Schedule
Date: 13 May, 2025 (Last talk of the Spring Semester)
Speaker: Eveline Legendre (Université Claude Bernard Lyon 1)
Title : The CR-Yamabe invariants and K-semistability.
Abstract: I will present a joint work in progress with Abdellah Lahdili and Carlo Scarpa, where we explore in more detail the results of a previously established correspondence between the constant scalar curvature Kähler problem (cscK) on a polarized manifold and a family of CR-Yamabe problems on the associated circle bundle. We show, among other things, that the supremum of the associated CR-Yamabe invariants detects K-semistability.
Past Talks
.Date: 29 Apr., 2025
Speaker: Gerard Misiolek (University of Notre Dame)
Title: Diffeomorphism groups and information geometry
Abstract: The pioneering work of V. Arnold in the 1960s introduced spaces of diffeomorphisms into the study of hydrodynamics. Motions of an ideal fluid in a fixed domain were recognized as geodesics in the group of volume-preserving diffeomorphisms equipped with an L^2 ("kinetic energy") metric. Subsequently, the space of densities was shown to play a similar role in the geometric description of optimal mass transport with the Kantorovich-Wasserstein distance. It turns out that information geometry with its attendant Fisher-Rao metric and the Hellinger distance can be placed in the same infinite-dimensional differential geometric framework and seen as an analogue of optimal transport albeit with respect to a higher-order Sobolev metric. In the talk I will describe this framework and give an application to integrable systems.
.Date: 22 Apr., 2025
Speaker: Yoshinori Hashimoto (Osaka Metropolitan University）
Title: Coupled Kähler--Einstein metrics and coupled Ding stability
Abstract: A foundational theorem in Kähler geometry states that a Kähler--Einstein metric exists on a Fano manifold (with discrete automorphisms) if and only if it is uniformly Ding stable, which happens if and only if the stability threshold is larger than 1. In the absence of Kähler--Einstein metrics, we can seek coupled Kähler--Einstein metrics defined in terms of a decomposition of the anticanonical bundle. The main result of this talk is the equivalence between the coupled Ding stability, the coupled stability threshold larger than 1 (once they are appropriately defined), and the existence of coupled Kähler--Einstein metrics. This is a joint work with Kento Fujita.
.Date: 15 Apr., 2025
Speaker: Eleonora Di Nezza (IMJ-PRG, Sorbonne Université)
Title: Almost convexity of the Mabuchi functional in singular settings
Abstract: The Mabuchi functional M was introduced by Mabuchi in the 80's in relation to the existence of canonical metrics on a compact Kähler manifold. The critical points of M are indeed constant scalar curvature Kähler (cscK) metrics. Recently, Chen and Cheng proved that the existence of a (smooth) cscK metric is equivalent to the properness of such functional. In order to look for singular metrics, it is then natural to study the properties of the Mabuchi functional in singular settings. In this talk we prove that this functional is (almost) convex in the very general "big case". This is based on a joint work with Trapani and Trusiani.
.Date: 11 Apr., 2025 (4PM-5PM, Friday) (In person, 静斋 105)
Speaker: Wilderich Tuschmann (Karlsruhe Institute of Technology)
Title: Moduli Spaces of Metrics and their Compactifications
Abstract: Focussing on lower curvature bounds, I will present recent results and open questions about the global topological properties of moduli spaces of metrics on manifolds and RCD spaces, and also discuss metric approaches to obtain and study suitable compactifications of the former.
.Date: 18 Mar., 2025
Speaker: Yanir Rubinstein (University of Maryland)
Title: Statistics meets algebraic geometry
Abstract: This 30-minute talk will describe a serendipitous encounter of notions from statistics (e.g., quantiles, tail distributions, cdfs) to the study of thresholds and Okounkov bodies in algebraic geometry. An unexpected twist in the plot is provided by Weierstrass gap theory.
.Date: 11 Mar., 2025
Speaker: Kartick Ghosh (Tata Institute of Fundamental Research, Mumbai)
Title: Computing the α-Futaki character
Abstract: Alvarez-Consul–Garcia-Fernandez–Garcia-Prada introduced the Kahler-Yang-Mills equations (coupled equations for a Kahler metric and a metric on a holomorphic vector bundle ). They also introduced the α-Futaki character, an analog of the Futaki character, as an obstruction to the existence of solutions of the Kahler-Yang-Mills equations. In this talk, we will prove a formula for the α-Futaki character for certain ample line bundles over toric manifolds. We will then compute the α-Futaki character in some cases and compare it with the known existence result of Keller–Tønnesen-Friedman.
.Date: 25 Feb., 2025
Speaker: Marco Mazzucchelli (ENS Lyon)
Title: Closed geodesics and the first Betti number
Abstract: In this talk, based on a joint work with Gonzalo Contreras, I will present the following result: on a closed Riemannian manifold of dimension at least two with non-trivial first Betti number, the existence of a minimal closed geodesic, in the sense of Aubry-Mather theory, implies the existence of a transverse homoclinic, and thus of a horseshoe, for the geodesic flow of a suitable -close Riemannian metric. As a corollary of this result and of existing literature, we infer that on any closed manifold of dimension at least two with non-trivial first Betti number, a generic Riemannian metric has infinitely many closed geodesics, and indeed closed geodesics of arbitrarily large length.
