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Geometry Colloquium
Seminar information archive ～04/18｜Next seminar｜Future seminars 04/19～
| Date, time & place | Friday 10:00 - 11:30 126Room #126 (Graduate School of Math. Sci. Bldg.) |
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Seminar information archive
2014/11/07
10:00-11:30 Room #126 (Graduate School of Math. Sci. Bldg.)
Shinpei KOBAYASHI (Hokkaido University)
Harmonic maps into the hyperbolic plane and their applications to surface theory (Japanese)
Shinpei KOBAYASHI (Hokkaido University)
Harmonic maps into the hyperbolic plane and their applications to surface theory (Japanese)
[ Abstract ]
Harmonic maps from two-dimensional Riemannian manifolds into the hyperbolic plane have been well studied. Since constant mean curvature surfaces in the Minkowski space have harmonic Gauss maps into the hyperbolic plane, there exist applications to surface theory.
In 1998, Dorfmeister, Pedit and Wu established the construction method of harmonic maps into symmetric spaces via loop group method. Recently, harmonic maps into the hyperbolic plane appear in various classes of surfaces, e.g., minimal surfaces in the Heisenberg group,
maximal surfaces in the anti-de Sitter space or constant Gaussian curvature surfaces in the hyperbolic space. In this talk I will talk about the general construction method of harmonic maps from surfaces into symmetric spaces via loop group method and the case of the hyperbolic plane in details.
Harmonic maps from two-dimensional Riemannian manifolds into the hyperbolic plane have been well studied. Since constant mean curvature surfaces in the Minkowski space have harmonic Gauss maps into the hyperbolic plane, there exist applications to surface theory.
In 1998, Dorfmeister, Pedit and Wu established the construction method of harmonic maps into symmetric spaces via loop group method. Recently, harmonic maps into the hyperbolic plane appear in various classes of surfaces, e.g., minimal surfaces in the Heisenberg group,
maximal surfaces in the anti-de Sitter space or constant Gaussian curvature surfaces in the hyperbolic space. In this talk I will talk about the general construction method of harmonic maps from surfaces into symmetric spaces via loop group method and the case of the hyperbolic plane in details.
2014/10/17
10:00-11:30 Room #126 (Graduate School of Math. Sci. Bldg.)
Yu Kitabeppu (Kyoto University)
A finite diameter theorem on RCD spaces (JAPANESE)
Yu Kitabeppu (Kyoto University)
A finite diameter theorem on RCD spaces (JAPANESE)
[ Abstract ]
I will talk about a finite diameter theorem on RCD spaces of (possibly) infinite dimension. An RCD space is a generalization of a concept of a manifold with bounded Ricci curvature. Savar¥'e proves the "self-improving property" on RCD spaces via the Gamma calculus. Because of his work and Kuwada’s duality argument, we are able to get the L^{¥infty}-contraction of heat kernels. I will show the main result by combining the contraction property and a simple lemma.
I will talk about a finite diameter theorem on RCD spaces of (possibly) infinite dimension. An RCD space is a generalization of a concept of a manifold with bounded Ricci curvature. Savar¥'e proves the "self-improving property" on RCD spaces via the Gamma calculus. Because of his work and Kuwada’s duality argument, we are able to get the L^{¥infty}-contraction of heat kernels. I will show the main result by combining the contraction property and a simple lemma.
2014/07/17
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Jingyi Chen (University of British Columbia)
The space of compact shrinking solutions to Lagrangian mean curvature flow in $C^2$ (ENGLISH)
Jingyi Chen (University of British Columbia)
The space of compact shrinking solutions to Lagrangian mean curvature flow in $C^2$ (ENGLISH)
[ Abstract ]
We will discuss compactness and rigidity of compact surfaces which are shrinking solutions to Lagrangian mean curvature flow. This is recent joint work with John Ma.
We will discuss compactness and rigidity of compact surfaces which are shrinking solutions to Lagrangian mean curvature flow. This is recent joint work with John Ma.
2014/06/26
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Kazumasa Kuwada (Tokyo Institute of Technology)
Entropic curvature-dimension condition and Bochner’s inequality (JAPANESE)
Kazumasa Kuwada (Tokyo Institute of Technology)
Entropic curvature-dimension condition and Bochner’s inequality (JAPANESE)
[ Abstract ]
As a characterization of "lower Ricci curvature bound and upper dimension bound”, there appear several conditions which make sense even on singular spaces. In this talk we show the equivalence in complete generality between two major conditions: a reduced version of curvature-dimension bounds of Sturm-Lott-Villani via entropy and optimal transport and Bakry–¥'Emery's one via Markov generator or the associated heat semigroup. More precisely, it holds for metric measure spaces where Cheeger's L^2-energy functional is a quadratic form. In particular, we establish the full Bochner inequality, which originally comes from the Bochner-Weitzenb¥"ock formula, on such spaces. This talk is based on a joint work with M. Erbar and K.-T. Sturm (Bonn).
