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代数幾何学セミナー
過去の記録 ～11/26｜次回の予定｜今後の予定 11/27～
| 開催情報 | 金曜日 13:30～15:00 数理科学研究科棟(駒場) 118号室 |
|---|---|
| 担当者 | 權業 善範、河上 龍郎 、榎園 誠 |
過去の記録
2025年10月31日(金)
13:00-14:30 数理科学研究科棟(駒場) 118号室
いつもと開始時間が異なります。
Miguel Angel Barja 氏 (UPC-Barcelona)
Asymptotic and continuous constructions in the geography of fibred varieties
いつもと開始時間が異なります。
Miguel Angel Barja 氏 (UPC-Barcelona)
Asymptotic and continuous constructions in the geography of fibred varieties
[ 講演概要 ]
Given a fibred variety $X$ onto a smooth variety $T$ it is possible to consider different types of inequalities between birational invariants associated to a line bundle $L$, such as Noether, Slope or Severi inequalities. Most of these inequalities are closely related through asymptotic constructions and/or continuous functions that suggest the use of some new invariants. We will survey different constructions both in characteristic 0 and positive characteristic, and will focus in the case of varieties of maximal Albanese dimension, fibred over curves. If time permits, we will also give some ideas on fibrations over surfaces.
Given a fibred variety $X$ onto a smooth variety $T$ it is possible to consider different types of inequalities between birational invariants associated to a line bundle $L$, such as Noether, Slope or Severi inequalities. Most of these inequalities are closely related through asymptotic constructions and/or continuous functions that suggest the use of some new invariants. We will survey different constructions both in characteristic 0 and positive characteristic, and will focus in the case of varieties of maximal Albanese dimension, fibred over curves. If time permits, we will also give some ideas on fibrations over surfaces.
2025年10月31日(金)
15:00-16:30 数理科学研究科棟(駒場) 118号室
いつもと開始時間が異なります。
服部真史 氏 (ノッティンガム大学)
Normal stable degeneration of Noether-Horikawa surfaces: Deformation Part
いつもと開始時間が異なります。
服部真史 氏 (ノッティンガム大学)
Normal stable degeneration of Noether-Horikawa surfaces: Deformation Part
[ 講演概要 ]
Koll’ar and Shepherd-Barron constructed a general theory for a canonical geometric compactification of moduli of smooth surfaces with ample canonical class by adding degenerations with only semi log canonical singularities. Their moduli is now called the KSBA moduli and degenerations are called stable degenerations. It has been a long standing question to classify all stable degenerations for smooth canonically polarized surfaces. In this talk, we focus on Q-Gorenstein deformation theory on Horikawa surfaces, which are minimal surfaces of general type in the case where the Noether inequality $K^2\geq 2p_g-4$ is an equality. This talk is based on the joint work (arXiv:2507:17633) with Hiroto Akaike, Makoto Enokizono, and Yuki Koto.
Koll’ar and Shepherd-Barron constructed a general theory for a canonical geometric compactification of moduli of smooth surfaces with ample canonical class by adding degenerations with only semi log canonical singularities. Their moduli is now called the KSBA moduli and degenerations are called stable degenerations. It has been a long standing question to classify all stable degenerations for smooth canonically polarized surfaces. In this talk, we focus on Q-Gorenstein deformation theory on Horikawa surfaces, which are minimal surfaces of general type in the case where the Noether inequality $K^2\geq 2p_g-4$ is an equality. This talk is based on the joint work (arXiv:2507:17633) with Hiroto Akaike, Makoto Enokizono, and Yuki Koto.
2025年10月10日(金)
10:00-11:30 数理科学研究科棟(駒場) 122号室
いつもと時間・部屋が異なります。
Yuri Tschinkel 氏 (New York University)
Equivariant birational geometry
いつもと時間・部屋が異なります。
Yuri Tschinkel 氏 (New York University)
Equivariant birational geometry
[ 講演概要 ]
I will report on new results and constructions in higher-dimensional birational geometry in presence of actions of finite groups.
I will report on new results and constructions in higher-dimensional birational geometry in presence of actions of finite groups.