.Date: 17 Dec., 2024
Speaker: Thomas Schick (Goettingen)
Time：Tues., 15:30-16:30, Dec.17, 2024
Venue：Jingzhai 105（静斋 105）, Tsinghua University
Title: Equivariant T-duality
Abstract: T-duality is a a concept inspired by certain aspects of string theory (the T stand for ``target space''). The physics predicts that one can pass from a torus bundle over a base space, equipped with some background B-field to a ``dual'' torus bundle with dual background B-field without changing the physical content of the theory. Strominger-Yau-Zaslow even propose that mirror symmetry is a special case of T-duality. To understand the mathematical underpinnings of this duality, the concept of topological T-duality has been introduced. We present the fundamental ideas of this concept: what are the objects we put in duality? what is the precise meaning of ``being dual''? Among the most striking mathematical consequences, we present the T-duality isomorphism between twisted K-theory of the two dual spaces. The last part of the talk will present new developments which concern the investigation of additional symmetries, encoded in the action of a compact group G. We discuss the meaning of ``equivariant T-duality'' and we show that one obtains a T-duality isomorphism in the full Atiyah-Segal equivariant K-theory.
.Date: 17 Dec., 2024(Last talk of the Winter Semester) （cancelled）
Speaker: Marco Mazzucchelli (ENS Lyon)
Time: Tues., 21:00-22:00, Dec. 17, 2024
Venue: Online
Title: Closed geodesics and the first Betti number
Abstract: In this talk, based on a joint work with Gonzalo Contreras, I will present the following result: on a closed Riemannian manifold of dimension at least two with non-trivial first Betti number, the existence of a minimal closed geodesic, in the sense of Aubry-Mather theory, implies the existence of a transverse homoclinic, and thus of a horseshoe, for the geodesic flow of a suitable -close Riemannian metric. As a corollary of this result and of existing literature, we infer that on any closed manifold of dimension at least two with non-trivial first Betti number, a generic Riemannian metric has infinitely many closed geodesics, and indeed closed geodesics of arbitrarily large length.
.Date: 10 Dec., 2024
Speaker: Ruobing Zhang (UW-Madison)
Title: Geometry of asymptotically hyperbolic Poincaré-Einstein manifolds
Abstract: This talk will summarize recent progress towards understanding some fundamental questions in Poincaré-Einstein manifolds. We will discuss existence, uniqueness, and compactness questions of the Poincaré-Einstein metrics with prescribed conformal infinity.
.Date: 3 Dec., 2024
Speaker: Yifan Chen (Berkeley)
Title: More Complete Calabi-Yau Metrics of Calabi Type
Abstract: We construct more complete Calabi-Yau metrics asymptotic to Calabi ansatz. They are the higher-dimensional analogues of two dimensional ALH* gravitational instantons. Our work builds on and generalizes the results of Tian-Yau and Hein-Sun-Viaclovski-Zhang, creating Calabi-Yau metrics that are only polynomially close to the model space. We also prove the uniqueness of such metrics in a given cohomology class with fixed asymptotic behavior.
.Date: 26 Nov., 2024
Speaker: Lucas Ambrozio (IMPA )
Title: Riemannian manifolds that are determined by their widths
Abstract: A fascinating subject in Geometric Analysis is whether or not a compact Riemannian manifold can be totally reconstructed from the sole knowledge of its Laplace spectrum. Recent developments in the min-max theory of the area functional on the space of codimension one flat cycles made it possible to ask a similar question regarding the area spectrum. We will present some recent work done in collaboration with A. Neves (UChicago) and F. Marques (Princeton) in that direction. The focus will be on the complete characterization of the class of Riemannian metrics on the three dimensional sphere whose four spherical widths are equal. Time permitting, we will also discuss how these ideas can be further developed, yielding the complete characterization of the real projective plane by its length spectrum.
.Date: 19 Nov., 2024
Speaker: Simone Diverio (Sapienza Università di Roma)
Title: A birational version of Gromov's Kähler hyperbolicity
Abstract: In his wonderful book "Shafarevich maps and automorphic forms", J. Kollár asked almost 30 years ago for a "good" birational version of the notion of Kähler hyperbolicity introduced by M. Gromov in the early '90s. Kähler hyperbolic manifolds are compact Kähler manifold admitting a Kähler form whose pull-back to the universal cover becomes d-exact and moreover with a bounded primitive. Such manifolds are of general type (more than this: with ample canonical bundle), Kobayashi hyperbolic, and with large fundamental group.
We shall report on the recently intruduced class of weakly Kähler hyperbolic manifolds which indeed provides such a birational generalization. Among other things, we shall explain that these manifolds are of general type, satisfy a precise quantitative version of the Green-Griffiths conjecture and have generically arbitrarily large fundamental group. Moreover, their properties permit to verify Lang's conjecture for Kähler hyperbolic manifolds. This is a joint work with F. Bei, B. Claudon, P. Eyssidieux, and S. Trapani.
.Date: 12 Nov., 2024
Speaker: Lars Martin Sektnan (University of Gothenburg)
Title: Constant scalar curvature Kähler metrics and semistable vector bundles
Abstract: A central question in Kähler geometry is if a Kähler manifold admits a canonical metric, such as a Kähler-Einstein metric or more generally a constant scalar curvature Kähler (cscK) metric, in a given Kähler class. The Yau-Tian-Donaldson conjecture predicts that this is equivalent to an algebraic notion of stability. In this talk, I will discuss a necessary and sufficient condition for the projectivisation of a slope semistable vector bundle to admit cscK metrics in adiabatic classes, when the base admits a cscK metric. In particular, this shows that the existence of cscK metrics is equivalent to K-stability in this setting. Moreover, our construction reduces K-stability to a finite dimensional criterion in terms of intersection numbers associated to the vector bundle. This is joint work with Annamaria Ortu.
