## 代数幾何学セミナー

過去の記録 ～03/31｜次回の予定｜今後の予定 04/01～

開催情報 | 火曜日 10:30～11:30 or 12:00 数理科学研究科棟(駒場) ハイブリッド開催/002号室 |
---|---|

担当者 | 權業 善範・中村 勇哉・田中公 |

**過去の記録**

### 2018年10月16日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

A countable characterisation of smooth algebraic plane curves, and generalisations (English)

**Tuyen Truong 氏**(Oslo)A countable characterisation of smooth algebraic plane curves, and generalisations (English)

[ 講演概要 ]

Given a smooth algebraic curve X in C^3, I will present a way to construct a sequence of algebraic varieties (whose ideals are explicitly determined from the ideal defining X), whose solution set is non-empty iff the curve X can be algebraically embedded into C^2.

Various other questions, such as whether two given algebraic varieties are birational, can be similarly treated. Some related conjectures are stated.

Given a smooth algebraic curve X in C^3, I will present a way to construct a sequence of algebraic varieties (whose ideals are explicitly determined from the ideal defining X), whose solution set is non-empty iff the curve X can be algebraically embedded into C^2.

Various other questions, such as whether two given algebraic varieties are birational, can be similarly treated. Some related conjectures are stated.

### 2018年10月09日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Stability conditions on threefolds with nef tangent bundles (English)

**小関直紀 氏**(東大数理/IPMU)Stability conditions on threefolds with nef tangent bundles (English)

[ 講演概要 ]

The construction of Bridgeland stability conditions on threefolds

is an open problem in general.

The problem is reduced to proving

the so-called Bogomolov-Gieseker (BG) type inequality conjecture,

proposed by Bayer, Macrí, and Toda.

In this talk, I will explain how to prove the BG type inequality

conjecture

for threefolds in the title.

The construction of Bridgeland stability conditions on threefolds

is an open problem in general.

The problem is reduced to proving

the so-called Bogomolov-Gieseker (BG) type inequality conjecture,

proposed by Bayer, Macrí, and Toda.

In this talk, I will explain how to prove the BG type inequality

conjecture

for threefolds in the title.

### 2018年07月18日(水)

15:30-17:00 数理科学研究科棟(駒場) 122号室

普段と違う水曜日にセミナーを行います。The seminar will be held on Wednesday. This is a different day from usual.

Normal Legendrian singularities (English)

普段と違う水曜日にセミナーを行います。The seminar will be held on Wednesday. This is a different day from usual.

**Jun-Muk Hwang 氏**(KIAS)Normal Legendrian singularities (English)

[ 講演概要 ]

A germ of a Legendrian subvariety in a holomorphic contact manifold

is called a Legendrian singularity. Legendrian singularities are usually not normal.

We look at some examples of normal Legendrian singularities and discuss their rigidity under deformation.

A germ of a Legendrian subvariety in a holomorphic contact manifold

is called a Legendrian singularity. Legendrian singularities are usually not normal.

We look at some examples of normal Legendrian singularities and discuss their rigidity under deformation.

### 2018年07月10日(火)

15:30-17:00 数理科学研究科棟(駒場) 002号室

いつもと部屋が違います。The room is different from usual.

The effective bound of anticanonical volume of Fano threefolds (English)

いつもと部屋が違います。The room is different from usual.

**賴青瑞 氏**(国立成功大学)The effective bound of anticanonical volume of Fano threefolds (English)

[ 講演概要 ]

According to Mori's program, varieties covered by rational curves are

built up from anti-canonically polarized varieties, aka Fano varieties. After fixed the

dimension and singularity type, Fano varieties form a bounded family by Birkar's proof (2016)

of Borisov-Alexeev-Borisov conjecture, which In particular implies that the anticanonical

volume -K^\dim is bounded. In this talk, we focus on canonical Fano threefolds,

where boundedness was established by Koll\'ar-Miyaoka-Mori-Takagi (2000).

Our aim is to find an effective bound of the anticanonical volume -K^3, which is

not explicit either from the work of Koll\'ar-Miyaoka-Mori-Takagi or Birkar. We will discuss

some effectiveness results related to this problem and prove that -K_X^3\leq 72 if \rho(X)\leq 2.

This partially extends early work of Mori, Mukai, Y. Prokhorov, et al.

