## Seminar on Geometric Complex Analysis

Seminar information archive ～02/07｜Next seminar｜Future seminars 02/08～

Date, time & place | Monday 10:30 - 12:00 128Room #128 (Graduate School of Math. Sci. Bldg.) |
---|---|

Organizer(s) | Kengo Hirachi, Shigeharu Takayama, Ryosuke Nomura |

**Seminar information archive**

### 2018/07/09

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Rigidity results for symplectic curvature flow (ENGLISH)

**Casey Kelleher**(Princeton University)Rigidity results for symplectic curvature flow (ENGLISH)

[ Abstract ]

We continue studying a parabolic flow of almost Kähler structure introduced by Streets and Tian which naturally extends Kähler-Ricci flow onto symplectic manifolds. In a system consisting primarily of quantities related to the Chern connection we establish clean formulas for the evolutions of canonical objects. Using this we give an extended characterization of fixed points of the flow.

We continue studying a parabolic flow of almost Kähler structure introduced by Streets and Tian which naturally extends Kähler-Ricci flow onto symplectic manifolds. In a system consisting primarily of quantities related to the Chern connection we establish clean formulas for the evolutions of canonical objects. Using this we give an extended characterization of fixed points of the flow.

### 2018/07/02

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Rigidity of certain groups of circle homeomorphisms and Teichmueller spaces (JAPANESE)

**Katsuhiko Matsuzaki**(Waseda University)Rigidity of certain groups of circle homeomorphisms and Teichmueller spaces (JAPANESE)

[ Abstract ]

In this talk, I explain a complex analytic method and its applications

for the study of quasisymmetric homeomorphisms of the circle by extending them to the unit disk quasiconformally.

In RIMS conference "Open Problems in Complex Geometry'' held in 2010,

I gave a talk entitled "Problems on infinite dimensional Teichmueller spaces", and

mentioned several problems on the fixed points of group actions on

the universal Teichmueller space and its subspaces, and the rigidity of conjugation of

certain groups of circle homeomorphisms.

I will report on the development of these problems since then.

In this talk, I explain a complex analytic method and its applications

for the study of quasisymmetric homeomorphisms of the circle by extending them to the unit disk quasiconformally.

In RIMS conference "Open Problems in Complex Geometry'' held in 2010,

I gave a talk entitled "Problems on infinite dimensional Teichmueller spaces", and

mentioned several problems on the fixed points of group actions on

the universal Teichmueller space and its subspaces, and the rigidity of conjugation of

certain groups of circle homeomorphisms.

I will report on the development of these problems since then.

### 2018/06/25

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Cornered Asymptotically Hyperbolic Spaces

**Stephen McKeown**(Princeton University)Cornered Asymptotically Hyperbolic Spaces

[ Abstract ]

This talk will concern cornered asymptotically hyperbolic spaces, which have a finite boundary in addition to the usual infinite boundary. I will first describe the construction a normal form near the corner for these spaces. Then I will discuss formal existence and uniqueness, near the corner, of asymptotically hyperbolic Einstein metrics, with a CMC-umbilic condition imposed on the finite boundary. This is analogous to the Fefferman-Graham construction for the ordinary, non-cornered setting. Finally, I will present work in progress regarding scattering on such spaces.

This talk will concern cornered asymptotically hyperbolic spaces, which have a finite boundary in addition to the usual infinite boundary. I will first describe the construction a normal form near the corner for these spaces. Then I will discuss formal existence and uniqueness, near the corner, of asymptotically hyperbolic Einstein metrics, with a CMC-umbilic condition imposed on the finite boundary. This is analogous to the Fefferman-Graham construction for the ordinary, non-cornered setting. Finally, I will present work in progress regarding scattering on such spaces.

### 2018/06/11

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Cohomology of non-pluriharmonic loci (JAPANESE)

**Yusaku Tiba**(Ochanomizu University)Cohomology of non-pluriharmonic loci (JAPANESE)

[ Abstract ]

In this talk, we study a pseudoconvex counterpart of the Lefschetz hyperplane theorem.

We show a relation between the cohomology of a pseudoconvex domain and the cohomology of the non-pluriharmonic locus of an exhaustive plurisubharmonic function.

In this talk, we study a pseudoconvex counterpart of the Lefschetz hyperplane theorem.

We show a relation between the cohomology of a pseudoconvex domain and the cohomology of the non-pluriharmonic locus of an exhaustive plurisubharmonic function.

