Seminar on Geometric Complex Analysis

Seminar information archive ~02/27Next seminarFuture seminars 02/28~

Date, time & place Monday 10:30 - 12:00 128Room #128 (Graduate School of Math. Sci. Bldg.)
Organizer(s) Kengo Hirachi, Shigeharu Takayama

Seminar information archive

2023/12/11

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Ryo Matsuda (Kyoto Univeristy)
On Partial deformations and Bers embedding (Japanese)
[ Abstract ]
The Teichmüller space of the Riemann surface S is the space of deformations of the complex structure of S. For complex analysis on Teich(S), it is biholomorphic embedded into a bounded set of the space of complex Banach spaces, denoted as B(S). This embedding is known as the Bers embedding. Additionally, when S is of infinite type, considering partial deformations can reveal properties of Teich(S). Earle-Gardiner-Lakic prove that asymptotically conformal deformations correspond to subspaces where the norm of the embedding decays at infinity. In this talk, we generalize this result, showing that deformations that become asymptotically conformal at some end correspond to spaces where the norm decays at that end. Finally, using this result and the David map, a generalization of quasiconformal maps, I’ll give that in the Bers boundary of infinite-type Riemann surface satisfying the Shiga condition, Maximal cusps are not dense.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/11/27

11:00-12:30   Room #128 (Graduate School of Math. Sci. Bldg.)
Satoshi Ogawa (Osaka Metropolitan University)
On a holomorphic tubular neighborhood of a compact complex curve and Brjuno condition (Japanese)
[ Abstract ]
Let $C$ be a compact complex curve holomorphically embedded in a non-singular complex surface $M$ with a unitary flat normal bundle $N_{C/M}$ and let $\mathcal{U}$ be a finite open cover of $C$. Gong--Stolovitch posed a sufficient condition for the existence of a holomorphic tubular neighborhood of $C$ in $M$ expressed with operator norms of Čech coboundary maps $\delta$ on $\check{C}^0(\mathcal{U}, \mathcal{O}_C(N_{C/M}^\nu))$ and $\check{C}^0(\mathcal{U}, \mathcal{O}_C(T_C \otimes N_{C/M}^\nu))$.
In this talk, we introduce some estimates of the operator norms of $\delta$. As a result, we see the Brjuno condition appears as a sufficient condition for the existence of a holomorphic tubular neighborhood.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/10/30

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Shin-Ichi Matsumura (Tohoku Univeristy)
The Nonvanishing problem for varieties with nef anticanonical bundle
[ Abstract ]
In the framework of the minimal model program for generalized pairs, the abundance conjecture does not hold. However, interestingly, the generalized nonvanishing conjecture is expected to hold. This conjecture asks whether the canonical divisor of generalized pairs can be represented by an effective divisor in its numerical class. In this talk, we discuss the nonvanishing conjecture for generalized LC pairs in three dimensions and prove that the conjecture is true for the nef anti-canonical divisors.
This talk is based on joint work with V. Lazic, Th. Peternell, N. Tsakanikas, and Z. Xie.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/10/16

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Hajime Tsuji (Sophia University)
The limit of Kähler-Ricci flows
[ Abstract ]
In this talk, I would like to present the (normalized) limit of Kähler-Ricci flows for compact Kähler manifolds with intermediate Kodaira dimesion under the condition that the canonical bundle is abundant.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/07/10

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Ken-Ichi Yoshikawa (Kyoto University)
Degenerations of Riemann surfaces and small eigenvalues of the Laplacian (日本語)
[ Abstract ]
In this talk, we consider a proper surjective holomorphic map from a smooth projective surface to a compact Riemann surface. Near a singular fiber, this is viewed as a one-parameter degeneration of compact Riemann surfaces. We fix a Kähler metric on the projective surface and consider the Kähler metric on the fibers induced from this metric. In this setting, for each regular fiber, we can consider the Laplacian acting on the functions on the fiber. It is known that for any k, the k-th eigenvalue of the Laplacian extends to a continuous function on the base curve. In particular, if the singular fiber is not irreducible, some eigenvalues of the Laplacian of the regular fiber converge to zero as the regular fiber approaches to the singular fiber. We call such eigenvalues small eigenvalues. In this talk, when the singular fiber is reduced, we will explain the asymptotic behavior of the product of all small eigenvalues of the Laplacian.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/07/03

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Katsutoshi Yamanoi (Osaka University)
Hyperbolicity and fundamental groups of complex quasi-projective varieties
[ Abstract ]
This talk is based on a joint work with Benoit Cadorel and Ya Deng. arXiv:2212.12225
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/06/26