.Date: 5 Nov., 2024
Speaker: Ronan Conlon (University of Texas at Dallas)
Title: A family of Kahler flying wing steady Ricci solitons
Abstract: Steady Kahler-Ricci solitons are eternal solutions of the Kahler-Ricci flow. I will present new examples of such solitons with strictly positive sectional curvature that live on C^n and provide an answer to an open question of H.-D. Cao in complex dimension n>2. This is joint work with Pak-Yeung Chan and Yi Lai.
.Date: 29 Oct., 2024
Speaker: Yusuke Kawamoto (ETH)
Title: Donaldson divisors and quantitative Floer homology
Abstract: I will explain a comparison of quantitative information of Floer homologies of a symplectic manifold and a Donaldson divisor therein, which leads to a solution to a question of Borman posed in 2012.
.Date: 22 Oct., 2024
Speaker: Rohil Prasad (Berkeley)
Title: Low-action pseudoholomorphic curves and invariant sets.
Abstract: Punctured pseudoholomorphic curves have been a key tool in Hamiltonian dynamics since a breakthrough 1993 paper by Hofer. I'll describe a new compactness theorem for sequences of punctured pseudoholomorphic curves that holds without the traditional technical assumption of "bounded Hofer energy". In the limit, one extracts a family of compact invariant subsets that are invariant under the Hamiltonian flow. This has various dynamical applications. I will briefly mention these applications, but I will mostly focus on the geometric analysis behind the compactness theorem. Most of the talk is based on joint work with Dan Cristofaro-Gardiner.
.Date: 15 Oct., 2024
Speaker: Bram Mesland（Universiteit Leiden)
Title: Curvature and Weitzenbock formulae for spectral triples
Abstract: In this talk I will present a new, operator theoretic construction of the Levi-Civita connection on a Riemannian manifold, based on the two projection problem in Hilbert modules. The construction allows us to deduce the mild technical assumptions needed for the existence and uniqueness of the Levi-Civita connection on the module of differential 1-forms over a noncommutative algebra. In particular the construction applies to a large class of noncommutative manifolds (spectral triples). The well-known algebraic theory of curvature for bimodule connections can then be applied to derive a Weitzenbock formula. Examples include toric non commutative manifolds and the Podles quantum sphere. This is joint work with Adam Rennie.
.Date: 8 Oct., 2024
Speaker: Yueqing Feng (Berkeley)
Title: A gluing construction of constant scalar curvature Kähler metrics of Poincaré type
Abstract: In this talk, we construct new examples of constant scalar curvature Kähler(cscK) metrics of Poincaré type from existing cscK ones. The construction is obtained via gluing a cscK metric on a compact Kähler manifold to a complete scalar-flat Kähler metric of Poincaré type on $\mathbb{C}^n$ removing the origin. Assuming the compact Kähler manifold has no non-trivial holomorphic vector field, we prove the existence of cscK metrics of Poincaré type on this compact manifold removing finitely many points.
.Date: 24 Sept., 2024
Speaker: Sébastien Picard (UBC)
Title: Non-Kahler Degenerations of Calabi-Yau Threefolds
Abstract: It was proposed in the works of Clemens, Reid, Friedman to connect Calabi-Yau threefolds with different topologies by a process which degenerates 2-cycles and introduces new 3-cycles. This operation may produce a non-Kahler complex manifold with trivial canonical bundle. In this talk, we will discuss the geometrization of this process by special non-Kahler metrics. This talk will survey joint works with T.C. Collins, B. Friedman, C. Suan, and S.-T. Yau.
.Date: 10 Sept., 2024
Speaker: Vamsi Pritham Pingali (Indian Institute of Science)
Title: Abelian and non-abelian gravitating vortices
Abstract: The gravitating vortex (GV) equations are a dimensional reduction of the Kahler-Yang-Mills-Higgs equations to a Riemann surface. The Abelian GV equations contain as a special case, the Einstein-Bogomol'yni equations modelling (the as of now, hypothetical) cosmic strings in the Bogomolo'yni phase in the early universe. The non-abelian GV equations are expected to play a similar role for Electroweak cosmic strings. Moreover, these equations might have interesting moduli spaces and arise out of a moment map interpretation. I shall describe our existence, uniqueness, and obstruction (including a Donaldson-Uhlenbeck-Yau-Kobayashi-Hitchin correspondence) results (joint with L. Alvarez-Consul, M. Garcia-Fernandez, O. Garcia-Prada, and C. Yao) for Abelian GV equations, and my recent existence result for the non-abelian case.
.Date: 11 Jun., 2024 (Last talk of the Spring Semester)
Speaker: Matthew Tointon (University of Bristol)
Title: Sharp bounds in a finitary version of Gromov’s polynomial-growth theorem
Abstract: A famous theorem of Gromov states that if a finitely generated group G has polynomial growth (i.e. there is a polynomial p such that the ball of radius n in some Cayley graph of G always contains at most p(n) vertices) then G has a nilpotent subgroup of finite index. Breuillard, Green and Tao proved a finitary refinement of this theorem, stating that if *some* ball of radius n contains at most eps n^d vertices then that ball contains a normal subgroup H such that G/H has a nilpotent subgroup with index at most f(d), where f is some non-explicit function. I will describe joint work with Romain Tessera in which we obtain explicit and even optimal bounds on both the index and the dimension of this nilpotent subgroup. I will also describe an application to random walks on groups.