According to Mori's program, varieties covered by rational curves are

built up from anti-canonically polarized varieties, aka Fano varieties. After fixed the

dimension and singularity type, Fano varieties form a bounded family by Birkar's proof (2016)

of Borisov-Alexeev-Borisov conjecture, which In particular implies that the anticanonical

volume -K^\dim is bounded. In this talk, we focus on canonical Fano threefolds,

where boundedness was established by Koll\'ar-Miyaoka-Mori-Takagi (2000).

Our aim is to find an effective bound of the anticanonical volume -K^3, which is

not explicit either from the work of Koll\'ar-Miyaoka-Mori-Takagi or Birkar. We will discuss

some effectiveness results related to this problem and prove that -K_X^3\leq 72 if \rho(X)\leq 2.

This partially extends early work of Mori, Mukai, Y. Prokhorov, et al.

### 2018年07月03日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Surface automorphisms and Salem numbers (English)

**Xun Yu 氏**(Tianjin University)Surface automorphisms and Salem numbers (English)

[ 講演概要 ]

The entropy of a surface automorphism is either zero or the

logarithm of a Salem number.

In this talk, we will discuss which Salem numbers arise in this way. We

will show that any

supersingular K3 surface in odd characteristic has an automorphism the

entropy of which is

the logarithm of a Salem number of degree 22. In particular, such

automorphisms are

not geometrically liftable to characteristic 0.

The entropy of a surface automorphism is either zero or the

logarithm of a Salem number.

In this talk, we will discuss which Salem numbers arise in this way. We

will show that any

supersingular K3 surface in odd characteristic has an automorphism the

entropy of which is

the logarithm of a Salem number of degree 22. In particular, such

automorphisms are

not geometrically liftable to characteristic 0.

### 2018年06月26日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Varieties with nef diagonal (English)

**渡辺究 氏**(埼玉)Varieties with nef diagonal (English)

[ 講演概要 ]

For a smooth projective variety $X$, we consider when the diagonal $Δ _X$ is nef as a

cycle on $X \times X$. In particular, we give a classication of complete intersections and smooth

del Pezzo varieties where the diagonal is nef. We also study the nefness of the diagonal for

spherical varieties. This is a joint work with Taku Suzuki.

For a smooth projective variety $X$, we consider when the diagonal $Δ _X$ is nef as a

cycle on $X \times X$. In particular, we give a classication of complete intersections and smooth

del Pezzo varieties where the diagonal is nef. We also study the nefness of the diagonal for

spherical varieties. This is a joint work with Taku Suzuki.

### 2018年06月19日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Dormant Miura opers and Tango structures (Japanese (writing in English))

**若林 泰央 氏**(東工大)Dormant Miura opers and Tango structures (Japanese (writing in English))

[ 講演概要 ]

Tango（丹後）構造とは, 正標数の代数曲線で定義された然るべき直線束であり, （小平消滅定理の反例となるなどの, いわゆる）「病理的な」正標数の代数多様体を構成するうえで基本的な概念です. 本発表では, このTango構造と, 一見すると無関係にも見える幾つかのトピック（dormant Miura oper, 3正則グラフのナンバリング, Gaudin模型の対角化etc.）との結びつきや関連する結果について説明させていただきたく予定です.（各トピックについて発表者はそれほど詳しくありませんので, この機会にぜひ皆さまの知見を伺えればと思っております.）

Tango（丹後）構造とは, 正標数の代数曲線で定義された然るべき直線束であり, （小平消滅定理の反例となるなどの, いわゆる）「病理的な」正標数の代数多様体を構成するうえで基本的な概念です. 本発表では, このTango構造と, 一見すると無関係にも見える幾つかのトピック（dormant Miura oper, 3正則グラフのナンバリング, Gaudin模型の対角化etc.）との結びつきや関連する結果について説明させていただきたく予定です.（各トピックについて発表者はそれほど詳しくありませんので, この機会にぜひ皆さまの知見を伺えればと思っております.）

### 2018年06月12日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Ample canonical heights for endomorphisms on projective varieties (English or Japanese)

**柴田 崇広 氏**(京都大学)Ample canonical heights for endomorphisms on projective varieties (English or Japanese)

[ 講演概要 ]

Given a smooth projective variety on a number field and an

endomorphism on it, we would like to know how the height of a point

grows by iteration of the action of the endomorphism. When the

endomorphism is polarized, Call and Silverman construct the canonical

height, which is an important tool for the calculation of growth of

heights. In this talk, we will give a generalization of the Call-

Silverman canonical heights for not necessarily polarized endomorphisms,

ample canonical heights, and propose an analogue of the Northcott

finiteness theorem as a conjecture. We will see that the conjecture

holds when the variety is an abelian variety or a surface.