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Masataka IWAI (Osaka Univeristy)
Miyaoka type inequality for terminal weak Fano varieties
[ Abstract ]
In this talk, we show that $c_2(X)c_1(X)^{n-2}$ is positive for any $n$-dimensional terminal weak Fano varieties $X$. As a corollary, we obtain some inequalities (Miyaoka type inequalities) with respect to $c_2(X)c_1(X)^{n-2}$ and $c_1(X)^{n}$. This is joint work with Chen Jiang and Haidong Liu (arXiv:2303.00268).
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/06/19

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Nobuhiro Honda (Tokyo University of Technology)
A new construction method for 3-dimensional indefinite Zoll manifolds
[ Abstract ]
The Penrose correspondence gives correspondences between special geometric structures on manifolds and complex manifolds, one of which is between Einstein-Weyl structures on 3-manifolds and complex surfaces. The latter complex surfaces are called mini-Twister spaces. In this talk, I will show that compact mini-Zeister spaces can be constructed in a natural way from hyperelliptic curves of arbitrary species, and that the resulting 3-manifolds have a remarkable geometric property called the Zoll property, which means that all geodesics are closed. A typical example is a sphere. The three-dimensional Einstein-Weyl manifold obtained in this study is indefinite, and the geodesics considered are spatial. These Einstein-Weyl manifolds can be regarded as generalizations of those given in arXiv:2208.13567.

Translated with www.DeepL.com/Translator (free version)
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/05/29

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Takato UEHARA (Okayama University)
On dynamical degrees of birational maps
[ Abstract ]
A birational map on a projective surface defines its dynamical degree, which measures the complexity of dynamical behavior of the map. The set of dynamical degrees, called the dynamical spectrum, has properties similar to that of volumes of hyperbolic 3-manifolds, shown by Thurston. In this talk, we will explain the properties of the dynamical spectrum.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/05/22

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Masanari Adachi (Shizuoka Univeristy)
A residue formula for meromorphic connections and applications to stable sets of foliations
[ Abstract ]
We discuss a proof for Brunella’s conjecture: a codimension one holomorphic foliation on a compact complex manifold of dimension > 2 has no exceptional minimal set if its normal bundle is ample. The main idea is the localization of the first Chern class of the normal bundle of the foliation via a holomorphic connection. Although this localization was done via that of the first Atiyah class in our previous proof, we shall explain that this can be shown more directly by a residue formula. If time permits, we also discuss a nonexistence result of Levi flat hypersurfaces with transversely affine Levi foliation. This talk is based on joint works
with S. Biard and J. Brinkschulte.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/05/15

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Yuya Takeuchi (Tsukuba Univeristy)
$\mathcal{I}'$-curvatures and the Hirachi conjecture (Japanese)
[ Abstract ]
Hirachi conjecture deals with a relation between the integrals of local pseudo-Hermitian invariants and global CR invariants. This is a CR analogue of the Deser-Schwimmer conjceture, which was proved by Alexakis. In this talk, I would like to explain some results on the Hirachi conjecture. In particular, I'll introduce the $\mathcal{I}'$-curvatures and prove that these produce counterexamples to the Hirachi conjecture in higher dimensions. This talk is based on joint work with Jeffrey S. Case.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/05/08

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Hisashi Kasuya (Osaka Univeristy)
Non-Kähler Hodge theory and resolutions of cyclic orbifolds (日本語)
[ Abstract ]
This talk is based on the joint works with Jonas Stelzig (LMU München). We discuss the Hodge theory of non-Kähler compact complex manifolds. In this term, we think several types of compact complex manifolds and compact Kähler manifolds are considered as the "simplest”. We give a way of constructing simply connected compact complex non-Kähler manifolds of certain types by using resolutions of cyclic orbifolds.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/04/24

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
On site+Zoom
Takeo Ohsawa (Nogoya Universiry)
Guan's theorems on optimal strong openness and concavity of minimal $L^2$ integrals (日本語)
[ Abstract ]
Motivated by a question of approximating plurisubharmonic (=psh) functions by those with tame singularities, Demailly and Kollar asked several basic questions on the singularities of psh functions. Guan solved two of them effectively in a paper published in 2019. One of their corollaries says the following.

THEOREM. Let $\Omega$ be a pseudoconvex domain in $\mathbb{C}^n$ and let $\varphi$ be a negative psh function on $\Omega$ such that $\int_\Omega{e^{-\varphi}}<\infty$. Then, $e^{-p\varphi}\in L^1_{\text{loc}}$ around $x$ for any $x\in\Omega$ and $p>1$ satisfying the inequality $$
\frac{p}{p-1}>\frac{\int_\Omega{e^{-\varphi}}}{K_\Omega(x)},
$$ where $K_\Omega$ denotes the diagonalized Bergman kernel of $\Omega$.

