Purdue Topology Seminar
In Spring 2025, the Purdue Topology Seminar will be held on Wednesdays 11:30am - 12:30pm EST at BRNG B206 (if we meet in person) unless otherwise noted. Some of the talks will be online through Zoom. If you want to be added to our email list please contact Manuel Rivera (manuelr at purdue.edu).
We have begun to record some of our online seminars. We publish them on our YouTube-Channel.
Spring 2025
January 15 (in person)
Title: An equivariant pair-of-paints product from higher Hochschild cochains
Abstract: Hochschild homology and cohomology are both generally defined in the settings of A-infinity algebras/categories. In general, Hochschild homology does not inherently possess a ring structure, unlike its cohomological counterpart. Rather, it is a module over Hochschild cohomology. However, when a (homologically) smooth A-infinity algebra/category, introduced by Kontsevich-Soibelman, is equipped with a smooth Calabi-Yau structure, a product structure emerges on Hochschild homology, introducing a richer algebraic framework. In the presence of group actions (for instance, Z/2 in our case), a natural question arises: ``Can this (non-equivariant) product structure, called pair-of-pants product, often be extended or adapted to its equivariant counterpart?’'.
In this talk, I will show that there is a canonical equivariant refinement, called algebraic equivariant pair-of-pants product, of the (non-equivariant) product structure provided that a canonical lift, of a Calabi-Yau map, induced by a (strong) smooth Calabi-Yau structure exists. I will also give the motivations behind the question via a conjectural diagram related to open-closed maps, fixed point Floer cohomology and Hochschild homology of Fukaya categories and Smith-type inequalities (modulo technical details) as long as time permits. This is joint work with Sheel Ganatra.
January 22 (online)
Maximilian Stegemeyer (Freiburg)
Title: Intersection multiplicity in loop spaces and the string topology coproduct
Abstract: String topology is the study of algebraic structures on the free loop space of a closed oriented manifold. The Goresky-Hingston coproduct is geometrically defined by cutting loops apart at points of self-intersection. A result by Hingston and Wahl makes this intuitive description precise and sates that if a homology class can be represented by loops without basepoint self-intersections then the coproduct of this class vanishes. In this talk I will talk about some results where the concept of intersection multiplicity is used to conclude the triviality of the string topology coproduct. We will also encounter the relation between the homology of the free loop space and the closed geodesics in the underlying manifold. The triviality results for the coproduct open up a number of questions.
January 29 (online)
Shuhei Maruyama (Kanazawa University)
Title: McDuff's secondary class and the Euler class of foliated sphere bundles
Abstract: Tsuboi proved that the Calabi invariant transgresses to the Euler class of foliated circle bundles. The Calabi invariant is a first cohomology class of the group of area-preserving diffeomorphisms of a closed disk that are identity on the boundary.
In this talk, I will introduce a group cocycle representative of McDuff's secondary class: a (2n−1)-dimensional cocycle of the group of volume-preserving diffeomorphisms of a 2n-dimensional closed ball that are identity on the boundary. I will also discuss a higher-dimensional analog of Tsuboi's theorem, which states that this cohomology class transgresses to the Euler class of foliated sphere bundles.
February 5 (in-person)
Liam Morgan Ashton (Purdue University)
Title: The algebra of strings
Abstract: One may associate to any surface a Lie bialgebra structure constructed by transversally intersecting, concatenating, and splitting free homotopy classes of loops. Deep geometric results have been proven using this structure; however there are still many mysteries and open questions around it. In this talk, I will describe the Goldman bracket and the Turaev cobracket on homotopy classes of free loops (or strings) and give a few examples to show how they work and interact. I will then discuss several statements and open questions that relate these operations to minimal intersections and minimal self-intersections of strings. This is my advanced topics exam.
Thursday, February 6 (Time: 2:00pm, Room: MATH 431)
Title: A pleasant surprise of the cyclotomic character
Abstract: Abstract: In 1990, V. Drinfeld introduced the Grothendieck-Teichmueller group GT. This group receives a homomorphism from the absolute Galois group G_Q of rational numbers, and this homomorphism is injective due to Belyi's theorem. In his 1990 ICM talk, Y. Ihara posed a very hard question about the surjectivity of this homomorphism from G_Q to GT. In my talk, I will introduce the groupoid of GT-shadows and show how this groupoid is related to the group GT. I will formulate a version of Ihara's question for GT-shadows and describe a family of objects of the groupoid for which this question has a positive answer. My talk is loosely based on the joint paper https://arxiv.org/abs/2405.11725 with I. Bortnovskyi, B. Holikov and V. Pashkovskyi.
February 12 (in person)
Ben McReynolds (Purdue University)
Title: Nilpotent representations and fundamental groups
Abstract: I will discuss a construction for producing groups with the same nilpotent representation theory. The construction is motivated by a theorem of Stallings. I will discuss some applications to Kleinian groups and a conjectural result on finite groups that could have important implications.
February 19 (in person)
Calvin McPhail-Snyder (Duke University)
Title: Geometric quantum invariants and the volume conjecture
Abstract: This talk is about recent joint work with N. Reshetikhin defining a new family of geometric quantum link invariants. Here "geometric" means that they depend on an additional choice of a flat 𝔰𝔩₂ connection (i.e. a generalized hyperbolic structure) on the link complement. When this structure is trivial we recover Kashaev's invariants, which are particular values of the colored Jones polynomials. In this talk I will explain how our geometric invariants naturally appear in the context of the Volume Conjecture, then sketch their construction and how they relate to both classical and quantum Chern-Simons theory for the complex group SL₂(ℂ).
February 26 (online)
Martin Tancer (Charles University)
Title: Simpler algorithmically unrecognizable 4-manifolds
Abstract: Markov proved that there exists an unrecognizable 4-manifold, that is, a 4-manifold for which the homeomorphism problem is undecidable. In this talk we consider the question how close we can get to S^4 with an unrecognizable manifold. We sketch a way to remove so-called Markov's trick from the proof of existence of such a manifold. This trick contributes to the complexity of the resulting manifold. We also show how to decrease the deficiency (or the number of relations) in so-called Adian-Rabin set which is another ingredient that contributes to the complexity of the resulting manifold. Altogether, our approach allows to show that the connected sum #_9(S^2 x S^2) is unrecognizable while the previous best result is the unrecognizability of #_12(S^2 x S^2) due to Gordon.
March 12 (online)
Amit Kumar (Louisiana State University, IMSc)
March 19 is a university holiday (spring break)
March 24 (in person, Monday 11:30-12:30, Room 631)
Peter Patzt (University of Oklahoma)
March 26 (online)
Title: Defining extended TQFTs via handle attachments
Abstract: Extended Topological Quantum Field Theories model some physical theories where time evolution doesn't depend on any metric, but only on the topology of space-time. Using Morse theory, one can decompose any space-time into a series of "events" corresponding to handle attachments. We will reformulate this idea into a presentation of the bicategory of cobordisms, i.e. space-times. If time allows, I will also discuss some applications in dimension 4 coming from skein theory.
April 2 (in person)
Jim Davis (Indiana University)
April 16 (in person)
Kevin Piterman (Vrije Universiteit)