Given a smooth projective variety on a number field and an

endomorphism on it, we would like to know how the height of a point

grows by iteration of the action of the endomorphism. When the

endomorphism is polarized, Call and Silverman construct the canonical

height, which is an important tool for the calculation of growth of

heights. In this talk, we will give a generalization of the Call-

Silverman canonical heights for not necessarily polarized endomorphisms,

ample canonical heights, and propose an analogue of the Northcott

finiteness theorem as a conjecture. We will see that the conjecture

holds when the variety is an abelian variety or a surface.

### 2018年05月29日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Nikulin configurations on Kummer surfaces (English)

**Alessandra Sarti 氏**(Universit\'e de Poitiers)Nikulin configurations on Kummer surfaces (English)

[ 講演概要 ]

A Nikulin configuration is the data of

16 disjoint smooth rational curves on a K3 surface.

According to results of Nikulin this means that the K3 surface

is a Kummer surface and the abelian surface in the Kummer structure

is determined by the 16 curves. An old question of Shioda is about the

existence of non isomorphic Kummer structures on the same Kummer K3

surface.

The question was positively answered and studied by several authors, and

it was shown that the number of non-isomorphic Kummer structures is

finite,

but no explicit geometric construction of such structures was given.

In the talk I will show how to construct explicitely non isomorphic

Kummer structures on generic Kummer K3 surfaces.

This is a joint work with X. Roulleau.

A Nikulin configuration is the data of

16 disjoint smooth rational curves on a K3 surface.

According to results of Nikulin this means that the K3 surface

is a Kummer surface and the abelian surface in the Kummer structure

is determined by the 16 curves. An old question of Shioda is about the

existence of non isomorphic Kummer structures on the same Kummer K3

surface.

The question was positively answered and studied by several authors, and

it was shown that the number of non-isomorphic Kummer structures is

finite,

but no explicit geometric construction of such structures was given.

In the talk I will show how to construct explicitely non isomorphic

Kummer structures on generic Kummer K3 surfaces.

This is a joint work with X. Roulleau.

### 2018年05月25日(金)

15:30-17:00 数理科学研究科棟(駒場) 122号室

普段と違う金曜日にセミナーを行います。The seminar will be held on Friday. This is a different day from usual.

Endomorphisms of normal projective variety and equivariant-MMP (English)

普段と違う金曜日にセミナーを行います。The seminar will be held on Friday. This is a different day from usual.

**De Qi Zhang 氏**(Singapore)Endomorphisms of normal projective variety and equivariant-MMP (English)

[ 講演概要 ]

We report some recent joint works on polarized or int-amplified endomorphisms f on a normal projective variety X with mild singularities, and prove the pseudo-effectivity of the anti-canonical divisor of X, and the f-equivariance, after replacing f by its power, for every minimal model program starting from X. Fano varieties and Q-abelian varieties turn out to be building blocks having such symmetries. The ground field is closed and of characteristic 0 or at least 7.

We report some recent joint works on polarized or int-amplified endomorphisms f on a normal projective variety X with mild singularities, and prove the pseudo-effectivity of the anti-canonical divisor of X, and the f-equivariance, after replacing f by its power, for every minimal model program starting from X. Fano varieties and Q-abelian varieties turn out to be building blocks having such symmetries. The ground field is closed and of characteristic 0 or at least 7.

### 2018年05月21日(月)

15:30-17:00 数理科学研究科棟(駒場) 122号室

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

Towards the termination of flips. (English)

https://www.math.utah.edu/~hacon/

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

**Christopher Hacon 氏**(Utah/Kyoto)Towards the termination of flips. (English)

[ 講演概要 ]

The minimal model program (MMP) predicts that if $X$ is a smooth complex projective variety which is not uniruled, then there is a finite sequence of "elementary" birational maps

$X=X_0-->X_1-->X_2-->...-->X_n$ known as divisorial contractions and flips whose output $\bar X=X_n$ is a minimal model so that $K_{\bar X}$ is a nef $Q$-divisor i.e it intersects all curves $C\subset \bar X$ non-negatively: $K_{\bar X}\cdot C\geq 0$.