This remarkable result is a consequence of a basic property of the minimal $L^2$ integrals (=MLI). The main purpose of the talk is to give an outline of the proof of Theorem by explaining the relation between several notions including the MLI which measure the singularities of psh functions. It will also be mentioned that the proof of Theorem is essentially based on the optimal Ohsawa-Takegoshi type extension theorem, which leads to a concavity property of MLI. Recent papers by Guan and his students will be reviewed, too.
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/02/13

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Junjiro Noguchi (The University of Tokyo)
On a presentation to introduce function theory of several variables (Japanese)
[ Abstract ]
微積分は,主に1変数の理論を講義するが,後半で多変数の内容を入れる.同じ様に,複素解析(函数論)でも,一変数の後につなぎよく,多変数の講義を段差なく行えるようにしたい.
モデルケースとして'リーマンの写像定理'がある.現在多くの教科書に書かれているモンテルの定理による初等的な証明(1922, Fejér--Riesz)まで,もとのリーマンの学位論文(1851)から約70年の歳月がかかている.
岡理論・多変数関数論基礎についてみると,Oka IX (1953)より本年でやはり70年たつが,あまり'初等化'の方面へは進展していないように思う.こここでは,学部の複素解析のコースで'リーマンの写像定理'の後に,段差無く完全証明付きで岡理論・多変数関数論基礎を講義する展開を考える.
初等化には,岡のオリジナル法(1943未発表, IX 1953)を第1連接定理に基づき展開するのが適当であることを紹介したい.学部講義の数学内容に日本人による成果が入ることで,学生のモチベーションに好効果を与えるであろうことも期待したい.
時間が許せば,擬凸問題解決の岡のオリジナル法と別証明とされるGrauertの証明との間のFredholm定理をめぐる類似性についても述べたい.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2023/01/16

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Takayuki Koike (Osaka Metropolitan University)
Holomorphic foliation associated with a semi-positive class of numerical dimension one (Japanese)
[ Abstract ]
Let $X$ be a compact Kähler manifold and $\alpha$ be a Dolbeault cohomology class of bidegree $(1,1)$ on $X$.
When the numerical dimension of $\alpha$ is one and $\alpha$ admits at least two smooth semi-positive representatives, we show the existence of a family of real analytic Levi-flat hypersurfaces in $X$ and a holomorphic foliation on a suitable domain of $X$ along whose leaves any semi-positive representative of $\alpha$ is zero.

As an application, we give the affirmative answer to a conjecture on the relation between the semi-positivity of the line bundle $[Y]$ and the analytic structure of a neighborhood of $Y$ for a smooth connected hypersurface $Y$ of $X$.

As an application, we give the affirmative answer to a conjecture on the relation between the semi-positivity of the line bundle $[Y]$ and the analytic structure of a neighborhood of $Y$ for a smooth connected hypersurface $Y$ of $X$.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/12/12

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Takahiro Inayama (Tokyo University of Science)
$L^2$-extension index and its applications (Japanese)
[ Abstract ]
In this talk, we introduce a new concept of $L^2$-extension indices. By using this notion, we propose a new way to study the positivity of curvature. We prove that there is an equivalence between how sharp the $L^2$-extension is and how positive the curvature is. As applications, we study Prekopa-type theorems and the positivity of a certain direct image sheaf.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/12/05

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Shota Kikuchi (National Institute of Technology, Suzuka College)
On sharper estimates of Ohsawa--Takegoshi $L^2$-extension theorem in higher dimensional case (Japanese)
[ Abstract ]
Hosono proposed an idea of getting an $L^2$-estimate sharper than the one of Berndtsson--Lempert type $L^2$-extension theorem by allowing constants depending on weight functions in $\mathbb{C}$.

In this talk, I explain the details of "sharper estimates" and the higher dimensional case of it. Also, I explain my recent studies related to it.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/11/21

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Joe Kamimoto (Kyushu University)
Resolution of singularities for $C^{\infty}$ functions and meromorphy of local zeta functions (Japanese)
[ Abstract ]
In this talk, we attempt to resolve the singularities of the zero variety of a $C^{\infty}$ function of two variables as much as possible by using ordinary blowings up. As a result, we formulate an algorithm to locally express the zero variety in the “almost” normal crossings form, which is close to the normal crossings form but may include flat functions. As an application, we investigate analytic continuation of local zeta functions associated with  $C^{\infty}$ functions of two variables.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/11/14

15:00-16:30   Online
Hideki Miyach (Kanazawa University)
The double holomorphic tangent space of the Teichmueller spaces (Japanese)
[ Abstract ]
The double holomorphic tangent space of a complex manifold is the holomorphic tangent space of the holomorphic tangent bundle of the complex manifold. In this talk, we will give an intrinsic description of the double tangent spaces of the Teichmueller spaces of closed Riemann surfaces of genus at least 2.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/10/31