### 2018/06/04

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

(JAPANESE)

**Junjiro Noguchi**(The University of Tokyo)(JAPANESE)

### 2018/05/28

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

A generalization of Kähler Einstein metrics for Fano manifolds with non-vanishing Futaki invariant (JAPANESE)

**Satoshi Nakamura**(Tohoku University)A generalization of Kähler Einstein metrics for Fano manifolds with non-vanishing Futaki invariant (JAPANESE)

[ Abstract ]

The existence problem of Kähler Einstein metrics for Fano manifolds was one of the central problems in Kähler Geometry. The vanishing of the Futaki invariant is known as an obstruction to the existence of Kähler Einstein metrics. Generalized Kähler Einstein metrics (GKE for short), introduced by Mabuchi in 2000, is a generalization of Kähler Einstein metrics for Fano manifolds with non-vanishing Futaki invariant. In this talk, we give the followings:

(i) The positivity for the Hessian of the Ricci Calabi functional which characterizes GKE as its critical points, and its application.

(ii) A criterion for the existence of GKE on toric Fano manifolds from view points of an algebraic stability and an analytic stability.

The existence problem of Kähler Einstein metrics for Fano manifolds was one of the central problems in Kähler Geometry. The vanishing of the Futaki invariant is known as an obstruction to the existence of Kähler Einstein metrics. Generalized Kähler Einstein metrics (GKE for short), introduced by Mabuchi in 2000, is a generalization of Kähler Einstein metrics for Fano manifolds with non-vanishing Futaki invariant. In this talk, we give the followings:

(i) The positivity for the Hessian of the Ricci Calabi functional which characterizes GKE as its critical points, and its application.

(ii) A criterion for the existence of GKE on toric Fano manifolds from view points of an algebraic stability and an analytic stability.

### 2018/05/21

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Kähler-Ricci soliton, K-stability and moduli space of Fano

manifolds (JAPANESE)

**Eiji Inoue**(The University of Tokyo)Kähler-Ricci soliton, K-stability and moduli space of Fano

manifolds (JAPANESE)

[ Abstract ]

Kähler-Ricci soliton is a kind of canonical metrics on Fano manifolds and is a natural generalization of Kähler-Einstein metric in

view of Kähler-Ricci flow.

In this talk, I will explain the following good geometric features of Fano manifolds admitting Kähler-Ricci solitons:

1. Volume minimization, reductivity and uniqueness results established by Tian&Zhu.

2. Relation to algebraic (modified) K-stability estabilished by Berman&Witt-Niström and Datar&Székelyhidi.

3. Moment map picture for Kähler-Ricci soliton (‘real side’)

4. Moduli stack (‘virtual side’) and moduli space of them

A result in 1 is indispensable for the formulation in 3 and 4, and explains why we should consider solitons, beyond Einstein metrics. I also show an essential idea in the construction of the moduli space of Fano manifolds admitting Kähler-Ricci solitons and give some remarks on technical key point.

Kähler-Ricci soliton is a kind of canonical metrics on Fano manifolds and is a natural generalization of Kähler-Einstein metric in

view of Kähler-Ricci flow.

In this talk, I will explain the following good geometric features of Fano manifolds admitting Kähler-Ricci solitons:

1. Volume minimization, reductivity and uniqueness results established by Tian&Zhu.

2. Relation to algebraic (modified) K-stability estabilished by Berman&Witt-Niström and Datar&Székelyhidi.

3. Moment map picture for Kähler-Ricci soliton (‘real side’)

4. Moduli stack (‘virtual side’) and moduli space of them

A result in 1 is indispensable for the formulation in 3 and 4, and explains why we should consider solitons, beyond Einstein metrics. I also show an essential idea in the construction of the moduli space of Fano manifolds admitting Kähler-Ricci solitons and give some remarks on technical key point.