The existence of these birational maps has been established, but in order to complete the MMP, it is necessary to show that flips terminate i.e. there are no infinite sequences of flips. In this talk we will discuss recent results towards the termination of flips.

[ 参考URL ]The minimal model program (MMP) predicts that if $X$ is a smooth complex projective variety which is not uniruled, then there is a finite sequence of "elementary" birational maps

$X=X_0-->X_1-->X_2-->...-->X_n$ known as divisorial contractions and flips whose output $\bar X=X_n$ is a minimal model so that $K_{\bar X}$ is a nef $Q$-divisor i.e it intersects all curves $C\subset \bar X$ non-negatively: $K_{\bar X}\cdot C\geq 0$.

The existence of these birational maps has been established, but in order to complete the MMP, it is necessary to show that flips terminate i.e. there are no infinite sequences of flips. In this talk we will discuss recent results towards the termination of flips.

https://www.math.utah.edu/~hacon/

### 2018年05月21日(月)

13:30-15:00 数理科学研究科棟(駒場) 122号室

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

Perverse sheaves of categories and birational geometry (English)

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

**Will Donovan 氏**(IPMU)Perverse sheaves of categories and birational geometry (English)

[ 講演概要 ]

Kapranov and Schechtman have initiated a program to study perverse sheaves of categories, or perverse schobers. It is expected that examples arise from birational geometry, in particular from webs of flops. I explain progress towards constructing these objects for Grothendieck resolutions (work of the above authors with Bondal), and for 3-folds (joint work of myself and Wemyss).

Kapranov and Schechtman have initiated a program to study perverse sheaves of categories, or perverse schobers. It is expected that examples arise from birational geometry, in particular from webs of flops. I explain progress towards constructing these objects for Grothendieck resolutions (work of the above authors with Bondal), and for 3-folds (joint work of myself and Wemyss).

### 2018年05月08日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Higher order families of lines and Fano manifolds covered by linear

spaces

(Japanese (writing in English))

**鈴木拓 氏**(宇都宮大)Higher order families of lines and Fano manifolds covered by linear

spaces

(Japanese (writing in English))

[ 講演概要 ]

In this talk, for an embedded Fano manifold $X$, we introduce higher

order families of lines and a new invariant $S_X$. They are line

versions of higher order minimal families of rational curves and the

invariant $N_X$ which were introduced in my previous talk on 4th

November 2016. In addition, $S_X$ is related to the dimension of

covering linear spaces. Our goal is to classify Fano manifolds $X$ which

have large $S_X$.

In this talk, for an embedded Fano manifold $X$, we introduce higher

order families of lines and a new invariant $S_X$. They are line

versions of higher order minimal families of rational curves and the

invariant $N_X$ which were introduced in my previous talk on 4th

November 2016. In addition, $S_X$ is related to the dimension of

covering linear spaces. Our goal is to classify Fano manifolds $X$ which

have large $S_X$.

### 2018年04月24日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

BIRATIONAL BOUNDEDNESS OF RATIONALLY CONNECTED CALABI–YAU 3-FOLDS

(English)

**陳韋中 氏**(東大数理)BIRATIONAL BOUNDEDNESS OF RATIONALLY CONNECTED CALABI–YAU 3-FOLDS

(English)

[ 講演概要 ]

Firstly, we show that rationally connected Calabi–Yau 3- folds with kawamata log terminal (klt) singularities form a birationally bounded family, or more generally, rationally connected 3-folds of ε-CY type form a birationally bounded family for ε > 0. Then we focus on ε-lc log Calabi–Yau pairs (X, B) such that coefficients of B are bounded from below away from zero. We show that such pairs are log bounded modulo flops. As a consequence, we show that rationally connected klt Calabi–Yau 3-folds with mld bounding away from 1 are bounded modulo flops.

