Hochschild Cohomology and Higher Centers

Speaker: 

Sonja Farr

Institution: 

University of Nevada, Reno

Time: 

Monday, April 21, 2025 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340P

Kontsevich’s formality theorem in deformation theory asserts that the Hochschild-Kostant-Rosenberg map lifts to a quasi-isomorphism of homotopy Gerstenhaber algebras between polyvector fields and polydifferential operators. By exhibiting the sheaf of polydifferential operators as a universal enveloping algebra, this can be interpreted as a type of Duflo isomorphism. In this talk, I will present my work towards understanding this geometric Duflo isomorphism in the context of ∞-operads and higher centers. In particular, I will explain how higher centers can be used to equip the Hochschild cochain complex with a universal property.

Computations in Representation Stability

Speaker: 

Emil Geisler

Institution: 

UCLA

Time: 

Monday, April 7, 2025 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340P

We recall the notion of representation stability in the context of the cohomology of ordered configuration space of the plane. We give examples of the computation of stable multiplicities by arithmetic methods using the Grothendieck-Lefschetz fixed point formula and describe how these methods lead to a general algorithm and proofs of specific asymptotics. 

Norms and Hermitian K-theory

Speaker: 

Brian Shin

Institution: 

UCLA

Time: 

Monday, February 3, 2025 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340N

Over the past century, cohomology operations have played a crucial role in homotopy theory and its applications. A powerful framework for constructing such operations is the theory of commutative (i.e. $\mathbb{E}_\infty$) ring spectra. In this talk, I will discuss an algebro-geometric analogue of this framework, called the theory of normed motivic ring spectra. As a particular example of interest, I'll show that (very effective) Hermitian K-theory can be equipped with a normed ring structure.

Renormalized energy, harmonic maps and random matrices

Speaker: 

Antoine Song

Institution: 

Caltech

Time: 

Monday, March 3, 2025 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340N

I will introduce a geometric way to study unitary representations of surface groups. I will discuss a notion of renormalized energy, its corresponding harmonic maps into sphere, and their asymptotic or random behavior. The results connect harmonic maps to random matrix theory and representation theory

Cohomology of definable coherent sheaves and definable Picard groups

Speaker: 

Patrick Brosnan

Institution: 

Maryland

Time: 

Monday, January 13, 2025 - 4:00pm to 5:00pm

Host: 

Location: 

Rh 340N

Definable coherent sheaves (with respect to an o-minimal structure) were introduced by Bakker, Brunebarbe and Tsimerman  (BBT) and used as an essential tool in their proof of Griffiths' conjecture that the image of the period map is algebraic.   The category of these definable sheaves on a complex algebraic variety X sits in between the category of algebraic and analytic sheaves.  More precisely, there is a definablization functor taking coherent algebraic sheaves to definable coherent sheaves and an analytification functor going from the category of definable coherent sheaves to the category of coherent analytic sheaves.  This makes them useful for answering questions about analytic maps involving algebraic varieties.  I'll explain these two functors and the concept of o-minimality necessary to define the BBT category of definable coherent sheaves.  Then I'll state a couple of results I obtained recently with Adam Melrod on the cohomology groups of definable coherent sheaves both in the case where X is projective (when, for reasonable o-minimal structures,  the groups are the same as the usual cohomology groups) and the general case (when they very much aren't).

Algebraic points on curves

Speaker: 

Bianca Viray

Institution: 

University of Washington

Time: 

Monday, March 10, 2025 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340N

The Mordell Conjecture (proved by Faltings in 1983) is a landmark result exemplifying the philosophy "Geometry controls arithmetic". It states that the genus of an algebraic curve, a purely topological invariant that can be computed over the complex numbers, determines whether the curve may have infinitely many rational points. However, it also implies that we can never hope to understand the arithmetic of a higher genus curve solely by studying its rational points over a fixed number field. In this talk, we will introduce the concepts of parametrized points and density degree sets and show how they, together with the Mordell-Lang conjecture (proved by Faltings in 1994), allow us to organize all algebraic points on a curve.