### 2018/05/14

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Harmonic map and the Einstein equation in five dimension (JAPANESE)

**Sumio Yamada**(Gakushuin University)Harmonic map and the Einstein equation in five dimension (JAPANESE)

[ Abstract ]

We present a new method in constructing 5-dimensional stationary solutions to the vacuum Einstein equation. In 1917, H. Weyl expressed the Schwarzschild black hole solution using a cylindical coordinate system, and consequently realized that the metric is completely determined by a harmonic function. Since then, the relation between harmonic maps and the Einstein equation has been explored mostly by physicists, which they call the sigma model of the Einstein equation. In this talk, after explaining the historical background, we demonstrate that in 5D, the Einstein spacetimes can have a wide range of black hole horizons in their topological types. In particular we establish an existence theorem of harmonic maps, which subsequently leads to constructions of 5D spacetimes with black hole horizons of positive Yamabe types, namely $S^3$, $S^2 \times S^1$, and the lens space $L(p,q)$. This is a joint work with Marcus Khuri and Gilbert Weinstein.

We present a new method in constructing 5-dimensional stationary solutions to the vacuum Einstein equation. In 1917, H. Weyl expressed the Schwarzschild black hole solution using a cylindical coordinate system, and consequently realized that the metric is completely determined by a harmonic function. Since then, the relation between harmonic maps and the Einstein equation has been explored mostly by physicists, which they call the sigma model of the Einstein equation. In this talk, after explaining the historical background, we demonstrate that in 5D, the Einstein spacetimes can have a wide range of black hole horizons in their topological types. In particular we establish an existence theorem of harmonic maps, which subsequently leads to constructions of 5D spacetimes with black hole horizons of positive Yamabe types, namely $S^3$, $S^2 \times S^1$, and the lens space $L(p,q)$. This is a joint work with Marcus Khuri and Gilbert Weinstein.

### 2018/05/07

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Proper holomorphic mappings and generalized pseudoellipsoids (JAPANESE)

**Atsushi Hayashimoto**(National Institute of Technology, Nagano College)Proper holomorphic mappings and generalized pseudoellipsoids (JAPANESE)

[ Abstract ]

We study the classification of proper holomorphic mappings between generalized pseudoellipsoids of different dimensions.

Huang proved some classification theorems of proper holomorphic mappings between balls of different dimensions, which are called gap theorems. Our present theorems are their weakly pseudoconvex versions.

In the theorem, classified mapping is so-called a variables splitting mapping and each component is derived from a homogeneous proper polynomial mapping between balls.

The essential methods are the ''good'' decompositions of CR vector bundle and reduction the mapping under consideration to the mapping of balls. By this reduction, we can apply Huang's gap theorem.

We study the classification of proper holomorphic mappings between generalized pseudoellipsoids of different dimensions.

Huang proved some classification theorems of proper holomorphic mappings between balls of different dimensions, which are called gap theorems. Our present theorems are their weakly pseudoconvex versions.

In the theorem, classified mapping is so-called a variables splitting mapping and each component is derived from a homogeneous proper polynomial mapping between balls.

The essential methods are the ''good'' decompositions of CR vector bundle and reduction the mapping under consideration to the mapping of balls. By this reduction, we can apply Huang's gap theorem.

### 2018/04/23

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Degeneration and bifurcation of quadratic endomorphisms of $\mathbb{P}^2$ towards a Hénon map (JAPANESE)

**Yûsuke Okuyama**(Kyoto Institute of Technology)Degeneration and bifurcation of quadratic endomorphisms of $\mathbb{P}^2$ towards a Hénon map (JAPANESE)

[ Abstract ]

The space of quadratic holomorphic endomorphisms of P^2 (over C) is

canonically identified with the complement of the zero locus of the

resultant form on P^{17}, and all H¥'enon maps, which are (the only)

interesting ones among all the quadratic polynomial automorphisms of C^2,

live in this zero locus.

We will talk about our joint work with Fabrizio Bianchi (Imperial College,

London) on the (algebraic) degeneration of quadratic endomorphisms of C^2

towards H¥'enon maps in terms of Berteloot-Bianchi-Dupont's

bifurcation/unstability theory of holomorphic families of endomorphisms of P^k,

which mostly generalizes Ma¥~n¥'e-Sad-Sullivan, Lyubich, and DeMarco's seminal

and similar theory on P^1.

Some preliminary knowledge on ergodic theory and pluripotential theory

would be desirable, but not be assumed.

The space of quadratic holomorphic endomorphisms of P^2 (over C) is

canonically identified with the complement of the zero locus of the

resultant form on P^{17}, and all H¥'enon maps, which are (the only)

interesting ones among all the quadratic polynomial automorphisms of C^2,

live in this zero locus.