As a characterization of "lower Ricci curvature bound and upper dimension bound”, there appear several conditions which make sense even on singular spaces. In this talk we show the equivalence in complete generality between two major conditions: a reduced version of curvature-dimension bounds of Sturm-Lott-Villani via entropy and optimal transport and Bakry–¥'Emery's one via Markov generator or the associated heat semigroup. More precisely, it holds for metric measure spaces where Cheeger's L^2-energy functional is a quadratic form. In particular, we establish the full Bochner inequality, which originally comes from the Bochner-Weitzenb¥"ock formula, on such spaces. This talk is based on a joint work with M. Erbar and K.-T. Sturm (Bonn).
2014/06/19
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Takashi Sakai (Tokyo Metropolitan University)
Antipodal structure of the intersection of real forms and its applications (JAPANESE)
Takashi Sakai (Tokyo Metropolitan University)
Antipodal structure of the intersection of real forms and its applications (JAPANESE)
[ Abstract ]
A subset A of a Riemannian symmetric space is called an antipodal set if the geodesic symmetry s_x fixes all points of A for each x in A. This notion was first introduced by Chen and Nagano. Tanaka and Tasaki proved that the intersection of two real forms L_1 and L_2 in a Hermitian symmetric space of compact type is an antipodal set of L_1 and L_2. As an application, we calculate the Lagrangian Floer homology of a pair of real forms in a monotone Hermitian symmetric space. Then we obtain a generalization of the Arnold-Givental inequality. We expect to generalize the above results to the case of complex flag manifolds. In fact, using the k-symmetric structure, we can describe an antipodal set of a complex flag manifold. Moreover we can observe the antipodal structure of the intersection of certain real forms in a complex flag manifold.
This talk is based on a joint work with Hiroshi Iriyeh and Hiroyuki Tasaki.
A subset A of a Riemannian symmetric space is called an antipodal set if the geodesic symmetry s_x fixes all points of A for each x in A. This notion was first introduced by Chen and Nagano. Tanaka and Tasaki proved that the intersection of two real forms L_1 and L_2 in a Hermitian symmetric space of compact type is an antipodal set of L_1 and L_2. As an application, we calculate the Lagrangian Floer homology of a pair of real forms in a monotone Hermitian symmetric space. Then we obtain a generalization of the Arnold-Givental inequality. We expect to generalize the above results to the case of complex flag manifolds. In fact, using the k-symmetric structure, we can describe an antipodal set of a complex flag manifold. Moreover we can observe the antipodal structure of the intersection of certain real forms in a complex flag manifold.
This talk is based on a joint work with Hiroshi Iriyeh and Hiroyuki Tasaki.
2014/05/22
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Boris Hasselblatt (Tufts Univ)
Godbillon-Vey invariants for maximal isotropic foliations (ENGLISH)
Boris Hasselblatt (Tufts Univ)
Godbillon-Vey invariants for maximal isotropic foliations (ENGLISH)
[ Abstract ]
The combination of a contact structure and an orientable maximal isotropic foliation gives rise to m+1 Godbillon-Vey invariants for an m+1-dimensional maximal isotropic foliation that are of interest with respect to geometric rigidity: by studying these jointly, we give new proofs of famous "rigidity'' results from the 1980s that require only a very few simple lines of reasoning rather than the elaborate original proofs.
The combination of a contact structure and an orientable maximal isotropic foliation gives rise to m+1 Godbillon-Vey invariants for an m+1-dimensional maximal isotropic foliation that are of interest with respect to geometric rigidity: by studying these jointly, we give new proofs of famous "rigidity'' results from the 1980s that require only a very few simple lines of reasoning rather than the elaborate original proofs.
2014/05/15
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Homare TADANO (Osaka University)
Gap theorems for compact gradient Sasaki Ricci solitons (JAPANESE)
Homare TADANO (Osaka University)
Gap theorems for compact gradient Sasaki Ricci solitons (JAPANESE)
[ Abstract ]
In this talk we give some necessary and sufficient conditions for compact gradient Sasaki-Ricci solitons to be Sasaki-Einstein. Our result may be considered as a Sasaki geometry version of recent works by H. Li, and M. Fern¥'andez-L¥'opez-E. Garc¥'ia-Rio.
In this talk we give some necessary and sufficient conditions for compact gradient Sasaki-Ricci solitons to be Sasaki-Einstein. Our result may be considered as a Sasaki geometry version of recent works by H. Li, and M. Fern¥'andez-L¥'opez-E. Garc¥'ia-Rio.