2025年08月19日(火)
13:30-15:00 数理科学研究科棟(駒場) 117号室
いつもと曜日・部屋が異なります。
Trung Tuyen Truong 氏 (University of Oslo)
Some new results concerning Tate's questions and generalisations
いつもと曜日・部屋が異なります。
Trung Tuyen Truong 氏 (University of Oslo)
Some new results concerning Tate's questions and generalisations
[ 講演概要 ]
In the 1960s, Tate formulated (inspired by Weil's conjectures and a result of Serre on compact Kahler manifolds) a couple of questions concerning eigenvalues for pullback on cohomology of polarized endomorphisms. Grothendieck and Bombieri proposed Standard conjectures to solve these questions by Tate. The speaker, inspired by complex dynamics, proposed a generalisation of one of Tate's questions to rational maps and dynamical correspondences. This talk presents some new results and approaches (which are less demanding than the Standard conjectures, in that Standard Conjecture of Hodge type is not required) concerning these Tate's questions and generalisation. The talk includes joint works with Fei Hu and Junyi Xie.
In the 1960s, Tate formulated (inspired by Weil's conjectures and a result of Serre on compact Kahler manifolds) a couple of questions concerning eigenvalues for pullback on cohomology of polarized endomorphisms. Grothendieck and Bombieri proposed Standard conjectures to solve these questions by Tate. The speaker, inspired by complex dynamics, proposed a generalisation of one of Tate's questions to rational maps and dynamical correspondences. This talk presents some new results and approaches (which are less demanding than the Standard conjectures, in that Standard Conjecture of Hodge type is not required) concerning these Tate's questions and generalisation. The talk includes joint works with Fei Hu and Junyi Xie.
2025年07月25日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
金光秋博 氏 (東京都立大学)
Quintic del Pezzo threefolds in positive and mixed characteristic
金光秋博 氏 (東京都立大学)
Quintic del Pezzo threefolds in positive and mixed characteristic
[ 講演概要 ]
We will show that, over any base scheme, (families of) quintic del Pezzo threefolds V5 are classified by non-degenerate ternary symmetric bilinear forms.
As applications, we will discuss (1) the geometry of quintic del Pezzo threefolds in positive characteristic, especially in characteristic two, and (2) finiteness results of V5 over number fields/rings of integers.
(Based on joint work with Tetsushi Ito, Teppei Takamatsu, Yuuji Tanaka)
We will show that, over any base scheme, (families of) quintic del Pezzo threefolds V5 are classified by non-degenerate ternary symmetric bilinear forms.
As applications, we will discuss (1) the geometry of quintic del Pezzo threefolds in positive characteristic, especially in characteristic two, and (2) finiteness results of V5 over number fields/rings of integers.
(Based on joint work with Tetsushi Ito, Teppei Takamatsu, Yuuji Tanaka)
2025年06月20日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
岡村 郁弥 氏 (中央大学)
Moduli spaces of rational curves on Artin-Mumford double solids
岡村 郁弥 氏 (中央大学)
Moduli spaces of rational curves on Artin-Mumford double solids
[ 講演概要 ]
Artin-Mumford double solids were originally constructed as examples of unirational but irrational varieties. Their method showed that the Brauer group gives an obstruction to rationality. Later, Voisin observed that this obstruction measures the difference between algebraic and numerical equivalence for 1-cycles.
In this talk, we study the moduli spaces of rational curves on Artin-Mumford double solids. First, we discuss the relation between the spaces of lines on these varieties and certain Enriques surfaces known as Reye congruences. Then, we classify all irreducible components of the moduli spaces of rational curves of each degree, and prove Geometric Manin's Conjecture in this setting.
Artin-Mumford double solids were originally constructed as examples of unirational but irrational varieties. Their method showed that the Brauer group gives an obstruction to rationality. Later, Voisin observed that this obstruction measures the difference between algebraic and numerical equivalence for 1-cycles.
In this talk, we study the moduli spaces of rational curves on Artin-Mumford double solids. First, we discuss the relation between the spaces of lines on these varieties and certain Enriques surfaces known as Reye congruences. Then, we classify all irreducible components of the moduli spaces of rational curves of each degree, and prove Geometric Manin's Conjecture in this setting.
2025年06月10日(火)
14:00-15:30 数理科学研究科棟(駒場) 122号室
いつもと日時・部屋が異なるのでご注意ください。
Meng Chen 氏 (Fudan University)
Some new methods in estimating the lower bound of the canonical volume of 3-folds of general type
いつもと日時・部屋が異なるのでご注意ください。
Meng Chen 氏 (Fudan University)
Some new methods in estimating the lower bound of the canonical volume of 3-folds of general type
[ 講演概要 ]
We introduce some new advance in estimating the lower bound of the canonical volume of 3-folds of general type with very small geometric genus. This topic covers a joint work with Jungkai A. Chen, Yong Hu and Chen Jiang.
We introduce some new advance in estimating the lower bound of the canonical volume of 3-folds of general type with very small geometric genus. This topic covers a joint work with Jungkai A. Chen, Yong Hu and Chen Jiang.