We will talk about our joint work with Fabrizio Bianchi (Imperial College,

London) on the (algebraic) degeneration of quadratic endomorphisms of C^2

towards H¥'enon maps in terms of Berteloot-Bianchi-Dupont's

bifurcation/unstability theory of holomorphic families of endomorphisms of P^k,

which mostly generalizes Ma¥~n¥'e-Sad-Sullivan, Lyubich, and DeMarco's seminal

and similar theory on P^1.

Some preliminary knowledge on ergodic theory and pluripotential theory

would be desirable, but not be assumed.

### 2018/04/16

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Metrics on a closed surface which maximize the first eigenvalue of the Laplacian (JAPANESE)

**Shin Nayatani**(Nagoya University)Metrics on a closed surface which maximize the first eigenvalue of the Laplacian (JAPANESE)

[ Abstract ]

In this talk, I will focus on recent progress on metrics that maximize the first eigenvalue of the Laplacian (under area normalization) on a closed surface. First, I introduce Hersch-Yang-Yau's inequality (1970, 1980), which was the starting point of the above problem. This is an inequality indicating that the first eigenvalue (precisely, the product of it with the area) is bounded from above by a constant depending only on the genus of the surface. Then I will outline the recent progress on the existence problem for maximizing metrics together with the relation with minimal surfaces in the sphere. Finally, I will discuss Jacobson-Levitin-Nadirashvili-Nigam-Polterovich's conjecture, which explicitly predicts maximizing metrics in the case of genus two, and the affirmative resolution of it (joint work with Toshihiro Shoda).

In this talk, I will focus on recent progress on metrics that maximize the first eigenvalue of the Laplacian (under area normalization) on a closed surface. First, I introduce Hersch-Yang-Yau's inequality (1970, 1980), which was the starting point of the above problem. This is an inequality indicating that the first eigenvalue (precisely, the product of it with the area) is bounded from above by a constant depending only on the genus of the surface. Then I will outline the recent progress on the existence problem for maximizing metrics together with the relation with minimal surfaces in the sphere. Finally, I will discuss Jacobson-Levitin-Nadirashvili-Nigam-Polterovich's conjecture, which explicitly predicts maximizing metrics in the case of genus two, and the affirmative resolution of it (joint work with Toshihiro Shoda).

### 2018/01/22

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Recent topics on the study of the Gauss images of minimal surfaces

**Yu Kawakami**(Kanazawa University)Recent topics on the study of the Gauss images of minimal surfaces

[ Abstract ]

In this talk, we give a survey of recent advances on the study of the images of the Gauss maps of complete minimal surfaces in Euclidean space.

In this talk, we give a survey of recent advances on the study of the images of the Gauss maps of complete minimal surfaces in Euclidean space.

### 2018/01/15

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Vanishing theorems of $L^2$-cohomology groups on Hessian manifolds

**Shinya Akagawa**(Osaka University)Vanishing theorems of $L^2$-cohomology groups on Hessian manifolds

[ Abstract ]

A Hessian manifold is a Riemannian manifold whose metric is locally given by the Hessian of a function with respect to flat coordinates. In this talk, we discuss vanishing theorems of $L^2$-cohomology groups on complete Hessian Manifolds coupled with flat line bundles. In particular, we obtain stronger vanishing theorems on regular convex cones with the Cheng-Yau metrics. Further we show that the Cheng-Yau metrics on regular convex cones give rise to harmonic maps to the positive symmetric matrices.

A Hessian manifold is a Riemannian manifold whose metric is locally given by the Hessian of a function with respect to flat coordinates. In this talk, we discuss vanishing theorems of $L^2$-cohomology groups on complete Hessian Manifolds coupled with flat line bundles. In particular, we obtain stronger vanishing theorems on regular convex cones with the Cheng-Yau metrics. Further we show that the Cheng-Yau metrics on regular convex cones give rise to harmonic maps to the positive symmetric matrices.

### 2017/12/18

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Gradient flow of the Ding functional

**Tomoyuki Hisamoto**(Nagoya University)Gradient flow of the Ding functional

[ Abstract ]

This is a joint work with T. Collins and R. Takahashi. We introduce the flow in the title to study the stability of a Fano manifold. The first result is the long-time existence of the flow. In the stable case it then converges to the Kähler-Einstein metric. In general the flow is expected to produce the optimally destabilizing degeneration of a Fano manifold. We confirm this expectation in the toric case.