.Date: 4 Jun., 2024
Speaker: Alex Mramor (University of Copenhagen)
Title: The mean curvature flow, singularities, and entropy
Abstract: The mean curvature flow is an example of a geometric flow, where in this case one deforms a submanifold according to its mean curvature vector. Like many such flows though the mean curvature flow will develop singularities, where the flow “pinches.” The entropy, in the sense of Colding and Minicozzi, is an interesting area-like monotone quantity under the flow, for one because it can constrain what sorts of singularity models may arise, and has played an important role in many recent developments. In this talk after introducing the relevant notions we’ll discuss some of these results, including some joint work with S. Wang.
.Date: 28 May, 2024
Speaker: Michael Alexander Hallam (Aarhus University）
Title: Stability of weighted extremal manifolds through blowups
Abstract: The weighted extremal Kähler metrics introduced by Lahdili provide a vast generalisation of Calabi's extremal Kähler metrics, encompassing many examples of canonical metrics in geometry. In this talk, I will give a quick introduction to these metrics, and discuss the proof that weighted extremal manifolds are relatively weighted K-polystable, in a suitable sense. The proof is along the lines of Stoppa--Szekelyhidi's argument that an extremal polarised manifold is relatively K-polystable, which in particular exploits the existence of an extremal metric on the blowup of an extremal manifold at a suitable point.
.Date: 21 May, 2024 (No Talks)
.Date: 14 May, 2024
Speaker: Shaoyun Bai (Columbia University)
Title: Infinitude of Hamiltonian periodic orbits and GLSM
Abstract: Take an irrational rotation of the two-sphere; it only has the north and south poles as its periodic points. However, Franks proved that for any area-preserving diffeomorphism of the two-sphere, if it has more than two fixed points, then it must have infinitely many periodic points. I will discuss a generalization of this result with Guangbo Xu to all compact toric manifolds, for which the gauged linear sigma model (GLSM) plays a surprising role.
.Date: 7 May, 2024
Speaker: Jesús A. Álvarez López (University of Santiago de Compostela (USC))
Title: A trace formula for foliated flows
Abstract: In the lecture, I will try to explain the ideas of a recent paper on the trace formula for foliated flows, written in collaboration with Yuri Kordyukov and Eric Leichtnam. Let $\mathcal{F}$ be a transversely oriented foliation of codimension one on a closed manifold $M$, and let $\phi=\{\phi^t\}$ be a foliated flow on $(M,\mathcal{F})$ (it maps leaves to leaves). Assume the closed orbits of $\phi$ are simple and its preserved leaves are transversely simple. In this case, there are finitely many preserved leaves, which are compact. Let $M^0$ denote their union, and write $M^1=M\setminus M^0$ and $\mathcal{F}^1=\mathcal{F}|_{M^1}$. We consider two locally convex Hausdorff spaces, $I(\mathcal{F})$ and $I'(\mathcal{F})$, consisting of the leafwise currents on $M$ that are conormal and dual-conormal to $M^0$, respectively. They become topological complexes with the differential operator $d_{\mathcal{F}}$ induced by the de~Rham derivative on the leaves, and they have an $\mathbb{R}$-action $\phi^*=\{\phi^{t\,*}\}$ induced by $\phi$. Let $\bar H^\bullet I(\mathcal{F})$ and $\bar H^\bullet I'(\mathcal{F})$ denote the corresponding leafwise reduced cohomologies, with the induced $\mathbb{R}$-action $\phi^*=\{\phi^{t\,*}\}$. The spaces $\bar H^\bullet I(\mathcal{F})$ and $\bar H^\bullet I'(\mathcal{F})$ are shown to be the central terms of short exact sequences in the category of continuous linear maps between locally convex spaces, where the other terms are described using Witten's perturbations of the de~Rham complex on $M^0$ and leafwise Witten's perturbations for $\mathcal{F}^1$. This is used to define some kind of Lefschetz distribution $L_{\rm dis}(\phi)$ of the actions $\phi^*$ on both $\bar H^\bullet I(\mathcal{F})$ and $\bar H^\bullet I'(\mathcal{F})$, whose value is a distribution on $\mathbb{R}$. Its definition involves several renormalization procedures, the main one is the b-trace of some smoothing b-pseudodifferential operator on the compact manifold with boundary obtained by cutting $M$ along $M^0$. We also prove a trace formula describing $L_{\rm dis}(\phi)$ in terms of infinitesimal data from the closed orbits and preserved leaves. This solves a conjecture of C.~Deninger involving two leafwise reduced cohomologies instead of a single one.
Remark (by Deng): This talk is based on arXiv:2402.06671
.Date: 30 Apr., 2024
Speaker: Antonio Trusiani (Chalmers University of Technology）
Title: Singular cscK metrics on smoothable varieties
Abstract: We extend the notion of cscK metrics to singular varieties. We establish the existence of these canonical metrics on Q-Gorenstein smoothable klt varieties when the Mabuchi functional is coercive, these arise as a limit of cscK metrics on close-by fibres. The proof relies on developing a novel strong topology of pluripotential theory in families and establishing uniform estimates for cscK metrics. A key point is the lower semicontinuity of the coercivity threshold of Mabuchi functional along degenerate families of normal compact Kähler varieties with klt singularities. The latter suggests the openness of (uniform) K-stability for general polarized families of normal projective varieties. This is a joint work with Chung-Ming Pan and Tat Dat Tô.
.Date: 23 Apr., 2024
Speaker: Giovanni Gentili (Università di Torino)
Title: Special metrics in hypercomplex Geometry
Abstract: The existence and search of special metrics plays a remarkably important role in Differential Geometry. After a brief overview of basic definitions and facts in hypercomplex Geometry, we will discuss certain notions of special metrics in the hypercomplex setting, focusing on the consequences of their existence. We will also introduce an Einstein condition for hyperhermitian metrics and describe the similarities with the Kähler-Einstein case. The talk is based on a joint work with Elia Fusi.