2025年06月10日(火)
16:00-17:30 数理科学研究科棟(駒場) 122号室
いつもと日時・部屋が異なるのでご注意ください。
Xun Yu 氏 (Tianjin University)
On the real forms of smooth complex projective varieties
いつもと日時・部屋が異なるのでご注意ください。
Xun Yu 氏 (Tianjin University)
On the real forms of smooth complex projective varieties
[ 講演概要 ]
The real form problem asks how many different ways one can describe a given complex variety by polynomial equations with real coefficients, up to isomorphisms over the real number field. In this talk, I will discuss some recent results about real forms of smooth complex projective varieties. This talk is based on my joint works with T.-C. Dinh, C. Gachet, G. van der Geer, H.-Y. Lin, K. Oguiso, and L. Wang.
The real form problem asks how many different ways one can describe a given complex variety by polynomial equations with real coefficients, up to isomorphisms over the real number field. In this talk, I will discuss some recent results about real forms of smooth complex projective varieties. This talk is based on my joint works with T.-C. Dinh, C. Gachet, G. van der Geer, H.-Y. Lin, K. Oguiso, and L. Wang.
2025年06月06日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
吉田 智輝 氏 (早稲田大学)
Bridgeland Stability of Sheaves on del Pezzo Surface of Picard Rank Three
吉田 智輝 氏 (早稲田大学)
Bridgeland Stability of Sheaves on del Pezzo Surface of Picard Rank Three
[ 講演概要 ]
Bridgeland stability is a notion of stability for objects in a triangulated category, particularly in the bounded derived category of coherent sheaves. Unlike classical sheaf stability, it is often unclear whether fundamental sheaves, such as line bundles, are (semi)stable with respect to a given Bridgeland stability condition. In this talk, we focus on the del Pezzo surface of Picard rank three and study the Bridgeland stability of its line bundles and certain torsion sheaves. More precisely, we first determine the maximal destabilizing objects for line bundles and then outline our proof strategy in the torsion case.
This talk is based on arXiv:2502.18894, which is joint work with Yuki Mizuno.
Bridgeland stability is a notion of stability for objects in a triangulated category, particularly in the bounded derived category of coherent sheaves. Unlike classical sheaf stability, it is often unclear whether fundamental sheaves, such as line bundles, are (semi)stable with respect to a given Bridgeland stability condition. In this talk, we focus on the del Pezzo surface of Picard rank three and study the Bridgeland stability of its line bundles and certain torsion sheaves. More precisely, we first determine the maximal destabilizing objects for line bundles and then outline our proof strategy in the torsion case.
This talk is based on arXiv:2502.18894, which is joint work with Yuki Mizuno.
2025年05月23日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
宮本 拓哉 氏 (東京大学)
Pathology of formal locally-trivial
deformations in positive characteristic
宮本 拓哉 氏 (東京大学)
Pathology of formal locally-trivial
deformations in positive characteristic
[ 講演概要 ]
An infinitesimal deformation of an algebraic variety X is called (formally) locally trivial if it is Zariski-locally isomorphic to the trivial deformation. The locally trivial deformation functor of X is the subfunctor of the usual deformation functor associated with X consisting of locally trivial deformations. In this talk, I will construct an explicit example that is an algebraic curve in positive characteristic whose locally trivial deformation functor does not satisfy Schlessinger’s first condition (H_1), in contrast to the complex/characteristic 0 case. In particular, this provides a negative answer to a question posed by H. Flenner and S. Kosarew. I will also mention recent progress on the structure of fibers of locally trivial deformation functors.
An infinitesimal deformation of an algebraic variety X is called (formally) locally trivial if it is Zariski-locally isomorphic to the trivial deformation. The locally trivial deformation functor of X is the subfunctor of the usual deformation functor associated with X consisting of locally trivial deformations. In this talk, I will construct an explicit example that is an algebraic curve in positive characteristic whose locally trivial deformation functor does not satisfy Schlessinger’s first condition (H_1), in contrast to the complex/characteristic 0 case. In particular, this provides a negative answer to a question posed by H. Flenner and S. Kosarew. I will also mention recent progress on the structure of fibers of locally trivial deformation functors.
2025年05月16日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
後藤 慶太 氏 (東京大学)
Berkovich geometry and SYZ fibration
後藤 慶太 氏 (東京大学)
Berkovich geometry and SYZ fibration
[ 講演概要 ]
The SYZ fibration refers to a special Lagrangian torus fibration on a Calabi–Yau manifold and has been extensively studied in the context of mirror symmetry.
In particular, for a degenerating family of Calabi--Yau manifolds, a family of SYZ fibrations defined on each fiber, away from a subset of sufficiently small measure, plays a central role.