This is a joint work with T. Collins and R. Takahashi. We introduce the flow in the title to study the stability of a Fano manifold. The first result is the long-time existence of the flow. In the stable case it then converges to the Kähler-Einstein metric. In general the flow is expected to produce the optimally destabilizing degeneration of a Fano manifold. We confirm this expectation in the toric case.

### 2017/12/11

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Nishino's rigidity theorem and questions on locally pseudoconvex maps

**Takeo Ohsawa**(Nagoya University)Nishino's rigidity theorem and questions on locally pseudoconvex maps

[ Abstract ]

Nishino proved in 1969 that locally Stein maps with fibers $\cong \mathbb{C}$ are locally trivial. Yamaguchi gave an alternate proof of Nishino's theorem which later developed into a the theory of variations of the Bergman kernel. The proofs of Nishino and Yamaguchi will be reviewed and questions suggested by the result will be discussed. A new application of the $L^2$ extension theorem will be also presented in this context.

Nishino proved in 1969 that locally Stein maps with fibers $\cong \mathbb{C}$ are locally trivial. Yamaguchi gave an alternate proof of Nishino's theorem which later developed into a the theory of variations of the Bergman kernel. The proofs of Nishino and Yamaguchi will be reviewed and questions suggested by the result will be discussed. A new application of the $L^2$ extension theorem will be also presented in this context.

### 2017/11/27

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

On the proof of the optimal $L^2$ extension theorem by Berndtsson-Lempert and related results

**Genki Hosono**(The University of Tokyo)On the proof of the optimal $L^2$ extension theorem by Berndtsson-Lempert and related results

[ Abstract ]

We will present the recent progress on the Ohsawa-Takegoshi $L^2$ extension theorem. A version of the Ohsawa-Takegoshi $L^2$ extension with a optimal estimate has been proved by Blocki and Guan-Zhou. After that, by Berndtsson-Lempert, a new proof of the optimal $L^2$ extension theorem was given. In this talk, we will show an optimal $L^2$ extension theorem for jets of holomorphic functions by the Berndtsson-Lempert method. We will also explain the recent result about jet extensions by McNeal-Varolin. Their proof is also based on Berndtsson-Lempert, but there are some differences.

We will present the recent progress on the Ohsawa-Takegoshi $L^2$ extension theorem. A version of the Ohsawa-Takegoshi $L^2$ extension with a optimal estimate has been proved by Blocki and Guan-Zhou. After that, by Berndtsson-Lempert, a new proof of the optimal $L^2$ extension theorem was given. In this talk, we will show an optimal $L^2$ extension theorem for jets of holomorphic functions by the Berndtsson-Lempert method. We will also explain the recent result about jet extensions by McNeal-Varolin. Their proof is also based on Berndtsson-Lempert, but there are some differences.

### 2017/11/20

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Relative GIT stabilities of toric Fano manifolds in low dimensions

**Yasufumi Nitta**(Tokyo Institute of Technology)Relative GIT stabilities of toric Fano manifolds in low dimensions

[ Abstract ]

In 2000, Mabuchi extended the notion of Kaehler-Einstein metrics to Fano manifolds with non-vanishing Futaki invariant. Such a metric is called generalized Kaehler-Einstein metric or Mabuchi metric in the literature. Recently this metrics were rediscovered by Yao in the story of Donaldson's infinite dimensional moment map picture. Moreover, he introduced (uniform) relative Ding stability for toric Fano manifolds and showed that the existence of generalized Kaehler-Einstein metrics is equivalent to its uniform relative Ding stability. This equivalence is in the context of the Yau-Tian-Donaldson conjecture. In this talk, we focus on uniform relative Ding stability of toric Fano manifolds. More precisely, we determine all the uniformly relatively Ding stable toric Fano 3- and 4-folds as well as unstable ones. This talk is based on a joint work with Shunsuke Saito and Naoto Yotsutani.

In 2000, Mabuchi extended the notion of Kaehler-Einstein metrics to Fano manifolds with non-vanishing Futaki invariant. Such a metric is called generalized Kaehler-Einstein metric or Mabuchi metric in the literature. Recently this metrics were rediscovered by Yao in the story of Donaldson's infinite dimensional moment map picture. Moreover, he introduced (uniform) relative Ding stability for toric Fano manifolds and showed that the existence of generalized Kaehler-Einstein metrics is equivalent to its uniform relative Ding stability. This equivalence is in the context of the Yau-Tian-Donaldson conjecture. In this talk, we focus on uniform relative Ding stability of toric Fano manifolds. More precisely, we determine all the uniformly relatively Ding stable toric Fano 3- and 4-folds as well as unstable ones. This talk is based on a joint work with Shunsuke Saito and Naoto Yotsutani.

