UCSB Differential Geometry Seminar 2020-2021

The talks are held on every Friday 5-6 PM (Pacific Time) on Zoom.

Winter 2021

Friday, January 15: Detang Zhou (UFF-Universidade Federal Fluminense, Brazil)
Title: Volume growth of complete submanifolds in gradient Ricci Solitons

Abstract: It is well-known that the volume of geodesic balls has polynomial growth and at least linear growth on complete noncompact Riemannian manifolds of nonnegative Ricci curvature. We study properly immersed complete noncompact submanifolds in a complete shrinking gradient Ricci soliton with weighted mean curvature vector bounded in norm. We prove that such a submanifold must have polynomial volume growth under some mild assumption on the potential function. On the other hand, if the ambient manifold is of bounded geometry, we prove that such a submanifold must have at least linear volume growth. In particular, we show that a properly immersed complete noncompact hypersurface in the Euclidean space with bounded Gaussian weighted mean curvature must have polynomial volume growth and at least linear volume growth. This is a joint work with Xu Cheng and Matheus Vieira.

Friday, January 22: Anna Siffert (Max Planck Institute for Mathematics)
Title: Construction of explicit pp-harmonic functions

The study of pp-harmonic functions on Riemannian manifolds has invoked the interest of mathematicians and physicists for nearly two centuries. Applications within physics can for example be found in continuum mechanics, elasticity theory, as well as two-dimensional hydrodynamics problems involving Stokes flows of incompressible Newtonian fluids.
In my talk I will focus on the construction of explicit p-harmonic functions on rank- one Lie groups of Iwasawa type. This joint wok with Sigmundur Gudmundsson and Marko Sobak.

Friday, January 29: Laura Fredrickson
Title: TBA

Friday, February 5: Darong Chen
Title: TBA

Fall 2020

Friday, October 9: Ákos Nagy
Title: The asymptotic geometry of G2G_2-monopoles

Abstract: G2G_2-monopoles are special Yang-Mills-Higgs configurations on G2G_2-manifolds. Donaldson and Segal conjecture that one can construct invariants of noncompact G2G_2-manifolds by counting G2G_2-monopoles. The first steps in achieving this goal is understanding the analytic properties of these monopoles, such as asymptotic and decay properties. In this talk, I introduce the proper analytic setup for the problem, and present our results about the asymptotic forms of G2G_2-monopoles, with structure group being SU(2)SU(2), on Asymptotically Conical G2G_2-manifolds. This is a joint project with Daniel Fadel and Gonçalo Oliveira.

Friday, October 16: Jiayin Pan
Title: Nonnegative Ricci curvature and escape rate gap

Abstract: Let MM be an open nn-manifold of nonnegative Ricci curvature and let pMp\in M. We show that if (M,p)(M,p) has escape rate less than some positive constant ϵ(n)\epsilon(n), that is, minimal representing geodesic loops of π1(M,p)\pi_1(M,p) escape from any bounded balls at a small linear rate with respect to their lengths, then π1(M,p)\pi_1(M,p) is virtually abelian.

Friday, October 23: Fedya Manin
Title: Filling random cycles

Abstract: I will explain an average-case isoperimetric inequality for certain combinatorial models of random cycles in cubes and spheres. For example, take a knot built by connecting a sequence of NN uniformly random points in the unit cube (a model introduced by Ken Millett). The minimal area of a Seifert surface for this knot is on the order of NlogN\sqrt{N \log N} with high probability. For all the models analyzed, this growth rate is the same, depending only on the codimension of the cycles; I suspect that Fourier analysis can be used to explain this apparent coincidence.

Friday, October 30: Yeping Zhang
Title: Quillen metric, BCOV invariant and motivic integration

Abstract: Bershadsky, Cecotti, Ooguri and Vafa constructed a real valued invariant for Calabi-Yau manifolds, which is now called the BCOV invariant. The BCOV invariant is conjecturally related to the Gromov-Witten theory via mirror symmetry. In this talk, we prove the conjecture that birational Calabi-Yau manifolds have the same BCOV invariant. We also build an analogue between the BCOV invariant and the motivic integration. The result presented in this talk is a joint work with Lie Fu.

Friday, November 6: Siqi He
Title: The Behavior of Solutions to the Hitchin-Simpson Equations

Abstract: The Hitchin-Simpson equations defined over a Kähler manifold are first order, non-linear equations for a pair of connection on a Hermitian vector bundle and a 1-form with values in the endomorphism bundle. We will describe the behavior of solutions to the Hitchin-Simpson equations with norms of these 1-forms unbounded. In addition, we will discuss the relationship between this behavior with the Taubes’ Z2 harmonic spinor and Hitchin’s WKB problem.

Friday, November 13: Junrong Yan
Title: Witten Deformation on Non-compact Manifolds: Heat Kernel Expansion and Local Index Theorem

Abstract: Asymptotic expansions of heat kernels and heat traces of Schrödinger operators on noncompact spaces are rarely explored, and even for cases as simple as Cn\mathbb{C}^n with (quasi-homogeneous) polynomials potentials, it’s already very complicated. Motivated by path integral formulation of the heat kernel, we introduced a parabolic distance, which also appeared in Li-Yau’s famous work on parabolic Harnack estimate. With the help of the parabolic distance, we derive a pointwise asymptotic expansion of the heat kernel with a strong remainder estimate. In particular, we obtain an asymptotic expansion of the heat kernel of the Witten Laplacian for the Witten deformation. When the deformation parameter of Witten deformation and time parameter are coupled, we derive a small time asymptotic expansion for the trace of the heat kernel of the Witten Laplacian. We introduce a novel rescaling technique to compute the local index density explicitly. If time permits, I will also briefly explain how to define analytic torsion for Witten Laplacian on noncompact spaces. This is joint work with Xianzhe Dai.

Friday, November 20: Qin Deng
Title: Hölder continuity of tangent cones in RCD(K,N)RCD(K,N) spaces and applications to non-branching

Abstract: It is known by a result of Colding-Naber that for any two points in a Ricci limit space, there exists a minimizing geodesic where the geometry of small balls centred along the interior of the geodesic change in at most a Hölder continuous manner. This was shown using an extrinsic argument and had several key applications for the structure theory of Ricci limits. In this talk, I will discuss how to generalize this result to the setting of metric measure spaces satisfying the synthetic lower Ricci curvature bound condition RCD(K,N)RCD(K,N). As an application, I will show that all RCD(K,N)RCD(K,N) spaces are non-branching, a result which was previously unknown for Ricci limit spaces.

Friday, December 4 at 9 AM (Pacific Time): Szilárd Szabó
Title: Asymptotic Hodge theory in the Painlevé cases

Abstract: We will state two conjectures for moduli spaces of flat (irregular singular) connections and Higgs bundles on curves, namely the P=W conjecture (due to de Cataldo, Hausel and Migliorini) and its geometric counterpart (due to Simpson et al). We will explain our proof of these conjectures in some classical 4-dimensional cases called Painlevé cases.