2014/05/08
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Hajime Ono (Saitama University)
On non Hamiltonian volume minimizing H-stable Lagrangian tori (JAPANESE)
Hajime Ono (Saitama University)
On non Hamiltonian volume minimizing H-stable Lagrangian tori (JAPANESE)
[ Abstract ]
Y. –G. Oh investigated the volume of Lagrangian submanifolds in a Kaehler manifold and introduced the notion of Hamiltonian minimality, Hamiltonian stability and Hamiltonian volume minimizing property. For example, it is known that standard tori in complex Euclidean spaces and torus orbits in complex projective spaces are H-minimal and H-stable. In this talk I show that
1. Almost all of standard tori in the complex Euclidean space of dimension greater than two are not Hamiltonian volume minimizing.
2. There are non Hamiltonian volume minimizing torus orbits in any compact toric Kaehler manifold of dimension greater than two.
Y. –G. Oh investigated the volume of Lagrangian submanifolds in a Kaehler manifold and introduced the notion of Hamiltonian minimality, Hamiltonian stability and Hamiltonian volume minimizing property. For example, it is known that standard tori in complex Euclidean spaces and torus orbits in complex projective spaces are H-minimal and H-stable. In this talk I show that
1. Almost all of standard tori in the complex Euclidean space of dimension greater than two are not Hamiltonian volume minimizing.
2. There are non Hamiltonian volume minimizing torus orbits in any compact toric Kaehler manifold of dimension greater than two.
2014/04/24
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Hiraku Nozawa (Ritsumeikan University)
On rigidity of Lie foliations (JAPANESE)
Hiraku Nozawa (Ritsumeikan University)
On rigidity of Lie foliations (JAPANESE)
[ Abstract ]
If the leaves of a Lie foliation are isometric to a symmetric space of noncompact type of higher rank, then, by a theorem of Zimmer, the holonomy group of the Lie foliation has rigidity similar to that of lattices of semisimple Lie groups of higher rank. The main result of this talk is a generalization of Zimmer's theorem including the case of real rank one based on an application of a variant of Mostow rigidity. (This talk is based on a joint work with Ga¥"el Meigniez.)
If the leaves of a Lie foliation are isometric to a symmetric space of noncompact type of higher rank, then, by a theorem of Zimmer, the holonomy group of the Lie foliation has rigidity similar to that of lattices of semisimple Lie groups of higher rank. The main result of this talk is a generalization of Zimmer's theorem including the case of real rank one based on an application of a variant of Mostow rigidity. (This talk is based on a joint work with Ga¥"el Meigniez.)
2014/04/10
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Kotaro Kawai ( University of Tokyo)
Deformations of homogeneous Cayley cone submanifolds (JAPANESE)
Kotaro Kawai ( University of Tokyo)
Deformations of homogeneous Cayley cone submanifolds (JAPANESE)
[ Abstract ]
A Cayley submanifold is a minimal submanifold in a Spin(7)-manifold, and is a special class of calibrated submanifolds introduced by Harvey and Lawson. The deformation of calibrated submanifolds is first studied by Mclean. He studied the compact case, and many people try to generalize it to noncompact cases (conical case, asymptotically conical case etc.). In general, the moduli space of deformations of a Cayley cone is known not to be smooth. In this talk, we focus on the homogeneous Cayley cones in R^8, and study their deformation spaces explicitly.
A Cayley submanifold is a minimal submanifold in a Spin(7)-manifold, and is a special class of calibrated submanifolds introduced by Harvey and Lawson. The deformation of calibrated submanifolds is first studied by Mclean. He studied the compact case, and many people try to generalize it to noncompact cases (conical case, asymptotically conical case etc.). In general, the moduli space of deformations of a Cayley cone is known not to be smooth. In this talk, we focus on the homogeneous Cayley cones in R^8, and study their deformation spaces explicitly.
2013/12/06
17:00-18:00 Room #123 (Graduate School of Math. Sci. Bldg.)
Sadayoshi KOJIMA (Tokyo Institute of Technology)
Normalized entropy versus volume for pseudo-Anosovs (JAPANESE)
Sadayoshi KOJIMA (Tokyo Institute of Technology)
Normalized entropy versus volume for pseudo-Anosovs (JAPANESE)
[ Abstract ]
We establish an explicit linear inequality between the normalized entropy of pseudo-Anosov automorphisms and the hyperbolic volume of their mapping tori, based on a recent result by Jean-Marc Schlenker on renormalized volume of quasi-Fuchsian
manifolds. This is a joint work with Greg McShane.
We establish an explicit linear inequality between the normalized entropy of pseudo-Anosov automorphisms and the hyperbolic volume of their mapping tori, based on a recent result by Jean-Marc Schlenker on renormalized volume of quasi-Fuchsian
manifolds. This is a joint work with Greg McShane.