### 2017/11/13

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Relative Canonical Bundles for Families of Calabi-Yau Manifolds

**Georg Schumacher**(Philipps-Universität Marburg)Relative Canonical Bundles for Families of Calabi-Yau Manifolds

[ Abstract ]

We consider holomorphic families of Calabi-Yau manifolds (here being defined by the vanishing of the first real Chern class). We study induced hermitian metrics on the relative canonical bundle, which are related to the Weil-Petersson form on the base. Under a certain condition the total space possesses a Kähler form, whose restriction to fibers are equal to the Ricci flat metrics. Furthermore we prove an extension theorem for the Weil-Petersson form and give applications.

We consider holomorphic families of Calabi-Yau manifolds (here being defined by the vanishing of the first real Chern class). We study induced hermitian metrics on the relative canonical bundle, which are related to the Weil-Petersson form on the base. Under a certain condition the total space possesses a Kähler form, whose restriction to fibers are equal to the Ricci flat metrics. Furthermore we prove an extension theorem for the Weil-Petersson form and give applications.

### 2017/10/30

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Odd dimensional complex analytic Kleinian groups

**Masahide Kato**(Sophia University)Odd dimensional complex analytic Kleinian groups

[ Abstract ]

In this talk, I would explain an idea to construct a higher dimensional analogue of the classical Kleinian group theory. For a group $G$ of a certain class of discrete subgroups of $\mathrm{PGL}(2n+2,\mathbf{C})$ which act on $\mathbf{P}^{2n+1}$, there is a canonical way to define the region of discontinuity, on which $G$ acts properly discontinuously. General principle in the discussion is to regard $\mathbf{P}^{n}$ in $\mathbf{P}^{2n+1}$ as a single point. We can consider the quotient space of the discontinuity region by the action of $G$. Though the Ahlfors finiteness theorem is hopeless because of a counter example, the Klein combination theorem and the handle attachment can be defined similarly. Any compact quotients which appear here are non-Kaehler. In the case $n=1$, we explain a new example of compact quotients which is related to a classical Kleinian group.

In this talk, I would explain an idea to construct a higher dimensional analogue of the classical Kleinian group theory. For a group $G$ of a certain class of discrete subgroups of $\mathrm{PGL}(2n+2,\mathbf{C})$ which act on $\mathbf{P}^{2n+1}$, there is a canonical way to define the region of discontinuity, on which $G$ acts properly discontinuously. General principle in the discussion is to regard $\mathbf{P}^{n}$ in $\mathbf{P}^{2n+1}$ as a single point. We can consider the quotient space of the discontinuity region by the action of $G$. Though the Ahlfors finiteness theorem is hopeless because of a counter example, the Klein combination theorem and the handle attachment can be defined similarly. Any compact quotients which appear here are non-Kaehler. In the case $n=1$, we explain a new example of compact quotients which is related to a classical Kleinian group.

### 2017/10/23

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

On the proof of the optimal $L^2$ extension theorem by Berndtsson-Lempert and related results

**Genki Hosono**(The University of Tokyo)On the proof of the optimal $L^2$ extension theorem by Berndtsson-Lempert and related results

[ Abstract ]

We will present the recent progress on the Ohsawa-Takegoshi $L^2$ extension theorem. A version of the Ohsawa-Takegoshi $L^2$ extension with a optimal estimate has been proved by Blocki and Guan-Zhou. After that, by Berndtsson-Lempert, a new proof of the optimal $L^2$ extension theorem was given. In this talk, we will show an optimal $L^2$ extension theorem for jets of holomorphic functions by the Berndtsson-Lempert method. We will also explain the recent result about jet extensions by McNeal-Varolin. Their proof is also based on Berndtsson-Lempert, but there are some differences.

We will present the recent progress on the Ohsawa-Takegoshi $L^2$ extension theorem. A version of the Ohsawa-Takegoshi $L^2$ extension with a optimal estimate has been proved by Blocki and Guan-Zhou. After that, by Berndtsson-Lempert, a new proof of the optimal $L^2$ extension theorem was given. In this talk, we will show an optimal $L^2$ extension theorem for jets of holomorphic functions by the Berndtsson-Lempert method. We will also explain the recent result about jet extensions by McNeal-Varolin. Their proof is also based on Berndtsson-Lempert, but there are some differences.