Firstly, we show that rationally connected Calabi–Yau 3- folds with kawamata log terminal (klt) singularities form a birationally bounded family, or more generally, rationally connected 3-folds of ε-CY type form a birationally bounded family for ε > 0. Then we focus on ε-lc log Calabi–Yau pairs (X, B) such that coefficients of B are bounded from below away from zero. We show that such pairs are log bounded modulo flops. As a consequence, we show that rationally connected klt Calabi–Yau 3-folds with mld bounding away from 1 are bounded modulo flops.

### 2018年04月17日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

SNC log symplectic structures on Fano products (English/Japanese)

**奥村 克彦 氏**(早稲田理工)SNC log symplectic structures on Fano products (English/Japanese)

[ 講演概要 ]

In 2014, Lima and Pereira gave a characterization of the even-dimensional projective space in terms of log symplectic Poisson structures. After that Pym gave an another more algebraic proof. In this talk, we will extend the result of Lima and Pereira to the case that the variety is a product of Fano varieties with the cyclic Picard group. This will be proved by extending Pym's proof. As a corollary, we will obtain a characterization of the projective space of all dimensions.

In 2014, Lima and Pereira gave a characterization of the even-dimensional projective space in terms of log symplectic Poisson structures. After that Pym gave an another more algebraic proof. In this talk, we will extend the result of Lima and Pereira to the case that the variety is a product of Fano varieties with the cyclic Picard group. This will be proved by extending Pym's proof. As a corollary, we will obtain a characterization of the projective space of all dimensions.

### 2018年04月09日(月)

15:30-17:00 数理科学研究科棟(駒場) 122号室

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

Adjoint forms on algebraic varieties (English)

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

**Luca Rizzi 氏**(Udine)Adjoint forms on algebraic varieties (English)

[ 講演概要 ]

The so called adjoint theory was introduced by A. Collino and G.P. Pirola in the case of smooth algebraic curves and then generalized by G.P. Pirola and F. Zucconi in the case of smooth algebraic varieties of arbitrary dimension.

The main idea of this theory is to study particular differential forms, called adjoint forms, on an algebraic variety to obtain information on the infinitesimal deformations of the variety itself.

The natural context for the application of this theory is given by Torelli-type problems, in particular infinitesimal Torelli problems.

The so called adjoint theory was introduced by A. Collino and G.P. Pirola in the case of smooth algebraic curves and then generalized by G.P. Pirola and F. Zucconi in the case of smooth algebraic varieties of arbitrary dimension.

The main idea of this theory is to study particular differential forms, called adjoint forms, on an algebraic variety to obtain information on the infinitesimal deformations of the variety itself.

The natural context for the application of this theory is given by Torelli-type problems, in particular infinitesimal Torelli problems.

### 2018年04月09日(月)

13:30-15:00 数理科学研究科棟(駒場) 122号室

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

Commuting nilpotents, punctual Hilbert schemes and jet bundles (ENGLISH)

今週は月曜日にセミナーを行います。13:30-15:00と15:30-17:00の2講演あります。This week's seminar will be held on Monday and consist of two lectures: 13:30-15:00 and 15:30-17:00.

**David Hyeon 氏**(ソウル大学校)Commuting nilpotents, punctual Hilbert schemes and jet bundles (ENGLISH)

[ 講演概要 ]

Pairs of commuting nilpotent matrices have been extensively studied, especially from the view point of quivers, but the space of commuting nilpotents modulo simultaneous conjugation has not received any attention at all despite its moduli theory flavor. I will explain how a 'moduli space' can be constructed via two different methods and demonstrate many interesting properties of the space:

- It is isomorphic to an open subscheme of a punctual Hilbert scheme.

- Over the field of complex numbers, it is diffeomorphic to a direct sum of twisted tangent bundles over a projective space.

- It is isomorphic to a bundle of regular jets.

- It gives examples of affine space bundles that are not vector bundles.

This is a joint work with W. Haboush (Illinois) and G. Bérczi (Zurich).

Pairs of commuting nilpotent matrices have been extensively studied, especially from the view point of quivers, but the space of commuting nilpotents modulo simultaneous conjugation has not received any attention at all despite its moduli theory flavor. I will explain how a 'moduli space' can be constructed via two different methods and demonstrate many interesting properties of the space:

- It is isomorphic to an open subscheme of a punctual Hilbert scheme.

- Over the field of complex numbers, it is diffeomorphic to a direct sum of twisted tangent bundles over a projective space.