However, the existence of such fibrations remains an open problem, known as the metric SYZ conjecture.
To approach this problem, formal analytic techniques are particularly effective, and Berkovich geometry lies at their foundation.
In this talk, I will explain Yang Li’s "comparison property," a sufficient condition for the conjecture, and present some related results I have been involved in. Along the way, I will also introduce some foundational ideas in Berkovich geometry.
The SYZ fibration refers to a special Lagrangian torus fibration on a Calabi–Yau manifold and has been extensively studied in the context of mirror symmetry.
In particular, for a degenerating family of Calabi--Yau manifolds, a family of SYZ fibrations defined on each fiber, away from a subset of sufficiently small measure, plays a central role.
However, the existence of such fibrations remains an open problem, known as the metric SYZ conjecture.
To approach this problem, formal analytic techniques are particularly effective, and Berkovich geometry lies at their foundation.
In this talk, I will explain Yang Li’s "comparison property," a sufficient condition for the conjecture, and present some related results I have been involved in. Along the way, I will also introduce some foundational ideas in Berkovich geometry.
2025年04月25日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
河上龍郎 氏 (東京大学)
Higher F-injective singularities
河上龍郎 氏 (東京大学)
Higher F-injective singularities
[ 講演概要 ]
The theory of F-singularities is a field that studies singularities in positive characteristic defined via the Frobenius morphism. A particularly well-known aspect of the theory is its correspondence, via reduction, with singularities that appear in birational geometry in characteristic zero.
In recent years, higher versions of such singularities in characteristic zero--such as higher Du Bois singularities--have been actively studied. In this talk, I will discuss how a higher analogue of F-singularity theory can be developed in positive characteristic by using the Cartier operator, which serves as a higher version of the Frobenius morphism.
In particular, I will introduce higher F-injective singularities, which correspond to higher Du Bois singularities, and focus on how the correspondence via reduction can be established.
This is joint work with Jakub Witaszek.
The theory of F-singularities is a field that studies singularities in positive characteristic defined via the Frobenius morphism. A particularly well-known aspect of the theory is its correspondence, via reduction, with singularities that appear in birational geometry in characteristic zero.
In recent years, higher versions of such singularities in characteristic zero--such as higher Du Bois singularities--have been actively studied. In this talk, I will discuss how a higher analogue of F-singularity theory can be developed in positive characteristic by using the Cartier operator, which serves as a higher version of the Frobenius morphism.
In particular, I will introduce higher F-injective singularities, which correspond to higher Du Bois singularities, and focus on how the correspondence via reduction can be established.
This is joint work with Jakub Witaszek.
2025年01月22日(水)
13:30-15:00 数理科学研究科棟(駒場) 002号室
田中公 氏 (東京大学)
Liftability and vanishing theorems for Fano threefolds in positive characteristic (日本語)
田中公 氏 (東京大学)
Liftability and vanishing theorems for Fano threefolds in positive characteristic (日本語)
[ 講演概要 ]
Smooth Fano threefolds in positive characteristic satisfy Kodaira vanishing and lift to characteristic zero. This is joint work with Tatsuro Kawakami.
Smooth Fano threefolds in positive characteristic satisfy Kodaira vanishing and lift to characteristic zero. This is joint work with Tatsuro Kawakami.
2024年12月20日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
榎園誠 氏 (東京大学)
Normal stable degenerations of Noether-Horikawa surfaces
榎園誠 氏 (東京大学)
Normal stable degenerations of Noether-Horikawa surfaces
[ 講演概要 ]
Noether-Horikawa surfaces are surfaces of general type satisfying the equation K2=2pg−4, which represents the boundary of the Noether inequality K2≥2pg−4 for surfaces of general type. In the 1970s, Horikawa conducted a detailed study of smooth Noether-Horikawa surfaces, providing a classification of these surfaces and describing their moduli spaces.
In this talk, I will present an explicit classification of normal stable degenerations of Noether-Horikawa surfaces. Specifically, I will discuss the following results:
(1) A preliminary classification of Noether-Horikawa surfaces with Q-Gorenstein smoothable log canonical singularities.
(2) Several criteria for determining the (global) Q-Gorenstein smoothability of the surfaces described in (1).
(3) Deformation results for Q-Gorenstein smoothable normal stable Noether-Horikawa surfaces, along with a description of the KSBA moduli spaces for these surfaces.
This is joint work with Hiroto Akaike, Masafumi Hattori and Yuki Koto.