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Eiji Inoue (RIKEN)
The non-archimedean μ-entropy in toric case (Japanese)
[ Abstract ]
The non-archimedean μ-entropy is a functional on the space of test configurations of a polarized variety. It plays a key role in μK-stability and can be interpreted as a dual functional to Perelman’s μ-entropy for Kahler metrics. The fundamental question on the non-archimedean μ-entropy is the existence and uniqueness of maximizers. To find its maximizers, it is natural to extend the functional to a suitable completion of the space of test configurations. For general polarized variety, we can realize such completion and extension based on the non-archimedean pluripotential theory.
In the toric case, the torus invariant subspace of the completion is identified with a suitable space of convex functions on the moment polytope and then the non-archimedean μ-entropy is simply expressed by integrations of convex functions on the polytope. I will show a compactness result in the toric case, by which we conclude the existence of maximizers for the toric non-archimedean μ-entropy.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/10/24

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Taro Fujisawa (Tokyo Denki University)
A new approach to the nilpotent orbit theorem via the $L^2$ extension theorem of Ohsawa-Takegoshi type (Japanese)
[ Abstract ]
I will talk about a new proof of (a part of) the nilpotent orbit theorem for unipotent variations of Hodge structure. This approach is largely inspired by the recent works of Deng and of Sabbah-Schnell. In my proof, the $L^2$ extension theorem of Ohsawa-Takegoshi type plays essential roles.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/07/11

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Yoshihiko Matsumoto (Osaka University)
The CR Killing operator and Bernstein-Gelfand-Gelfand construction in CR geometry (Japanese)
[ Abstract ]
In this talk, I introduce the CR Killing operator associated with compatible almost CR structures on contact manifolds, which describes trivial infinitesimal deformations generated by contact Hamiltonian vector fields, and discuss how it can also be reconstructed by the Bernstein-Gelfand-Gelfand construction in the general theory of parabolic geometries. The “modified” adjoint tractor connection defined by Cap (2008) plays a crucial role. If time permits, I’d also like to discuss what this observation might mean in relation to asymptotically complex hyperbolic Einstein metrics, which are bulk geometric structures for compatible almost CR structures at infinity.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/07/04

10:30-12:00   Online
Katsutoshi Yamanoi (Osaka University)
Bloch's principle for holomorphic maps into subvarieties of semi-abelian varieties (Japanese)
[ Abstract ]
We discuss a generalization of the logarithmic Bloch-Ochiai theorem about entire curves in subvarieties of semi-abelian varieties, in terms of sequences of holomorphic maps from the unit disc.

This generalization implies, among other things, that subvarieties of log general type in semi-abelian varieties are pseudo-Kobayashi hyperbolic.

As another application, we discuss an improvement of a classical theorem due to Cartan in 1920's about the system of nowhere vanishing holomorphic functions on the unit disc satisfying Borel's identity.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/06/20

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Taiji Marugame (The University of Electro-Communications)
Constructions of CR GJMS operators in dimension three (Japanese)
[ Abstract ]
CR GJMS operators are invariant differential operators on CR manifolds whose leading parts are powers of the sublaplacian. Such operators can be constructed by Fefferman's ambient metric or the Cheng-Yau metric, but the construction is obstructed at a finite order due to the ambiguity of these metrics. Gover-Graham constructed some higher order CR GJMS operators by using tractor calculus and BGG constructions.  In particular, they showed that three dimensional CR manifolds admit CR GJMS operators of all orders. In this talk, we give proofs to this fact in two different ways. One is by the use of self-dual Einstein ACH metric and the other is by the Graham-Hirachi inhomogeneous ambient metric adapted to the Fefferman conformal structure. We also state a conjecture on the relationship between these two metrics.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

2022/05/30

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Yusaku Tiba (Ochanomizu University)
Asymptotic estimates of holomorphic sections on Bohr-Sommerfeld Lagrangian submanifolds (Japanese)
[ Abstract ]
In this talk, we study an asymptotic estimate of holomorphic sections of a positive line bundle. Let $M$ be a complex manifold and $L$ be a positive line bundle over $M$ with a Hermitian metric $h$ whose Chern form is a Kähler form $\omega$. Let $X \subset M$ be a Lagrangian submanifold of $(M, \omega)$. When $X$ satisfies the Bohr-Sommerfeld condition, we prove a submean value theorem for holomorphic sections and we give an asymptotic estimate of $\inf_{x \in X}|f(x)|_{h^k}$ for $f \in H^0(M, L^k)$. This estimate provides an analog result about the leading term of the asymptotic series expansion formula of the Bergman kernel function.
[ Reference URL ]
https://forms.gle/hYT2hVhDE3q1wDSh6

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