Date: 16 April 2024
Speaker: Simion Filip (Chicago)
Title: The volume of a divisor and cusp excursions of geodesics in hyperbolic manifolds
Abstract: The volume of a divisor on an algebraic variety measures the growth rate of the dimension of the space of sections of tensor powers of the associated line bundle. In the case of certain Calabi-Yau N-folds possessing a large group of pseudo-automorphisms, we show that the behavior of the volume can be highly oscillatory as the divisor class approaches the boundary of the pseudo-effective cone. This is explained by relating the volume function to the dynamical behavior of geodesics on certain hyperbolic manifolds.
After providing an introduction and some context for the above notions, I will discuss some of the ideas that go into the proof. (joint work with John Lesieutre and Valentino Tosatti)
.Date: 9 Apr., 2024
Speaker: Charles Cifarelli (Stony Brook)
Title: Non-compact Kähler-Ricci solitons, toric fibrations, and weighted K-stability
Abstract: In recent years, it has been shown that various problems in Kähler geometry can be unified under the framework of the weighted cscK equation, introduced by Lahdili. One feature of this setup is that, if a given manifold Y admits a special kind of fibration structure with toric fiber M and base B, then many interesting equations for the metric on Y can be reduced to the weighted cscK equation on M. This can be thought of as a generalized form of the Calabi Ansatz, which one recovers by taking M = \mathbb{C}. I will present on recent work extending some of this picture to the non-compact setting, with particular attention on (shrinking) Kähler-Ricci solitons. In particular, if a non-compact toric manifold M admits a weighted cscK metric (for suitable choices of weights), then it must be weighted K-stable. I will explain the relationship with Kähler-Ricci solitons, and time permitting I will explain how this leads to a simple proof of a well-known result of Futaki-Wang on the existence of shrinking Kähler-Ricci solitons on the total space of certain line bundles over a compact Kähler-Einstein Fano base.
.Date: 2 Apr., 2024
Speaker: Tomoyuki Hisamoto (Tokyo Metropolitan University)
Title: Quantization of the Kähler-Ricci flow, the entropy, and the optimal degeneration for a Fano manifold
Abstract: In recent years the study of K-unstable Fano manifolds atracts people's attention. In this talk I will introduce the geometric quantization of the Kähler-Ricci flow and the associated entropy functional introduced by Weiyong He. The "quantized entropy" coincides with the terminology in the quantum information theory. We also show some convergence results and discuss about the finite-dimensional analogue of the optimal degeneration.
.Date: 26 Mar., 2024
Speaker: Jian Song (Rutgers University)
Title: Minimal slopes for complex Hessian type equations
Abstract: The existence of smooth solutions to a broad class of complex Hessian type equations is related to nonlinear Nakai type criteria on intersection numbers on Kahler manifolds. Such a Nakai criteria can be interpreted as a slope stability condition analogous to the slope stability for Hermitian vector bundles over Kahler manifolds. In the present work, we initiate a program to find canonical solutions to such equations in the unstable case when the Nakai criteria fails. Conjecturally such solutions should arise as limits of natural parabolic flows and should be minimisers of the corresponding moment-map energy functionals. We implement our approach for the J-equation and the deformed Hermitian Yang-Mills equation on surfaces and some examples with symmetry. We further present the bubbling phenomena for the J-equation by constructing minimizing sequences of the moment-map energy functionals.
.Date: 19 Mar., 2024
Speaker: Yaxiong Liu (University of Maryland)
Title: The eigenvalue problem of complex Hessian operators
Abstract: In a very recent pair of nice papers of Badiane and Zeriahi, they consider the eigenvalue problem of complex Monge-Ampere and complex Hessian, and show that the C^{1,\bar{1}}-regularity of eigenfunction for MA and C^alpha-regularity for complex Hessian. They posed a question about the C^{1,1}-regularity. We give a positive answer and show the C^{1,1}-regularity and uniqueness of the eigenfunction. We also derive a number of applications, including a bifurcation-type theorem and geometric bounds for the eigenvalue. This is a joint work with Jianchun Chu and Nicholas McCleerey.
.Date: 12 Mar., 2024 (Additional talks)
Speaker: Thorsten Hertl (Albert-Ludwigs-Universität Freiburg) 19:30-20:00 (Beijing Time)
Title: Moduli Spaces of Positive Curvature Metrics
Abstract: Besides the space of positive scalar curvature metrics, various moduli spaces have gained a lot of attention. Among those, the observer moduli space arguably has the best behaviour from a homotopy-theoretical perspective because the subgroup of observer diffeomorphisms acts freely on the space of Riemannian metrics if the underlying manifold is connected.
In this talk, I will present how to construct non-trivial elements in the second homotopy of the observer moduli space of positive scalar curvature metrics for a large class for four-manifolds. I will further outline how to adapt this construction to produce the first non-trivial elements in higher homotopy groups of the observer moduli space of positive sectional curvature metrics on complex projective spaces.
Speaker: Zhicheng Han (Georg-August-Universität) 20:00-20:30 (Beijing Time)
Title: Spectra of Lie groups and application to L^2-invariants
Abstract: In this talk, I will explore the Laplace operator and Dirac operator on semisimple Lie groups. While the parallel problem on symmetric spaces has been well-studied in the last century, the corresponding problem is much less understood in general homogeneous spaces. We will examine the obstacles in extending existing techniques and discuss how some of them can be resolved in the case of group manifolds. Towards the end, we will see how the spectra data shall aid in computing certain topological L^2-invariants.