Noether-Horikawa surfaces are surfaces of general type satisfying the equation K2=2pg−4, which represents the boundary of the Noether inequality K2≥2pg−4 for surfaces of general type. In the 1970s, Horikawa conducted a detailed study of smooth Noether-Horikawa surfaces, providing a classification of these surfaces and describing their moduli spaces.
In this talk, I will present an explicit classification of normal stable degenerations of Noether-Horikawa surfaces. Specifically, I will discuss the following results:
(1) A preliminary classification of Noether-Horikawa surfaces with Q-Gorenstein smoothable log canonical singularities.
(2) Several criteria for determining the (global) Q-Gorenstein smoothability of the surfaces described in (1).
(3) Deformation results for Q-Gorenstein smoothable normal stable Noether-Horikawa surfaces, along with a description of the KSBA moduli spaces for these surfaces.
This is joint work with Hiroto Akaike, Masafumi Hattori and Yuki Koto.
2024年12月12日(木)
13:30-15:00 数理科学研究科棟(駒場) 128号室
Chenyang Xu 氏 (Princeton University)
Irreducible symplectic varieties with a large second Betti number
Chenyang Xu 氏 (Princeton University)
Irreducible symplectic varieties with a large second Betti number
[ 講演概要 ]
(joint with Yuchen Liu, Zhiyu Liu) We show that the Lagrangian fibration constructed by Iiiev-Manivel using intermediate Jacobians of cubic fivefolds containing a fixed cubic fourfold, admits a compactification as a terminal Q-factorial irreducible symplectic varieties. As far as I know, besides OG10, this is the second family of irreducible symplectic varieties with the second Betti number at least 24.
(joint with Yuchen Liu, Zhiyu Liu) We show that the Lagrangian fibration constructed by Iiiev-Manivel using intermediate Jacobians of cubic fivefolds containing a fixed cubic fourfold, admits a compactification as a terminal Q-factorial irreducible symplectic varieties. As far as I know, besides OG10, this is the second family of irreducible symplectic varieties with the second Betti number at least 24.
2024年11月22日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
入谷寛 氏 (京都大学)
Quantum cohomology of blowups
入谷寛 氏 (京都大学)
Quantum cohomology of blowups
[ 講演概要 ]
I will discuss a decomposition theorem for the quantum cohomology of a smooth projective variety blown up along a smooth subvariety. I will start with a general relationship between decomposition of quantum cohomology and extremal contractions, and then specialize to the case of blowups. Applications to birational geometry of this result have been announced by Katzarkov, Kontsevich, Pantev and Yu.
I will discuss a decomposition theorem for the quantum cohomology of a smooth projective variety blown up along a smooth subvariety. I will start with a general relationship between decomposition of quantum cohomology and extremal contractions, and then specialize to the case of blowups. Applications to birational geometry of this result have been announced by Katzarkov, Kontsevich, Pantev and Yu.
2024年11月15日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
谷本祥 氏 (名古屋大学)
The spaces of rational curves on del Pezzo surfaces via conic bundles
谷本祥 氏 (名古屋大学)
The spaces of rational curves on del Pezzo surfaces via conic bundles
[ 講演概要 ]
There have been extensive activities on counting functions of rational points of bounded height on del Pezzo surfaces, and one of prominent approaches to this problem is by the usage of conic bundle structures on del Pezzo surfaces. This leads to upper and lower bounds of correct magnitude for quartic del Pezzo surfaces.
In this talk, I will explain how conic bundle structures on del Pezzo surfaces induce fibration structures on the spaces of rational curves on such surfaces. Then I will explain applications of this structure which include:
1. upper bounds of correct magnitude for the counting function of rational curves on quartic del Pezzo surfaces over finite fields.
2. rationality of the space of rational curves on a quartic del Pezzo surface.
Finally, I will explain our ongoing proof of homological stability for the spaces of rational curves on quartic del Pezzo surfaces. This is joint work in progress with Ronno Das, Brian Lehmann, and Philip Tosteson.
There have been extensive activities on counting functions of rational points of bounded height on del Pezzo surfaces, and one of prominent approaches to this problem is by the usage of conic bundle structures on del Pezzo surfaces. This leads to upper and lower bounds of correct magnitude for quartic del Pezzo surfaces.
In this talk, I will explain how conic bundle structures on del Pezzo surfaces induce fibration structures on the spaces of rational curves on such surfaces. Then I will explain applications of this structure which include:
1. upper bounds of correct magnitude for the counting function of rational curves on quartic del Pezzo surfaces over finite fields.
2. rationality of the space of rational curves on a quartic del Pezzo surface.
Finally, I will explain our ongoing proof of homological stability for the spaces of rational curves on quartic del Pezzo surfaces. This is joint work in progress with Ronno Das, Brian Lehmann, and Philip Tosteson.