Contact topology and geometry in high dimensions

Speaker: 

Bahar Acu

Institution: 

Pitzer College and Claremont Graduate University

Time: 

Monday, November 4, 2024 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340N

A very useful strategy in studying topological manifolds is to factor them into “smaller" pieces. An open book decomposition of an n-manifold (the open book) is a fibration that helps us study our manifold in terms of its (n-1)-dimensional fibers (the pages) and (n-2)-dimensional boundary of these submanifolds (the binding). Open books provide a natural framework for studying topological properties of certain geometric structures on smooth manifolds such as "contact structures". Thanks to open books, contact manifolds, odd dimensional smooth manifolds carrying these geometric structures, can be studied from an entirely topological viewpoint. For example, every contact 3-manifold can be presented as an open book whose pages are surfaces and binding is a knot/link. In this talk, we will talk about higher-dimensional contact manifolds and provide a setting where we study these manifolds in terms of 3D open books. We also present various results along with examples concerning geometric and topological aspects of contact and symplectic manifolds along with upcoming work concerning these special fibrations.

L-space satellite operators and knot Floer homology

Speaker: 

Daren Chen

Institution: 

Caltech

Time: 

Monday, December 2, 2024 - 4:00pm

Location: 

RH 340N

Heegaard Floer homology is a package of invariants for 3 manifolds introduced by Ozsváth and Szabó, which is a symplectic alternative to more gauge theoretic invariants such as monopole Floer homology. A variation of this theory, called knot Floer homology, defines an invariant for knots in 3-manifolds. It was developed independently by Ozsváth and Szabó, and by Rasmussen. In this talk, we will outline the construction, some properties and applications of these invariants. If time permits, I will discuss my recent project to compute the knot Floer homology for a large class of satellite knots. This is joint work with Ian Zemke and Hugo Zhou.

Symplectic Geometry and Sheaves

Speaker: 

Christopher Kuo

Institution: 

USC

Time: 

Monday, November 25, 2024 - 4:00pm

Location: 

RH 340N

Sheaves have long been classical tools for studying the topology of manifolds. Symplectic geometry, which encodes topological information about a manifold via its cotangent bundle, has revealed a profound connection to sheaf theory through the microlocal framework developed by Kashiwara and Schapira. Remarkably, many important symplectic invariants can now be computed using sheaves. In this talk, I will survey several well-known applications of sheaf theory in symplectic geometry and also consider the reverse perspective: how symplectic geometry provides constructions and insights that deepen our understanding of sheaf theory. This latter viewpoint is central to obtaining a global version of the microlocal Riemann-Hilbert correspondence in joint work with Côté, Nadler, and Shende.

Classical Physics in Curved Space

Speaker: 

Steve Trettel

Institution: 

University of San Francisco

Time: 

Monday, November 18, 2024 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340N

When transitioning from studying Euclidean space to more Riemannian manifolds, one must first unlearn many special properties of the flat world. The same is true in physics: while one can make sense of classical physics on an arbitrary curved background space, many seemingly foundational concepts (like the center of mass) turn out to have no place in the general theory.  Freed from the constraints such properties induce, classical physics on a curved background space has many surprises in store.  In this talk I will share some stories related to joint work with Brian Day and Sabetta Matsumoto on understanding and simulating such situations, focusing on hyperbolic space when convenient.  To give a taste, here are two such surprises:
(1) there is no Galilean relativity: inside a sealed box in hyperbolic geometry it is possible to perform an experiment which detects your precise velocity.  And (2): it's possible to ‘swim’ in the vacuum in hyperbolic space - to move your arms and legs in a specific pattern that causes you to translate along a geodesic with no external forces.  The arguments for the former are readily accessible to beginning graduate students in geometry, and the latter illustrates a use of gauge theory in classical mechanics, following work of Wilczek and Montgomery.
 

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