism category.

In particular we will see how TQFTs are natural models for describing quantum gravity in low dimensions.

March 14   Pi Day. No seminar.
March 21 Manak Singh K-theory as a generalized cohomology theory

Topological K-theory is an assignment of algebraic data to a topological space. It is a homotopy invariant and satisfies the Eilenberg-Steenrod axioms for a generalized cohomology theory. In this talk I give a rapid introduction to vector bundles over compact Hausdorff spaces. I show how this helps generate useful algebraic data associated to a space, and how the resulting assignment takes the shape of a cohomology theory.

March 28 Martin Frankland Extended TQFTs and higher categories

The classification of 2-dimensional TQFTs was obtained by cutting up surfaces into disks, cylinders, and pairs of pants. In higher dimension, such a decomposition of n-manifolds is not available. We need to cut up manifolds not only along codimension 1 submanifolds, but also strata of codimension 2 or higher. Those refined cutting rules lead to the notion of an extended TQFT. I will sketch some attempts at making this notion precise, and how the data can be described using higher categories.

April 4 Manak Singh Homotopy types of cobordism categories

April 11
On Zoom
Brandon Doherty (Stockholm University) TBA

## Previous semesters

For more information, contact Martin Frankland.

Back to Martin Frankland's home page.
Back to the Department of Mathematics and Statistics.