2024年11月01日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
Gerard van der Geer 氏 (University of Amsterdam)
The cycle class of the supersingular locus (English)
Gerard van der Geer 氏 (University of Amsterdam)
The cycle class of the supersingular locus (English)
[ 講演概要 ]
Deuring gave a now classical formula for the number of supersingular elliptic curves
in characteristic p. We generalize this to a formula for the cycle class of the
supersingular locus in the moduli space of principally polarized abelian varieties
of given dimension g in characteristic p. The formula determines the class up to
a multiple and shows that it lies in the tautological ring. We also give the multiple
for g up to 4. This is joint work with S. Harashita.
Deuring gave a now classical formula for the number of supersingular elliptic curves
in characteristic p. We generalize this to a formula for the cycle class of the
supersingular locus in the moduli space of principally polarized abelian varieties
of given dimension g in characteristic p. The formula determines the class up to
a multiple and shows that it lies in the tautological ring. We also give the multiple
for g up to 4. This is joint work with S. Harashita.
2024年10月18日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
Jennifer Li 氏 (プリンストン大学)
Rational surfaces with a non-arithmetic automorphism group (英語)
Jennifer Li 氏 (プリンストン大学)
Rational surfaces with a non-arithmetic automorphism group (英語)
[ 講演概要 ]
In [Tot12], Totaro gave examples of a K3 surface such that its automorphism group is not commensurable with an arithmetic group, answering a question of Mazur. We give examples of rational surfaces with the same property. Our examples Y are log Calabi-Yau surfaces, i.e., there is a reduced normal crossing divisor D in Y such that KY+D=0. This is joint work with Sebastián Torres.
In [Tot12], Totaro gave examples of a K3 surface such that its automorphism group is not commensurable with an arithmetic group, answering a question of Mazur. We give examples of rational surfaces with the same property. Our examples Y are log Calabi-Yau surfaces, i.e., there is a reduced normal crossing divisor D in Y such that KY+D=0. This is joint work with Sebastián Torres.
2024年10月04日(金)
13:30-15:00 数理科学研究科棟(駒場) 118号室
高松哲平 氏 (京都大学)
Arithmetic finiteness of Mukai varieties of genus 7 (日本語)
高松哲平 氏 (京都大学)
Arithmetic finiteness of Mukai varieties of genus 7 (日本語)
[ 講演概要 ]
Fano threefolds over C with Picard number and index equal to 1 are known to be classified by their genus g (where 2≤g≤12 and g≠11). In particular, Mukai has shown that those with genus 7 can be described as hyperplane sections of a connected component of the 10-dimensional orthogonal Grassmannian.
In this talk, we discuss the arithmetic properties of these genus 7 threefolds and their higher-dimensional generalizations (called Mukai varieties of genus 7). More precisely, we consider the finiteness problem of varieties over a ring of S-integers (so called the Shafarevich conjecture), and the existence problem of varieties over the rational integer ring Z.
This talk is based on a joint work with Tetsushi Ito, Akihiro Kanemitsu, and Yuuji Tanaka.
Fano threefolds over C with Picard number and index equal to 1 are known to be classified by their genus g (where 2≤g≤12 and g≠11). In particular, Mukai has shown that those with genus 7 can be described as hyperplane sections of a connected component of the 10-dimensional orthogonal Grassmannian.
In this talk, we discuss the arithmetic properties of these genus 7 threefolds and their higher-dimensional generalizations (called Mukai varieties of genus 7). More precisely, we consider the finiteness problem of varieties over a ring of S-integers (so called the Shafarevich conjecture), and the existence problem of varieties over the rational integer ring Z.
This talk is based on a joint work with Tetsushi Ito, Akihiro Kanemitsu, and Yuuji Tanaka.
2024年07月04日(木)
13:00-14:30 数理科学研究科棟(駒場) ハイブリッド開催/118号室
Stefan Reppen 氏 (東京大学)
On a principle of Ogus: the Hasse invariant's order of vanishing and "Frobenius and the Hodge filtration'' (English)
Stefan Reppen 氏 (東京大学)
On a principle of Ogus: the Hasse invariant's order of vanishing and "Frobenius and the Hodge filtration'' (English)
[ 講演概要 ]
In joint work with W. Goldring we generalize a result of Ogus that, under certain technical conditions, the vanishing order of the Hasse invariant of a family $Y/X$ of $n$-dimensional Calabi-Yau varieties in characteristic $p$ at a point $x$ of $X$ equals the "conjugate line position" of $H^n_{\dR}(Y/X)$ at $x$, i.e. the largest $i$ such that the line of the conjugate filtration is contained in $\text{Fil}^i$ of the Hodge filtration. For every triple $(G,\mu,r)$ consisting of a connected, reductive $\mathbb{F}_p$-group $G$, a cocharacter $\mu \in X_*(G)$ and an $\mathbb{F}_p$-representation $r$ of $G$, we state a generalized Ogus Principle. If $\zeta:X \to \GZip^{\mu}$ is a smooth morphism, then the group theoretic Ogus Principle implies an Ogus Principle on $X$. We deduce an Ogus Principle for several Hodge and abelian-type Shimura varieties and the moduli space of K3 surfaces. In the talk I will present this work.