- It is isomorphic to a bundle of regular jets.

- It gives examples of affine space bundles that are not vector bundles.

This is a joint work with W. Haboush (Illinois) and G. Bérczi (Zurich).

### 2018年01月26日(金)

16:30-18:00 数理科学研究科棟(駒場) 122号室

On classification of prime Q-Fano 3-folds with only 1/2(1,1,1)-singularities and of genus less than 2

**髙木寛通 氏**(東大数理)On classification of prime Q-Fano 3-folds with only 1/2(1,1,1)-singularities and of genus less than 2

[ 講演概要 ]

I classified prime Q-Fano threefolds with only 1/2(1,1,1)-singularities and of genus greater than 1 (2002, Nagoya Math. J.).

In this talk, I will explain how the method in that paper can be extended to the case of genus less than 2. The method is so called two ray game. By this method, I can classify the possibilities of such Q-Fano's. The classification is not yet completed since constructions of examples in certain cases are difficult. I will also explain some pretty examples in this talk.

I classified prime Q-Fano threefolds with only 1/2(1,1,1)-singularities and of genus greater than 1 (2002, Nagoya Math. J.).

In this talk, I will explain how the method in that paper can be extended to the case of genus less than 2. The method is so called two ray game. By this method, I can classify the possibilities of such Q-Fano's. The classification is not yet completed since constructions of examples in certain cases are difficult. I will also explain some pretty examples in this talk.

### 2017年12月26日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

K-stability of log Fano hyperplane arrangements (English)

**藤田 健人 氏**(RIMS)K-stability of log Fano hyperplane arrangements (English)

[ 講演概要 ]

We completely determine which log Fano hyperplane arrangements are uniformly K-stable, K-stable, K-polystable, K-semistable or not.

We completely determine which log Fano hyperplane arrangements are uniformly K-stable, K-stable, K-polystable, K-semistable or not.

### 2017年12月14日(木)

15:30-17:00 数理科学研究科棟(駒場) 123号室

普段と曜日・部屋が異なります

Algebraic curves and modular forms of low degree (English)

普段と曜日・部屋が異なります

**Gerard van der Geer 氏**(Universiteit van Amsterdam)Algebraic curves and modular forms of low degree (English)

[ 講演概要 ]

For genus 2 and 3 modular forms are intimately connected with the moduli of curves of genus 2 and 3. We give an explicit way to describe such modular forms for genus 2 and 3 using invariant theory and give some applications. This is based on joint work with Fabien Clery and Carel Faber.

For genus 2 and 3 modular forms are intimately connected with the moduli of curves of genus 2 and 3. We give an explicit way to describe such modular forms for genus 2 and 3 using invariant theory and give some applications. This is based on joint work with Fabien Clery and Carel Faber.

### 2017年12月14日(木)

10:30-12:00 数理科学研究科棟(駒場) 123号室

Perfectoid test ideals (English)

**Linquan Ma 氏**(University of Utah)Perfectoid test ideals (English)

[ 講演概要 ]

Inspired by the recent solution of the direct summand conjecture

of Andre and Bhatt, we introduce perfectoid multiplier/test ideals in mixed

characteristic. As an application, we obtain a uniform bound on the growth

of symbolic powers in regular local rings of mixed characteristic analogous

to results of Ein--Lazarsfeld--Smith and Hochster--Huneke in equal

characteristic. This is joint work with Karl Schwede.

Inspired by the recent solution of the direct summand conjecture

of Andre and Bhatt, we introduce perfectoid multiplier/test ideals in mixed

characteristic. As an application, we obtain a uniform bound on the growth

of symbolic powers in regular local rings of mixed characteristic analogous

to results of Ein--Lazarsfeld--Smith and Hochster--Huneke in equal

characteristic. This is joint work with Karl Schwede.

### 2017年12月05日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Ascending chain condition for F-pure thresholds on a fixed strongly F-regular germ (English or Japanese)

**佐藤 謙太 氏**(東大数理)Ascending chain condition for F-pure thresholds on a fixed strongly F-regular germ (English or Japanese)

[ 講演概要 ]

For a germ of a variety in positive characteristic and a non-zero ideal sheaf on the variety, we can define the F-pure threshold of the ideal by using Frobenius morphisms, which measures the singularities of the pair. In this talk, I will show that the set of all F-pure thresholds on a fixed strongly F-regular germ satisfies the ascending chain condition. This is a positive characteristic analogue of the "ascending chain condition for log canonical thresholds" in characteristic 0, which was recently proved by Hacon, McKernan, and Xu.