2013/12/05
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Sumio Yamada (Gakushuin University)
Variational characterizations of exact solutions of the Einstein equation (JAPANESE)
Sumio Yamada (Gakushuin University)
Variational characterizations of exact solutions of the Einstein equation (JAPANESE)
[ Abstract ]
There are a set of well-known exact solutions to the Einstein equation. The most important one is the Schwarzschild metric, and it models a Ricci-flat space-time, which is asymptotically flat. In addition, there are the Reissner-Nordstrom metric and the Majumdar-Papapetrou metric, which satisfy the Einstein-Maxwell equation, instead of the vacuum Einstein equation. In a jointwork with Marcus Khuri and Gilbert Weinstein, it is shown that those metrics are characterized as the equality
cases of a set of so-called Penrose-type inequalities. The method of proof is a
conformal deformation of Riemannian metrics defined on the space-like slice of the space-time.
There are a set of well-known exact solutions to the Einstein equation. The most important one is the Schwarzschild metric, and it models a Ricci-flat space-time, which is asymptotically flat. In addition, there are the Reissner-Nordstrom metric and the Majumdar-Papapetrou metric, which satisfy the Einstein-Maxwell equation, instead of the vacuum Einstein equation. In a jointwork with Marcus Khuri and Gilbert Weinstein, it is shown that those metrics are characterized as the equality
cases of a set of so-called Penrose-type inequalities. The method of proof is a
conformal deformation of Riemannian metrics defined on the space-like slice of the space-time.
2013/11/28
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Shinichiroh MATSUO (Osaka University)
The prescribed scalar curvature problem for metrics with unit total volume (JAPANESE)
Shinichiroh MATSUO (Osaka University)
The prescribed scalar curvature problem for metrics with unit total volume (JAPANESE)
[ Abstract ]
In this talk I will talk about the modified Kazdan-Warner problem.
Kazdan and Warner in 1970's completely solved the prescribed scalar curvature problem. In particular, they proved that every function on a manifold with positive Yamabe invariant is the scalar curvature of some metric. Kobayashi in 1987 proposed the modified problem of finding metrics with prescribed scalar curvature and total volume 1. He proved that every function except positive constants on a manifold with positive Yamabe invariant is the scalar curvature of some metricwith total volume 1.
I have recently settled the remaining case. Applying Taubes tequniques to the scalar curvature equations, we can glue two Yamabe metrics to construct metrics with very large scalar curvature and unit total volume, and prove that every positive constant is the scalar curvature of some metric with total volume 1.
In this talk I will talk about the modified Kazdan-Warner problem.
Kazdan and Warner in 1970's completely solved the prescribed scalar curvature problem. In particular, they proved that every function on a manifold with positive Yamabe invariant is the scalar curvature of some metric. Kobayashi in 1987 proposed the modified problem of finding metrics with prescribed scalar curvature and total volume 1. He proved that every function except positive constants on a manifold with positive Yamabe invariant is the scalar curvature of some metricwith total volume 1.
I have recently settled the remaining case. Applying Taubes tequniques to the scalar curvature equations, we can glue two Yamabe metrics to construct metrics with very large scalar curvature and unit total volume, and prove that every positive constant is the scalar curvature of some metric with total volume 1.
2013/11/14
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Hiroshige Kajiura (Chiba University)
Homological mirror symmetry of torus fibrations and some deformations (JAPANESE)
Hiroshige Kajiura (Chiba University)
Homological mirror symmetry of torus fibrations and some deformations (JAPANESE)
[ Abstract ]
We consider pairs of symplectic torus fibrations equipped with foliation structures and noncommutative deformations of complex torus fibrations as some deformations of the formulation of mirror symmetry via torus fibrations by Strominger-Yau-Zaslow. In order to assert that these pairs are mirror dual pairs, we consider homological mirror symmetry. Namely, we define deformations of Fukaya categories on symplectic torus fibrations and deformations of derived categories on complex torus fibrations, and discuss some equivalences between them. (What are known to hold true for non-deformed setting hold true, too, for the deformed setting. )
We consider pairs of symplectic torus fibrations equipped with foliation structures and noncommutative deformations of complex torus fibrations as some deformations of the formulation of mirror symmetry via torus fibrations by Strominger-Yau-Zaslow. In order to assert that these pairs are mirror dual pairs, we consider homological mirror symmetry. Namely, we define deformations of Fukaya categories on symplectic torus fibrations and deformations of derived categories on complex torus fibrations, and discuss some equivalences between them. (What are known to hold true for non-deformed setting hold true, too, for the deformed setting. )
2013/10/24
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Yohsuke Imagi (Kyoto University)
Some Uniqueness Theorems for Smoothing Singularities in Special Lagrangian Geometry (JAPANESE)
Yohsuke Imagi (Kyoto University)
Some Uniqueness Theorems for Smoothing Singularities in Special Lagrangian Geometry (JAPANESE)
[ Abstract ]
Special Lagrangian submanifolds are area-minimizing Lagrangian submanifolds of Calabi--Yau manifolds. I'll talk mainly about the singularities of two special Lagrangian planes intersecting transversely. I'll determine a class of smoothing models for the singularities.