In joint work with W. Goldring we generalize a result of Ogus that, under certain technical conditions, the vanishing order of the Hasse invariant of a family $Y/X$ of $n$-dimensional Calabi-Yau varieties in characteristic $p$ at a point $x$ of $X$ equals the "conjugate line position" of $H^n_{\dR}(Y/X)$ at $x$, i.e. the largest $i$ such that the line of the conjugate filtration is contained in $\text{Fil}^i$ of the Hodge filtration. For every triple $(G,\mu,r)$ consisting of a connected, reductive $\mathbb{F}_p$-group $G$, a cocharacter $\mu \in X_*(G)$ and an $\mathbb{F}_p$-representation $r$ of $G$, we state a generalized Ogus Principle. If $\zeta:X \to \GZip^{\mu}$ is a smooth morphism, then the group theoretic Ogus Principle implies an Ogus Principle on $X$. We deduce an Ogus Principle for several Hodge and abelian-type Shimura varieties and the moduli space of K3 surfaces. In the talk I will present this work.
2024年06月28日(金)
13:30-15:00 数理科学研究科棟(駒場) ハイブリッド開催/117号室
吉野太郎 氏 (東京大学)
Stable rationality of hypersurfaces in mock toric varieties (日本語)
吉野太郎 氏 (東京大学)
Stable rationality of hypersurfaces in mock toric varieties (日本語)
[ 講演概要 ]
In recent years, there has been a development in approaching rationality problems through motivic methods. This approach requires the explicit construction of degeneration families over curves with favorable properties. However, the specific construction is generally difficult. Nicaise and Ottem combined combinatorial methods to construct degeneration families of hypersurfaces in toric varieties and mentioned the stable rationality of a very general hypersurface in projective spaces. In this talk, we mention the following two points: First, I introduce the notion of mock toric varieties, which are generalizations of toric varieties. Second, I combinatorially construct degeneration families of hypersurfaces in mock toric varieties, and I mention the irrationality of a very general hypersurface in the complex Grassmannian variety Gr(2, n).
In recent years, there has been a development in approaching rationality problems through motivic methods. This approach requires the explicit construction of degeneration families over curves with favorable properties. However, the specific construction is generally difficult. Nicaise and Ottem combined combinatorial methods to construct degeneration families of hypersurfaces in toric varieties and mentioned the stable rationality of a very general hypersurface in projective spaces. In this talk, we mention the following two points: First, I introduce the notion of mock toric varieties, which are generalizations of toric varieties. Second, I combinatorially construct degeneration families of hypersurfaces in mock toric varieties, and I mention the irrationality of a very general hypersurface in the complex Grassmannian variety Gr(2, n).
2024年06月21日(金)
13:30-15:00 数理科学研究科棟(駒場) ハイブリッド開催/056号室
Kien Nguyen Huu 氏 (Normandie Université/KU Leuven)
ON THE POWER SERIES OF DENEF AND LOESER'S MOTIVIC VANISHING CYCLES OF JET POLYNOMIALS (English)
Kien Nguyen Huu 氏 (Normandie Université/KU Leuven)
ON THE POWER SERIES OF DENEF AND LOESER'S MOTIVIC VANISHING CYCLES OF JET POLYNOMIALS (English)
[ 講演概要 ]
Let f be a non-constant polynomial in n variables over a field k of characteristic
0. Denef and Loeser introduced the notion of motivic vanishing cycles of f as an element in
the localization Mμˆ of the Grothendieck ring Kμˆ(Var ) of k-varieties with a good action of k0k
μˆ := lim μm by inverting the affne line equipped with the trivial action of μˆ, where μm
is the group scheme over k of mth roots of unity. In particular, if k is the field of complex
numbers then Denef and Loeser showed that their motivic vanishing cycles and the complex
φf [n − 1] has the same Hodge characteristic, where φf is the complex of vanishing cycles
in the usual sense. Motivated by the Igusa conjecture for exponential sums and the strong
monodromy conjecture, we introduce the notion of Poincaré series of Denef-Loeser's van-
ishing cycles of jet polynomials of f, where jet polynomials of f are polynomials appearing
naturally when we compute the jet schemes of f. By using Davison-Meinhardt's conjecture
which was proved by Nicaise and Payne in 2019, we can show that our Poincaré series is a
rational function over a quotient ring of Mμˆ by very natural relations. In particular, we can k
recovery Denef and Loeser's motivic vanishing cycles from our Poincaré series. Moreover, we can show that our Poincaré series owns a universal property in the sense that if k is a number field then the Igusa local zeta functions, the motivic Igusa zeta functions, the Poincaré series of exponential sums modulo pm of f can be obtained from our Poincaré se- ries by suitable specialization maps preserving the rationality. If time permits, I will present some initial consequences that have arisen during the study of our Poincaré series.