.Date: 12 Mar., 2024 21:00-22:00 (Beijing Time)
Speaker: Vyacheslav Lysov (London Institute for Mathematical Sciences)
Title: Chern-Gauss-Bonnet theorem via BV localization
Abstract: I will give a brief introduction to supersymmetric localization, BV formalism, and BV localization. I will show that the Euler class integral is a partition function for a zero-dimensional field theory with on-shell supersymmetry. I will describe the partition function as a BV integral, and deform the Lagrangian sub-manifold to evaluate the same integral as a sum over critical points for the Morse function.
Speaker: Ruadhaí Dervan (University of Glasgow)
Title: The universal structure of moment maps in complex geometry
Abstract: Much of complex geometry is motivated by linking the existence of solutions to geometric PDEs (producing "canonical metrics") to stability conditions in algebraic geometry. I will discuss a more basic question: what is the recipe to actually produce interesting geometric PDEs in complex geometry? The construction will be geometric, using a combination of universal families and tools from equivariant differential geometry. This is joint work with Michael Hallam.
Speaker: Xin Wang (KIAS)
Title: 5D Wilson Loops and Topological Strings on Fano-threefolds
Abstract: Geometric engineering provides a rich class of 5D supersymmetric gauge theories with eight supercharges, arising from M-theory compactification on non-compact Calabi-Yau threefolds. The counting of BPS states in the low-energy gauge theory is determined by the degeneracies of M2-branes wrapping holomorphic two-cycles in the Calabi-Yau threefold X. These degeneracies can also be calculated from the (refined) topological strings on the same manifold X. In this talk, I will explore the BPS spectrum of the 5D gauge theory with the insertion of a half-BPS Wilson loop operator. Utilizing M-theory realization, we derive the BPS expansion of the expectation value of the Wilson loop operator in terms of BPS sectors. It is found that the BPS sectors can be realized and computed from topological string theories. In the unrefined limit, the BPS sectors act as generating functions for Gromov-Witten invariants on compact (semi)-Fano threefolds constructed from X. We further conjecture a new refined holomorphic anomaly equations for the generating functions of the Wilson loops. These equations can completely solve the refined BPS invariants of Wilson loops for local \mathbb{P}^2 and local \mathbb{P}^1\times\mathbb{P}^1.
Speaker: Abdellah Lahdili (University of Aarhus) (Last talk of this semester)
Title: The Einstein-Hilbert functional and K-stability.
Abstract: In this talk, I will discuss a correspondence between the problem of finding a constant scalar curvature K\"ahler metric (cscK for short) on a polarized complex manifold and the CR-Yamabe problem on the associated circle bundle. I will then use a CR version of the Einstein-Hilbert functional to show K-semistability of polarised complex manifolds admitting cscK metrics. This is a joint work with Eveline Legendre and Carlo Scarpa.
Speaker: Denis Auroux (Harvard)
Title: Holomorphic discs and extended deformations in mirror symmetry
Abstract: Given a Lagrangian torus fibration on the complement of an anticanonical divisor in a Kahler manifold, one usually constructs a mirror space by gluing local charts (moduli spaces of local systems on generic torus fibers) via wall-crossing transformations determined by counts of Maslov index 0 holomorphic discs; this mirror also comes equipped with a regular function (the superpotential) which enumerates Maslov index 2 holomorphic discs. However, holomorphic discs of negative Maslov index deform this picture by introducing inconsistencies in the wall-crossing transformations; the geometric features of the resulting mirror can be understood in the language of extended deformations of Landau-Ginzburg models. We illustrate this phenomenon (and show that it actually occurs) on an explicit example (a 4-fold obtained by blowing up a Calabi-Yau toric variety), and discuss a family Floer approach to the geometry of the corrected mirror in this setting.
Speaker: Yuta Kusakabe (Kyoto University )
Title: Oka tubes in holomorphic line bundles
Abstract: A complex manifold is called an Oka manifold if continuous maps from Stein manifolds can be deformed into holomorphic maps with approximation and interpolation. Oka manifolds are characterized by a certain kind of ellipticity which is the opposite of Kobayashi-Eisenman-Brody hyperbolicity. Based on the fact that the zero section of a negative line bundle on a compact complex manifold admits a basis of pseudo-Kobayashi hyperbolic neighborhoods, we discuss the question of when the zero section of a line bundle admits a basis of Oka neighborhoods. This talk is based on joint work with Franc Forstnerič.
Speaker: Timothée Bénard (Warwick Mathematic Institute)
Title: Limit theorems on nilpotent Lie groups.
Abstract: I will present limit theorems for random walks on nilpotent Lie groups, obtained in a recent work with Emmanuel Breuillard. Most works on the topic assumed the step distribution of the walk to be centered in the abelianization of the group. Our main contribution is to authorize non-centered step distributions. In this case, new phenomena emerge: the large-scale geometry of the walk depends on the increment average, and the limit measure in the central limit theorem may not have full support in the ambient group.
Speaker: Martin de Borbon (Loughborough University)
Title: Bubbling of Kahler-Einstein metrics: examples and conjectures
Abstract: I will report on joint work with Cristiano Spotti. Chi Li's theory of normalized volumes permits the algebraic determination of tangent cones of singular Kahler-Einstein metrics. The general principle is to extend this to the dynamic setting of families and determine all possible rescaled limits using algebraic geometry. I will discuss examples in low dimensions, including polyhedral metrics on the sphere and Kahler-Einstein metrics on complex surfaces with cone singularities along curves.