By some results of Abouzaid and Smith one can determine the smoothing models up to quasi-isomorphism in a Fukaya category. I'll combine it with a technique of Thomas and Yau.
Special Lagrangian submanifolds are area-minimizing Lagrangian submanifolds of Calabi--Yau manifolds. I'll talk mainly about the singularities of two special Lagrangian planes intersecting transversely. I'll determine a class of smoothing models for the singularities.
By some results of Abouzaid and Smith one can determine the smoothing models up to quasi-isomorphism in a Fukaya category. I'll combine it with a technique of Thomas and Yau.
2013/10/03
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Masayuki ASAOKA (Kyoto University)
A rigidity lemma for cocycles over BS(1,k)-actions (JAPANESE)
Masayuki ASAOKA (Kyoto University)
A rigidity lemma for cocycles over BS(1,k)-actions (JAPANESE)
[ Abstract ]
Existence of an invariant geometric structure is persistent for many known examples of group actions on homogeneous spaces. In this talk, I would like to report an attempt to explain such a rigidity from a unified point of view. We will see that some rigidity results are reduced to a rigidity lemma on Diff(R^n,0)-valued cocycles over BS(1,k)-actions, where BS(1,k) is the Baumslag-Solitar group.
Existence of an invariant geometric structure is persistent for many known examples of group actions on homogeneous spaces. In this talk, I would like to report an attempt to explain such a rigidity from a unified point of view. We will see that some rigidity results are reduced to a rigidity lemma on Diff(R^n,0)-valued cocycles over BS(1,k)-actions, where BS(1,k) is the Baumslag-Solitar group
2013/07/11
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Changzheng Li (IPMU)
Primitive forms via polyvector fields (ENGLISH)
Changzheng Li (IPMU)
Primitive forms via polyvector fields (ENGLISH)
[ Abstract ]
The theory of primitive forms was introduced by Kyoji Saito in early 1980s, which was first known in singularity theory and has attracted much attention in mirror symmetry recently. In this talk, we will introduce a differential geometric approach to primitive forms, using compactly supported polyvector fields. We will first introduce the notion of primitive forms, making it acceptable to general audience. We will use the example of the mirror Laudau-Ginzberg model of P^1 to illustrate such approach. This is my joint work with Si Li and Kyoji Saito.
The theory of primitive forms was introduced by Kyoji Saito in early 1980s, which was first known in singularity theory and has attracted much attention in mirror symmetry recently. In this talk, we will introduce a differential geometric approach to primitive forms, using compactly supported polyvector fields. We will first introduce the notion of primitive forms, making it acceptable to general audience. We will use the example of the mirror Laudau-Ginzberg model of P^1 to illustrate such approach. This is my joint work with Si Li and Kyoji Saito.
2013/06/27
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
MASAI, Hidetoshi (Tokyo Institute of Technology)
On volume formulae in terms of orthospectrum (JAPANESE)
MASAI, Hidetoshi (Tokyo Institute of Technology)
On volume formulae in terms of orthospectrum (JAPANESE)
[ Abstract ]
Bridgeman-Kahn and Calegari derived formulae to compute the volumes of compact hyperbolic n-manifolds with totally geodesic boundary in terms of orthospectrum. Here the orthospectrum is the set of length of geodesics perpendicular to the boundary at both ends. The two formulae are obtained by apparently different methods. In this talk, we prove that the two volume formulae coincide. We also discuss some interesting relationship between two formulae. This work is a joint work with Greg McShane.
Bridgeman-Kahn and Calegari derived formulae to compute the volumes of compact hyperbolic n-manifolds with totally geodesic boundary in terms of orthospectrum. Here the orthospectrum is the set of length of geodesics perpendicular to the boundary at both ends. The two formulae are obtained by apparently different methods. In this talk, we prove that the two volume formulae coincide. We also discuss some interesting relationship between two formulae. This work is a joint work with Greg McShane.