Let f be a non-constant polynomial in n variables over a field k of characteristic
0. Denef and Loeser introduced the notion of motivic vanishing cycles of f as an element in
the localization Mμˆ of the Grothendieck ring Kμˆ(Var ) of k-varieties with a good action of k0k
μˆ := lim μm by inverting the affne line equipped with the trivial action of μˆ, where μm
is the group scheme over k of mth roots of unity. In particular, if k is the field of complex
numbers then Denef and Loeser showed that their motivic vanishing cycles and the complex
φf [n − 1] has the same Hodge characteristic, where φf is the complex of vanishing cycles
in the usual sense. Motivated by the Igusa conjecture for exponential sums and the strong
monodromy conjecture, we introduce the notion of Poincaré series of Denef-Loeser's van-
ishing cycles of jet polynomials of f, where jet polynomials of f are polynomials appearing
naturally when we compute the jet schemes of f. By using Davison-Meinhardt's conjecture
which was proved by Nicaise and Payne in 2019, we can show that our Poincaré series is a
rational function over a quotient ring of Mμˆ by very natural relations. In particular, we can k
recovery Denef and Loeser's motivic vanishing cycles from our Poincaré series. Moreover, we can show that our Poincaré series owns a universal property in the sense that if k is a number field then the Igusa local zeta functions, the motivic Igusa zeta functions, the Poincaré series of exponential sums modulo pm of f can be obtained from our Poincaré se- ries by suitable specialization maps preserving the rationality. If time permits, I will present some initial consequences that have arisen during the study of our Poincaré series.
2024年06月07日(金)
13:30-15:00 数理科学研究科棟(駒場) ハイブリッド開催/056号室
Ivan Cheltsov 氏 (University of Edinburgh)
K-stability of pointless Fano 3-folds (English)
Ivan Cheltsov 氏 (University of Edinburgh)
K-stability of pointless Fano 3-folds (English)
[ 講演概要 ]
In this talk we will show how to prove that all pointless smooth Fano 3-folds defined over a subfield of the field of complex numbers are Kahler-Einstein unless they belong to 8 exceptional deformation families. This is a joint work in progress with Hamid Abban (Nottingham) and Frederic Mangolte (Marseille).
In this talk we will show how to prove that all pointless smooth Fano 3-folds defined over a subfield of the field of complex numbers are Kahler-Einstein unless they belong to 8 exceptional deformation families. This is a joint work in progress with Hamid Abban (Nottingham) and Frederic Mangolte (Marseille).
2024年05月24日(金)
13:30-15:00 数理科学研究科棟(駒場) ハイブリッド開催/056号室
佐藤謙太 氏 (九州大学)
Boundedness of weak Fano threefolds with fixed Gorenstein index in positive characteristic
佐藤謙太 氏 (九州大学)
Boundedness of weak Fano threefolds with fixed Gorenstein index in positive characteristic
[ 講演概要 ]
In this talk, we give a partial affirmative answer to the BAB conjecture for 3-folds in characteristic p>5. Specifically, we prove that a set of weak Fano 3-folds over an uncountable algebraically closed field is bounded, if each element X satisfies certain conditions regarding the Gorenstein index, a complement and Kodaira type vanishing. In the course of the proof, we also study a uniform lower bound for Seshadri constants of nef and big invertible sheaves on projective 3-folds.
In this talk, we give a partial affirmative answer to the BAB conjecture for 3-folds in characteristic p>5. Specifically, we prove that a set of weak Fano 3-folds over an uncountable algebraically closed field is bounded, if each element X satisfies certain conditions regarding the Gorenstein index, a complement and Kodaira type vanishing. In the course of the proof, we also study a uniform lower bound for Seshadri constants of nef and big invertible sheaves on projective 3-folds.