### 2017/10/16

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Characterizations of hyperbolically $k$-convex domains in terms of hyperbolic metric

**Toshiyuki Sugawa**(Tohoku University)Characterizations of hyperbolically $k$-convex domains in terms of hyperbolic metric

[ Abstract ]

It is known that a plane domain $X$ with hyperbolic metric $h_X=h_X(z)|dz|$ of constant curvature $-4$ is (Euclidean) convex if and only if $h_X(z)d_X(z)\ge 1/2$, where $d_X(z)$ denotes the Euclidean distance from a point $z$ in $X$ to the boundary of $X$. We will consider spherical and hyperbolic versions of this result. More generally, we consider hyperbolic $k$-convexity (in the sense of Mejia and Minda) in the same line. A key is to observe a detailed behaviour of the hyperbolic density $h_X(z)$ near the boundary.

It is known that a plane domain $X$ with hyperbolic metric $h_X=h_X(z)|dz|$ of constant curvature $-4$ is (Euclidean) convex if and only if $h_X(z)d_X(z)\ge 1/2$, where $d_X(z)$ denotes the Euclidean distance from a point $z$ in $X$ to the boundary of $X$. We will consider spherical and hyperbolic versions of this result. More generally, we consider hyperbolic $k$-convexity (in the sense of Mejia and Minda) in the same line. A key is to observe a detailed behaviour of the hyperbolic density $h_X(z)$ near the boundary.

### 2017/10/02

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

The extension of holomorphic functions on a non-pluriharmonic locus

**Yusaku Tiba**(Ochanomizu University)The extension of holomorphic functions on a non-pluriharmonic locus

[ Abstract ]

Let $n \geq 4$ and let $\Omega$ be a bounded hyperconvex domain in $\mathbb{C}^{n}$. Let $\varphi$ be a negative exhaustive smooth plurisubharmonic function on $\Omega$. In this talk, we show that any holomorphic function defined on a connected open neighborhood of the support of $(i\partial \overline{\partial}\varphi)^{n-3}$ can be extended to the holomorphic function on $\Omega$.

Let $n \geq 4$ and let $\Omega$ be a bounded hyperconvex domain in $\mathbb{C}^{n}$. Let $\varphi$ be a negative exhaustive smooth plurisubharmonic function on $\Omega$. In this talk, we show that any holomorphic function defined on a connected open neighborhood of the support of $(i\partial \overline{\partial}\varphi)^{n-3}$ can be extended to the holomorphic function on $\Omega$.

### 2017/09/25

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Asymptotics of $L^2$ and Quillen metrics in degenerations of Calabi-Yau varieties

**Christophe Mourougane**(Université de Rennes 1)Asymptotics of $L^2$ and Quillen metrics in degenerations of Calabi-Yau varieties

[ Abstract ]

It is a joint work with Dennis Eriksson and Gerard Freixas i Montplet.

Our first motivation is to give a metric analogue of Kodaira's canonical bundle formula for elliptic surfaces, in the case of families of Calabi-Yau varieties. We consider degenerations of complex projective Calabi-Yau varieties and study the singularities of $L^2$, Quillen and BCOV metrics on Hodge and determinant bundles. The dominant and subdominant terms in the expansions of the metrics close to non-smooth fibres are shown to be related to well-known topological invariants of singularities, such as limit Hodge structures, vanishing cycles and log-canonical thresholds.

It is a joint work with Dennis Eriksson and Gerard Freixas i Montplet.

Our first motivation is to give a metric analogue of Kodaira's canonical bundle formula for elliptic surfaces, in the case of families of Calabi-Yau varieties. We consider degenerations of complex projective Calabi-Yau varieties and study the singularities of $L^2$, Quillen and BCOV metrics on Hodge and determinant bundles. The dominant and subdominant terms in the expansions of the metrics close to non-smooth fibres are shown to be related to well-known topological invariants of singularities, such as limit Hodge structures, vanishing cycles and log-canonical thresholds.