2013/06/06
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Hajime Fujita (Japan Women's University)
Equivariant local index and transverse index for circle action (JAPANESE)
Hajime Fujita (Japan Women's University)
Equivariant local index and transverse index for circle action (JAPANESE)
[ Abstract ]
In our joint work with Furuta and Yoshida we gave a formulation of index theory of Dirac-type operator on open Riemannian manifolds. We used a torus fibration and a perturbation by Dirac-type operator along fibers. In this talk we develop an equivariant version for circle action and apply it for Hamiltonian circle action case. We investigate the relation between our equivariant index and index of transverse elliptic operator/symbol developed by Atiyah, Paradan-Vergne and Braverman. We give a computation for the standard cylinder, which shows the difference between two equivariant indices.
In our joint work with Furuta and Yoshida we gave a formulation of index theory of Dirac-type operator on open Riemannian manifolds. We used a torus fibration and a perturbation by Dirac-type operator along fibers. In this talk we develop an equivariant version for circle action and apply it for Hamiltonian circle action case. We investigate the relation between our equivariant index and index of transverse elliptic operator/symbol developed by Atiyah, Paradan-Vergne and Braverman. We give a computation for the standard cylinder, which shows the difference between two equivariant indices.
2013/05/16
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Kota Hattori (University of Tokyo)
A generalization of Taub-NUT deformations (JAPANESE)
Kota Hattori (University of Tokyo)
A generalization of Taub-NUT deformations (JAPANESE)
[ Abstract ]
Taub-NUT metric on C^2 is a complete Ricci-flat Kaehler metric which is not flat. It is obtained by the Taub-NUT deformations of the Euclidean metric on C^2 using an S^1 action. Taub-NUT deformations are known to be defined for toric hyperKaehler manifolds, and deform ALE metrics to non-ALE metrics. In this talk, I explain a generalization of Taub-NUT deformations by using noncommutative Lie groups.
Taub-NUT metric on C^2 is a complete Ricci-flat Kaehler metric which is not flat. It is obtained by the Taub-NUT deformations of the Euclidean metric on C^2 using an S^1 action. Taub-NUT deformations are known to be defined for toric hyperKaehler manifolds, and deform ALE metrics to non-ALE metrics. In this talk, I explain a generalization of Taub-NUT deformations by using noncommutative Lie groups.
2013/05/09
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
KIDA Yoshikata (Kyoto University)
Rigidity for amalgamated free products and their envelopes (JAPANESE)
KIDA Yoshikata (Kyoto University)
Rigidity for amalgamated free products and their envelopes (JAPANESE)
[ Abstract ]
For a discrete countable group L, we mean by an envelope of L a locally compact second countable group having a lattice isomorphic to L. In general, it is quite hard to describe all envelopes of a given L. This problem is closely related to orbit equivalence between probability-measure-preserving actions of groups, and also related to Mostow type rigidity. I explain a fundamental idea to attack this problem, and give examples of groups for which the problem is solved. The examples contain mapping class groups of surfaces and certain amalgamated free products. An outline to get an answer for the latter groups will be discussed.
For a discrete countable group L, we mean by an envelope of L a locally compact second countable group having a lattice isomorphic to L. In general, it is quite hard to describe all envelopes of a given L. This problem is closely related to orbit equivalence between probability-measure-preserving actions of groups, and also related to Mostow type rigidity. I explain a fundamental idea to attack this problem, and give examples of groups for which the problem is solved. The examples contain mapping class groups of surfaces and certain amalgamated free products. An outline to get an answer for the latter groups will be discussed.
2013/04/18
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Yasuyuki Nagatomo (Meiji University)
Harmonic maps into Grassmannian manifolds (JAPANESE)
Yasuyuki Nagatomo (Meiji University)
Harmonic maps into Grassmannian manifolds (JAPANESE)
[ Abstract ]
A harmonic map from a Riemannian manifold into a Grassmannian manifold is characterized by a vector bundle, a space of sections of the bundle and the Laplace operator. This characterization can be considered as a generalization of Theorem of Takahashi on minimal immersions into a sphere (J.Math.Soc.Japan 18 (1966)) and implies the well-known fact that the Kodaira embedding is a harmonic map.
We apply the main result to generalize a Theorem of do Carmo and Wallach (Ann.of Math. 93 (1971)) and describe a moduli space of harmonic maps with constant energy densities and some properties about pull-back bundles and connections from a Riemannian homogeneous space into a Grassmannian. We give some applications including a rigidity of minimal immersions from the complex projective line to complex projective spaces (S.Bando and Y.Ohnita, J. Math. Soc. Japan 39 (1987)).
The ADHM-construction of instantons gives a family of maps into Grassmannians via monad theory on the twistor space. These maps are, in general, not harmonic maps, but are similar to maps obtained in our generalized do Carmo-Wallach theorem. We compare these two constructions of moduli spaces.