Speaker: Marc Troyanov (Institut de Mathématiques EPFL)
Title: Double Forms, Curvature Integrals and the Gauss-Bonnet Formula
Abstract: In this talk I will briefly present the historical development behind Chern's proof in 1944 of the higher dimensional Gauss-Bonnet formula, starting with the work of H. Hopf back in 1925. I will then introduce the notion of double forms (which basically are sums of tensor products of differential forms) and explain how these tensors help us reformulate the Gauss-Bonnet-Chern-Formula and in particular clarify the meaning of Chern's boundary term. I will give some application and if time permits, some perspectives on non compact and singular Riemannian manifolds.
Speaker: Fernando Galaz-García (Durham University)
Title: Decompositions of 3-dimensional Alexandrov spaces
Abstract: Alexandrov spaces (with curvature bounded below) are metric generalizations of complete Riemannian manifolds with a uniform lower sectional curvature bound. Instances of Alexandrov spaces include compact Riemannian orbifolds and orbit spaces of isometric compact Lie group actions on compact Riemannian manifolds. In addition to being objects of intrinsic interest, Alexandrov spaces play an important role in Riemannian geometry, for example, in Perelman's proof of the Poincaré Conjecture. In this talk, I will discuss the topology and geometry of 3-dimensional Alexandrov spaces, focusing on extensions of basic results in 3-manifold topology (such as the prime decomposition theorem) to general three-dimensional Alexandrov spaces. This is joint work with Luis Atzin Franco Reyna, José Carlos Gómez-Larrañaga, Luis Guijarro, and Wolfgang Heil.
Speaker: Jeffrey D. Streets (University of California, Irvine) (22:00-23:00 Beijing Time !)
Title: Formal structure of scalar curvature in generalized Kahler geometry
Abstract: The Fujiki-Donaldson moment map formulation of scalar curvature problem, and the attendant Mabuchi-Semmes-Donaldson geometry of a Kahler class, play a central role in addressing the existence and uniqueness of constant scalar curvature Kahler metrics. Generalized Kahler (GK) geometry is a natural extension of Kahler geometry arising from Hitchin’s generalized geometry program and mathematical physics, and forms a particularly well-structured extension of Kahler geometry. Recently Goto defined a notion of scalar curvature in GK geometry as the moment map of a particular Hamiltonian action on the space of generalized Kahler structures. In this talk I will describe joint work with Vestislav Apostolov and Yury Ustinovskiy where we give an explicit description of the scalar curvature, and define a natural generalization of the Mabuchi-Semmes-Donaldson metric, leading to a Calabi-Lichnerowicz-Matsushima obstruction, generalizations of Futaki’s invariants, and a conditional uniqueness result.
Speaker: Gonçalo Oliveira (Instituto Superior Técnicon Lisbon)
Title: Einstein metrics from Derdziński duality
Abstract: (joint work with Rosa Sena-Dias) A theorem of Derdziński from the 1980's establishes that certain Einstein metrics are conformal to Bach-flat extremal Kahler metrics. Using this result Rosa Sena-Dias and I classified conformally Kähler, U(2)-invariant, Einstein metrics on the total space of O(−m). This yields infinitely many 1-parameter families of metrics exhibiting several different behaviors including asymptotically hyperbolic metrics (more specifically of Poincaré type), ALF metrics, and metrics which compactify to a Hirzebruch surface with a cone singularity along the ''divisor at infinity''. As an application of these results, we find many interesting phenomena. For instance, we exhibit the Taub-bolt Ricci-flat ALF metric as a limit of cone angle Einstein metrics on the blow up of CP2 at a point (in the limit when the cone angle converges to zero). We also construct Einstein metrics which are asymptotically hyperbolic and conformal to a scalar-flat Kähler metric and cannot be obtained by applying Derdziński's theorem.
Speaker: Gabriel Katz (MIT)
Title: Recovering contact forms from boundary data
Abstract: Let $X$ be a compact connected smooth manifold with boundary. The paper deals with contact $1$-forms $\beta$ on $X$, whose Reeb vector fields $v_\b$ admit Lyapunov functions $f$. We prove that any odd-dimensional $X$ admits such a contact form.
We tackle the question: how to recover $X$ and $\beta$ from the appropriate data along the boundary $\partial X$? We describe such boundary data and prove that they allow for a reconstruction of the pair $(X, \beta)$, up to a diffeomorphism of $X$. We use the term ``holography" for the reconstruction. We say that objects or structures inside $X$ are {\it holographic}, if they can be reconstructed from their $v_\b$-flow induced ``shadows" on the boundary $\partial X$.
For a given $\beta$, we study the contact vector fields $u$ on $X$ that preserve it invariant. Integrating $u$, we get a $1$-parameter family of contactomorphisms $\{\Phi^t(u)\}_{t \in \mathbb R}$ which maps Reeb trajectories to Reeb trajectories. This leads to estimates from below of the number of $\Phi^t(u)$-fixed Reeb trajectories.
We also introduce numerical invariants that measure how ``wrinkled" the boundary $\partial X$ is with respect to the $v_\beta$-flow and study their holographic properties under the contact forms preserving embeddings of equidimensional contact manifolds with boundary. We get some ``non-squeezing results" about such contact embedding, which are reminiscent of Gromov's non-squeezing theorem in symplectic geometry.