### 2017/07/03

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Holomorphic isometric embeddings into Grassmannians of rank $2$

**Yasuyuki Nagatomo**(Meiji University)Holomorphic isometric embeddings into Grassmannians of rank $2$

[ Abstract ]

We suppose that Grassmannians are equipped with the standard Kähler metrics of Fubini-Study type. This means that the ${\it universal \ quotient}$ bundles over Grassmannians are provided with not only fibre metrics but also connections. Such connections are called the ${\it canonical}$ connection.

First of all, we classify $\text{SU}(2)$ equivariant holomorphic embeddings of the complex projective line into complex Grassmannians of $2$-planes. To do so, we focus our attention on the pull-back connection of the canonical connection, which is an $\text{SU}(2)$ invariant connection by our hypothesis. We use ${\it extensions}$ of vector bundles to classify $\text{SU}(2)$ invariant connections on vector bundles of rank $2$ over the complex projective line. Since the extensions are in one-to-one correspondence with $H^1(\mathbf CP^1;\mathcal O(-2))$, the moduli space of non-trivial invariant connections modulo gauge transformations is identified with the quotient space of $H^1(\mathbf CP^1;\mathcal O(-2))$ by $S^1$-action. The positivity of the mean curvature of the pull-back connection implies that the moduli spaces of $\text{SU}(2)$ equivariant holomorphic embeddings are the open intervals $(0,l)$, where $l$ depends only on the ${\it degree}$ of the map.

Next, we describe moduli spaces of holomorphic isometric embeddings of the complex projective line into complex quadric hypersurfaces of the projective spaces. A harmonic map from a Riemannian manifold into a Grassmannian is characterized by the universal quotient 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)). Due to this, we can generalize do Carmo-Wallach theory. We apply a generalized do Carmo-Wallach theory to obtain the moduli spaces. This method also gives a description of the moduli space of Einstein-Hermitian harmonic maps with constant Kähler angles of the complex projective line into complex quadrics. It turns out that the moduli space is diffeomorphic to the moduli of holomorphic isometric embeddings of the same degree.

We suppose that Grassmannians are equipped with the standard Kähler metrics of Fubini-Study type. This means that the ${\it universal \ quotient}$ bundles over Grassmannians are provided with not only fibre metrics but also connections. Such connections are called the ${\it canonical}$ connection.

First of all, we classify $\text{SU}(2)$ equivariant holomorphic embeddings of the complex projective line into complex Grassmannians of $2$-planes. To do so, we focus our attention on the pull-back connection of the canonical connection, which is an $\text{SU}(2)$ invariant connection by our hypothesis. We use ${\it extensions}$ of vector bundles to classify $\text{SU}(2)$ invariant connections on vector bundles of rank $2$ over the complex projective line. Since the extensions are in one-to-one correspondence with $H^1(\mathbf CP^1;\mathcal O(-2))$, the moduli space of non-trivial invariant connections modulo gauge transformations is identified with the quotient space of $H^1(\mathbf CP^1;\mathcal O(-2))$ by $S^1$-action. The positivity of the mean curvature of the pull-back connection implies that the moduli spaces of $\text{SU}(2)$ equivariant holomorphic embeddings are the open intervals $(0,l)$, where $l$ depends only on the ${\it degree}$ of the map.

Next, we describe moduli spaces of holomorphic isometric embeddings of the complex projective line into complex quadric hypersurfaces of the projective spaces. A harmonic map from a Riemannian manifold into a Grassmannian is characterized by the universal quotient 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)). Due to this, we can generalize do Carmo-Wallach theory. We apply a generalized do Carmo-Wallach theory to obtain the moduli spaces. This method also gives a description of the moduli space of Einstein-Hermitian harmonic maps with constant Kähler angles of the complex projective line into complex quadrics. It turns out that the moduli space is diffeomorphic to the moduli of holomorphic isometric embeddings of the same degree.

### 2017/06/26

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

Volume minimization and obstructions to geometric problems

**Akito Futaki**(The University of Tokyo)Volume minimization and obstructions to geometric problems

[ Abstract ]

We discuss on the volume minimization principle for conformally Kaehler Einstein-Maxwell metrics in the similar spirit as the Kaehler-Ricci solitons and Sasaki-Einstein metrics. This talk is base on a joint work with Hajime Ono.

We discuss on the volume minimization principle for conformally Kaehler Einstein-Maxwell metrics in the similar spirit as the Kaehler-Ricci solitons and Sasaki-Einstein metrics. This talk is base on a joint work with Hajime Ono.