A harmonic map from a Riemannian manifold into a Grassmannian manifold is characterized by a vector bundle, a space of sections of the bundle and the Laplace operator. This characterization can be considered as a generalization of Theorem of Takahashi on minimal immersions into a sphere (J.Math.Soc.Japan 18 (1966)) and implies the well-known fact that the Kodaira embedding is a harmonic map.
We apply the main result to generalize a Theorem of do Carmo and Wallach (Ann.of Math. 93 (1971)) and describe a moduli space of harmonic maps with constant energy densities and some properties about pull-back bundles and connections from a Riemannian homogeneous space into a Grassmannian. We give some applications including a rigidity of minimal immersions from the complex projective line to complex projective spaces (S.Bando and Y.Ohnita, J. Math. Soc. Japan 39 (1987)).
The ADHM-construction of instantons gives a family of maps into Grassmannians via monad theory on the twistor space. These maps are, in general, not harmonic maps, but are similar to maps obtained in our generalized do Carmo-Wallach theorem. We compare these two constructions of moduli spaces.
2013/04/11
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Jeff Viaclovsky (University of Wisconsin)
Critical metrics on connected sums of Einstein four-manifolds (ENGLISH)
Jeff Viaclovsky (University of Wisconsin)
Critical metrics on connected sums of Einstein four-manifolds (ENGLISH)
[ Abstract ]
I will discuss a gluing procedure designed to obtain critical metrics of quadratic Riemannian functionals on connected sums of certain Einstein four-manifolds. Start with two Einstein four-manifolds of positive scalar curvature which are "rigid". Using the Green's function for the conformal Laplacian, convert one of these into an asymptotically flat (AF) scalar-flat metric. A "naive" approximate critical metric is obtained by identifying the boundary of a large ball in the AF metric with the boundary of a small ball in the other compact Einstein metric, using cutoff functions to glue together the AF metric with a suitably scaled compact metric in order to obtain a smooth metric on the connected sum. It turns out that this naive approximate metric is too rough, and must be refined in order to compute the leading term of the Kuranishi map. The main application is an existence result using two well-known Einstein manifolds as building blocks: the Fubini-Study metric on $¥mathbb{CP}^2$ and the product metric on $S^2 ¥times S^2$. Using these factors in various gluing configurations, a zero of the Kuranishi map is then found for a specific quadratic Riemannian functional on certain connected sums. The exact functional depends on the geometry of the factors, and also on the mass of the AF metric. Using certain quotients of $S^2 ¥times S^2$ as one of the gluing factors, several non-simply connected examples are also obtained. This is joint work with Matt Gursky.
I will discuss a gluing procedure designed to obtain critical metrics of quadratic Riemannian functionals on connected sums of certain Einstein four-manifolds. Start with two Einstein four-manifolds of positive scalar curvature which are "rigid". Using the Green's function for the conformal Laplacian, convert one of these into an asymptotically flat (AF) scalar-flat metric. A "naive" approximate critical metric is obtained by identifying the boundary of a large ball in the AF metric with the boundary of a small ball in the other compact Einstein metric, using cutoff functions to glue together the AF metric with a suitably scaled compact metric in order to obtain a smooth metric on the connected sum. It turns out that this naive approximate metric is too rough, and must be refined in order to compute the leading term of the Kuranishi map. The main application is an existence result using two well-known Einstein manifolds as building blocks: the Fubini-Study metric on $¥mathbb{CP}^2$ and the product metric on $S^2 ¥times S^2$. Using these factors in various gluing configurations, a zero of the Kuranishi map is then found for a specific quadratic Riemannian functional on certain connected sums. The exact functional depends on the geometry of the factors, and also on the mass of the AF metric. Using certain quotients of $S^2 ¥times S^2$ as one of the gluing factors, several non-simply connected examples are also obtained. This is joint work with Matt Gursky.
2013/01/30
10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)
Ryoichi Kobayashi (Nagoya University)
Hamiltonian Volume Minimizing Property of Maximal Torus Orbits in the Complex Projective Space (JAPANESE)
Ryoichi Kobayashi (Nagoya University)
Hamiltonian Volume Minimizing Property of Maximal Torus Orbits in the Complex Projective Space (JAPANESE)
[ Abstract ]
We prove that any $U(1)^n$-orbit in $\\Bbb P^n$ is volume minimizing under Hamiltonian deformation.