Speaker: Jacopo Stoppa (Scuola Internazionale Superiore di Studi Avanzati)
Title: K-stability and large complex structure limits
Abstract: According to mirror symmetry, the geometry of a given Fano manifold endowed with some extra data, including an arbitrary Kähler class, should be reflected in a mirror Landau-Ginzburg model, i.e. a noncompact complex manifold endowed with a nonconstant holomorphic function. On the other hand, a fundamental notion for constructing moduli of Fano manifolds is K-polystability, i.e. positivity of the Donaldson-Futaki invariants for nonproduct test-configurations. In this talk I will introduce the problem of characterising K-polystable Kähler classes on a Fano in terms of their mirror Landau-Ginzburg models. I will then discuss some first concrete results in the case of slope stability for del Pezzo surfaces. The computations involve the particular “large complex structure limit” of the Landau-Ginzburg model corresponding to scaling the Kähler class on the Fano, which acts trivially on K-polystability.
Speaker: Mario Garcia Fernandez (Universidad Autónoma de Madrid)
Title: Futaki invariants and harmonic metrics for the Hull-Strominger system
Abstract: The Hull-Strominger system has been proposed as a geometrization tool for understanding the moduli space of Calabi-Yau threefolds with topology change (‘Reid’s Fantasy’). In this talk we will introduce obstructions to the existence of solutions for these equations which combine infinite-dimensional moment maps, ‘harmonic metrics’, and a holomorphic version of generalized geometry. We will discuss the implications of these new obstructions in relation to a conjecture by S.-T. Yau for the Hull-Strominger system. Based on joint work with Raúl Gonzalez Molina, in arXiv:2303.05274 and arXiv:2301.08236.
Speaker: James Bonifacio (University of Mississippi)
Title: Spectral bounds on hyperbolic 3-manifolds
Abstract: I will discuss some new bounds on the spectra of Laplacian operators on hyperbolic 3-manifolds. One example of such a bound is that the spectral gap of the Laplace-Beltrami operator on a closed orientable hyperbolic 3-manifold must be less than 47.32, or less than 31.57 if the first Betti number is positive. The bounds are derived using two approaches, both of which employ linear programming techniques: 1) the Selberg trace formula, and 2) identities derived from the associativity of spectral decompositions. The second approach is inspired by an area of physics called the conformal bootstrap. This talk is based on work done in collaboration with Dalimil Mazáč and Sridip Pal.
Speaker: Javier Martínez-Aguinaga (Universidad Complutense de Madrid)
Title: $h$-Principle for maximal growth distributions
Abstract: The existence and classification problem for maximal growth distributions on smooth manifolds has garnered much interest in the mathematical community in recent years. Prototypical examples of maximal growth distributions are contact structures on $3$-dimensional manifolds and Engel distributions on $4$-dimensional manifolds.
The existence and classification of maximal growth distributions on open manifolds follows from Gromov’s $h$-Principle for open manifolds. Nonetheless, not so much was known for the case of closed manifolds and several open questions have been posed in the literature throughout the years about this problem.
In our recent work [1] we show that maximal growth distributions of rank $> 2$ abide by a full h-principle in all dimensions, providing thus a classification result from a homotopic viewpoint. As a consequence we answer in the positive, for $k > 2$, the long-standing open question posed by M. Kazarian and B. Shapiro more than 25 years ago of whether any parallelizable manifold admits a $k$-rank distribution of maximal growth.
[1]. Martínez-Aguinaga, Javier. Existence and classification of maximal growth distributions . Preprint. arXiv:2308.10762
Speaker: Timothée Bénard (Warwick Mathematic Institute)
Title: Limit theorems on nilpotent Lie groups.
Abstract: I will present limit theorems for random walks on nilpotent Lie groups, obtained in a recent work with Emmanuel Breuillard. Most works on the topic assumed the step distribution of the walk to be centered in the abelianization of the group. Our main contribution is to authorize non-centered step distributions. In this case, new phenomena emerge: the large-scale geometry of the walk depends on the increment average, and the limit measure in the central limit theorem may not have full support in the ambient group.
Speaker: Kajal Das (Indian Statistical Institute)
Title: On super-rigidity of Gromov's random monster group （Slides)
Abstract: In my talk, I will speak on the super-rigidity of Gromov's random monster group. It is a finitely generated random group $\Gamma_\alpha$ ( $\alpha$ is in a probability space $\mathcal{A}$) constructed using an expander graph by M. Gromov in 2000. It provides a counterexample to the Baum-Connes conjecture for groups with coefficients in commutative $C^*$-algebra. It is already known that it has global fixed point property for isometric affine action on $L^p$ spaces for $1< p<\infty$ (in particular, Property (T) ) for a.e. $\alpha$ due to Gromov and Naor-Silberman. It is also hyperbolically rigid, i.e., any isometric action of the group on a Gromov-hyperbolic space is elementary for a.e. $\alpha$ (due to Gruber-Sisto-Tessera). In this talk, I will discuss the following type of super-rigidity problem (motivated by Margulis super-rigidity theorem): for which countable group $G$, any collection of homomorphisms $\phi_\alpha:\Gamma_\alpha\rightarrow G$ have a finite image for a.e. $\alpha$? This question was first addressed for linear groups by Naor-Silberman. The super-rigidity follows immediately from the literature for groups with a-$L^p$-menablity and K-amenable groups. In this talk, we will show that $\Gamma_\alpha$ has super-rigidity with respect to the following groups $G$: mapping class group $MCG(S_{g,p})$, braid group $B_n$, automorphism group of a free group $Aut(F_n)$, outer automorphism group of a free group $Out(F_N)$ . We will also show a stability result of the class of groups $G$.