For a germ of a variety in positive characteristic and a non-zero ideal sheaf on the variety, we can define the F-pure threshold of the ideal by using Frobenius morphisms, which measures the singularities of the pair. In this talk, I will show that the set of all F-pure thresholds on a fixed strongly F-regular germ satisfies the ascending chain condition. This is a positive characteristic analogue of the "ascending chain condition for log canonical thresholds" in characteristic 0, which was recently proved by Hacon, McKernan, and Xu.

### 2017年11月28日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Kodaira vanishing theorem for Witt canonical sheaves (English)

**田中 公 氏**(東大数理)Kodaira vanishing theorem for Witt canonical sheaves (English)

[ 講演概要 ]

We establish an analogue of the Kodaira vanishing theorem in terms of de Rham-Witt complex. More specifically, given a smooth projective variety over a perfect field of positive characteristic, we prove that the higher cohomologies vanish for the tensor product of the Witt canonical sheaf and the Teichmuller lift of an ample invertible sheaf.

We establish an analogue of the Kodaira vanishing theorem in terms of de Rham-Witt complex. More specifically, given a smooth projective variety over a perfect field of positive characteristic, we prove that the higher cohomologies vanish for the tensor product of the Witt canonical sheaf and the Teichmuller lift of an ample invertible sheaf.

### 2017年11月21日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

Orbifold rational connectedness (English)

**Frédéric Campana 氏**(Université de Lorraine/KIAS)Orbifold rational connectedness (English)

[ 講演概要 ]

The first step in the decomposition by canonical fibrations with fibres of `signed' canonical bundle of an arbitrary complex projective manifolds $X$ is its `rational quotient' (also called `MRC' fibration): it has rationally connected fibres and non-uniruled base. In general, the further steps (such as the Moishezon-Iitaka fibration) of this decomposition will require the consideration of 'orbifold base' of fibrations in order to deal with the multiple fibres (as seen already for elliptic surfaces). One thus needs to work in the larger category of (smooth) `orbifold pairs' $(X,D)$ to achieve this decomposition. The aim of the talk is thus to introduce the notions of Rational Connectedness and 'rational quotient' in this context, by means of suitable equivalent notions of negativity for the orbifold cotangent bundle (suitably defined. When $D$ is reduced, this is just the usual Log-version). The expected equivalence with connecting families of `orbifold rational curves' remains however presently open.

The first step in the decomposition by canonical fibrations with fibres of `signed' canonical bundle of an arbitrary complex projective manifolds $X$ is its `rational quotient' (also called `MRC' fibration): it has rationally connected fibres and non-uniruled base. In general, the further steps (such as the Moishezon-Iitaka fibration) of this decomposition will require the consideration of 'orbifold base' of fibrations in order to deal with the multiple fibres (as seen already for elliptic surfaces). One thus needs to work in the larger category of (smooth) `orbifold pairs' $(X,D)$ to achieve this decomposition. The aim of the talk is thus to introduce the notions of Rational Connectedness and 'rational quotient' in this context, by means of suitable equivalent notions of negativity for the orbifold cotangent bundle (suitably defined. When $D$ is reduced, this is just the usual Log-version). The expected equivalence with connecting families of `orbifold rational curves' remains however presently open.

### 2017年11月14日(火)

15:30-17:00 数理科学研究科棟(駒場) 122号室

A characterization of the birationality of 4-canonical maps of minimal 3-folds (English)

**Meng Chen 氏**(Fudan )A characterization of the birationality of 4-canonical maps of minimal 3-folds (English)

[ 講演概要 ]

We explain the following theorem: For any minimal 3-fold X of general type with p_g>4, the 4-canonical map is non-birational if and only if X is birationally fibred by a pencil of (1,2) surfaces. The statement fails in the case of p_g=4.

We explain the following theorem: For any minimal 3-fold X of general type with p_g>4, the 4-canonical map is non-birational if and only if X is birationally fibred by a pencil of (1,2) surfaces. The statement fails in the case of p_g=4.