The idea of the proof is :
- (1) We extend one $U(1)^n$-orbit to the momentum torus fibration $\\{T_t\\}_{t\\in\\Delta^n}$ and consider its Hamiltonian deformation $\\{\\phi(T_t)\\}_{t\\in\\Delta^n}$ where $\\phi$ is a Hamiltobian diffeomorphism of $\\Bbb P^n$,
and then :
- (2) We compare each $U(1)^n$-orbit and its Hamiltonian deformation by compaing the large $k$ asymptotic behavior of the sequence of projective embeddings defined, for each $k$, by the basis of $H^0(\\Bbb P^n,\\Cal O(k))$ obtained by semi-classical approximation of the $\\Cal O(k)$ Bohr-Sommerfeld tori of the Lagrangian torus fibration $\\{T_t\\}_{t\\in\\Delta^n}$ and its Hamiltonian deformation $\\{\\phi(T_t)\\}_{t\\in\\Delta^n}$.
We prove that any $U(1)^n$-orbit in $\\Bbb P^n$ is volume minimizing under Hamiltonian deformation.
The idea of the proof is :
- (1) We extend one $U(1)^n$-orbit to the momentum torus fibration $\\{T_t\\}_{t\\in\\Delta^n}$ and consider its Hamiltonian deformation $\\{\\phi(T_t)\\}_{t\\in\\Delta^n}$ where $\\phi$ is a Hamiltobian diffeomorphism of $\\Bbb P^n$,
and then :
- (2) We compare each $U(1)^n$-orbit and its Hamiltonian deformation by compaing the large $k$ asymptotic behavior of the sequence of projective embeddings defined, for each $k$, by the basis of $H^0(\\Bbb P^n,\\Cal O(k))$ obtained by semi-classical approximation of the $\\Cal O(k)$ Bohr-Sommerfeld tori of the Lagrangian torus fibration $\\{T_t\\}_{t\\in\\Delta^n}$ and its Hamiltonian deformation $\\{\\phi(T_t)\\}_{t\\in\\Delta^n}$.
2013/01/16
10:30-12:00 Room #122 (Graduate School of Math. Sci. Bldg.)
Yuuji Tanaka (Kyoto University)
A construction of Spin(7)-instantons (JAPANESE)
Yuuji Tanaka (Kyoto University)
A construction of Spin(7)-instantons (JAPANESE)
[ Abstract ]
Spin(7)-instantons are elliptic gauge fields on 8-dimensional Spin(7)-manifolds, which minimize the Yang-Mills action. Analytic properties of Spin(7)-instantons have been studied by Gang Tian and others, but little was known about the existence of examples of Spin(7)-instantons other than an Oxford Ph.D thesis by Christopher Lewis in 1998.
There are two known constructions of compact Spin(7)-manifolds both obtained by Dominic Joyce. The first one begins with a torus orbifold of a special kind with non-isolated singularities. The Spin(7)-manifold is obtained by resolving the singularities with the aid of algebraic geometry techniques. The second one begins with a Calabi-Yau four-orbifold with isolated singular points of a special kind and an anti-holomorphic involution fixing only the singular points. The Spin(7)-manifold is obtained by gluing ALE Spin(7)-manifolds with anti-holomorphic involutions fixing only the origins to each singular point.
Christopher Lewis studied the problem of constructing Spin(7)-instantons on Spin(7)-manifolds coming from Joyce's first construction.
This talk describes a general construction of Spin(7)-instantons on examples of compact Spin(7)-manifolds coming from Joyce's second construction. Starting with certain Hermitian-Einstein connections on the Calabi-Yau four-orbifold and on ALE Spin(7)-manifolds, we glue them together simultaneously with the underlying pieces to make a Spin(7)-instanton on the compact Spin(7)-manifold by Joyce.
Spin(7)-instantons are elliptic gauge fields on 8-dimensional Spin(7)-manifolds, which minimize the Yang-Mills action. Analytic properties of Spin(7)-instantons have been studied by Gang Tian and others, but little was known about the existence of examples of Spin(7)-instantons other than an Oxford Ph.D thesis by Christopher Lewis in 1998.
There are two known constructions of compact Spin(7)-manifolds both obtained by Dominic Joyce. The first one begins with a torus orbifold of a special kind with non-isolated singularities. The Spin(7)-manifold is obtained by resolving the singularities with the aid of algebraic geometry techniques. The second one begins with a Calabi-Yau four-orbifold with isolated singular points of a special kind and an anti-holomorphic involution fixing only the singular points. The Spin(7)-manifold is obtained by gluing ALE Spin(7)-manifolds with anti-holomorphic involutions fixing only the origins to each singular point.
Christopher Lewis studied the problem of constructing Spin(7)-instantons on Spin(7)-manifolds coming from Joyce's first construction.
This talk describes a general construction of Spin(7)-instantons on examples of compact Spin(7)-manifolds coming from Joyce's second construction. Starting with certain Hermitian-Einstein connections on the Calabi-Yau four-orbifold and on ALE Spin(7)-manifolds, we glue them together simultaneously with the underlying pieces to make a Spin(7)-instanton on the compact Spin(7)-manifold by Joyce.
