代数幾何学セミナー

過去の記録 ~02/28次回の予定今後の予定 02/29~

開催情報 金曜日 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
担当者 權業 善範、中村 勇哉、田中 公

過去の記録

2022年11月29日(火)

10:30-11:30   数理科学研究科棟(駒場) ハイブリッド開催/002号室
Thomas Hall 氏 (University of Nottingham)
The behaviour of Kahler-Einstein polygons under combinatorial mutation
(English)
[ 講演概要 ]
Combinatorial mutations play an important role in the mirror symmetry approach to the classification of Fano varieties. Another important notion for Fano varieties is that of K-polystability, which turns out to have a nice combinatorial characterisation in the toric case. In this talk, I will give an overview of how mutations work and sketch the key ideas used to explore its interaction with Kahler-Einstein polygons (i.e. the Fano polygons whose associated toric variety is K-polystable).

2022年11月22日(火)

10:30-12:00   数理科学研究科棟(駒場) 002号室
90分ハイブリッド開催です。
谷本祥 氏 (名古屋多元)
Non-free sections of Fano fibrations (日本語)
[ 講演概要 ]
Manin’s Conjecture predicts the asymptotic formula for the counting function of rational points over number fields or global function fields. In the late 80’s, Batyrev developed a heuristic argument for Manin’s Conjecture over global function fields, and the assumptions underlying Batyrev’s heuristics are refined and formulated as Geometric Manin’s Conjecture. Geometric Manin’s Conjecture is a set of conjectures regarding properties of the space of sections of Fano fibrations, and it consists of three conjectures: (i) Pathological components are controlled by Fujita invariants; (ii) For each nef algebraic class, a non-pathological component which should be counted in Manin’s Conjecture is unique (This component is called as Manin components); (iii) Manin components exhibit homological or motivic stability. In this talk we discuss our proofs of GMC (i) over complex numbers using theory of foliations and the minimal model program. Using this result, we prove that these pathological components are coming from a bounded family of accumulating maps. This is joint work in progress with Brian Lehmann and Eric Riedl.

2022年11月15日(火)

10:30-12:00   数理科学研究科棟(駒場) ハイブリッド開催/002号室
張 繼剛 氏 (NTU/東大数理)
Positivity of anticanonical divisors in algebraic fibre spaces (日本語)
[ 講演概要 ]
It is known that the positivity of the anti-canonical divisor is an important property that is closely related to the geometric structure of a variety. Given an algebraic fibre space f : X → Y between normal projective varieties with mild singularities, and let F be a general fibre of f. In this talk, we will introduce results relating the positivity of −KX and −KY under some conditions on the asymptotic base loci of −KX. In particular, we will obtain an inequality between the anti-canonical Iitaka dimensions κ(X, −KX) ≤ κ(F, −KF ) + κ(Y, −KY ) under the assumption that the stable base locus B(−KX) does not dominant over Y .

2022年11月01日(火)

10:30-12:00   数理科学研究科棟(駒場) ハイブリッド開催/002号室
河上龍郎 氏 (京大数学教室)
Extendability of differential forms via Cartier operators (Japanese)
[ 講演概要 ]
For a normal variety X, we say X satisfies the extension theorem if, for every proper birational morphism from Y, every differential form on the regular locus of X extends to Y. This is a basic property relating differential forms and singularities, and many results are known over the field of complex numbers.
In this talk, we discuss the extension theorem in positive characteristic. Existing studies depend on geometric tools such as log resolutions, (mixed) Hodge theory, the minimal model program, and vanishing theorems, which are not expected to be true or are not known for higher-dimensional varieties in positive characteristic.
For this reason, I introduce a new algebraic approach to the extension theorem using Cartier operators. I also talk about an application of the theory of quasi-F-splitting, which is studied in joint work with Takamatsu-Tanaka-Witaszek-Yobuko-Yoshikawa, to the extension problem.

2022年10月25日(火)

10:30-11:45   数理科学研究科棟(駒場) ハイブリッド開催/002号室
伊藤敦 氏 (岡山大学)
Projective normality of general polarized abelian varieties (Japanese)
[ 講演概要 ]
Projective normality is an important property of polarized varieties. Hwang and To prove that a general polarized abelian variety $(X,L)$ of dimension $g$ is projectively normal if $\chi(X,L) \geq (8g)^g/2g!$. In this talk, I will explain that their bound can be weaken as $\chi(X,L) > 2^{2g-1}$, which is sharp. A key tool in the proof is an invariant introduced by Jiang and Pareschi, which measures the basepoint freeness of $\mathbb{Q}$-divisors on abelian varieties.

2022年10月05日(水)

13:00-14:00   数理科学研究科棟(駒場) 056号室
今学期より対面ハイブリッドでセミナーを再開します。本セミナーは京大と共催です。オンライン情報はメーリングリストで公開しています。
Yuri Tschinkel 氏 (Mathematics and Physical Sciences Division, Simons Foundation/ Courant Institute, New York University)
Equivariant birational geometry (joint with A. Kresch) (English)
[ 講演概要 ]
Ideas from motivic integration led to the introduction of new invariants in equivariant birational geometry, the study of actions of finite groups on algebraic varieties, up to equivariant birational transformations.
These invariants allow us to distinguish actions in many new cases, shedding light on the structure of the Cremona group. The structure of the invariants themselves is also interesting: there are unexpected connections to modular curves and cohomology of arithmetic groups.

2021年07月21日(水)

15:00-16:00   数理科学研究科棟(駒場) zoom号室
キャンセルになりました。京大と共催
宮本恵介 氏 (大阪大学)
TBA (日本語)
[ 講演概要 ]
TBA

2021年07月05日(月)

16:00-17:00   数理科学研究科棟(駒場) zoom号室
いつもと日時が異なります。京大と共催
Paolo Cascini 氏 (Imperial College London)
Birational geometry of foliations (English)
[ 講演概要 ]
I will survey about some recent progress towards the Minimal Model Program for foliations on complex varieties, focusing mainly on the case of threefolds and the case of algebraically integrable foliations.

2021年07月01日(木)

10:00-11:00   数理科学研究科棟(駒場) 号室
いつもと日時が違います。京大と共催です。
鈴木文顕 氏 (UCLA)
An O-acyclic variety of even index


[ 講演概要 ]
I will construct a family of Enriques surfaces parametrized by P^1 such that any multi-section has even degree over the base P^1. Over the function field of a complex curve, this gives the first example of an O-acyclic variety (H^i(X,O)=0 for i>0) whose index is not equal to one, and an affirmative answer to a question of Colliot-Thélène and Voisin. I will also discuss applications to related problems, including the integral Hodge conjecture and Murre’s question on universality of the Abel-Jacobi maps. This is joint work with John Christian Ottem.

2021年06月14日(月)

17:00-18:00   数理科学研究科棟(駒場) 号室
京大と共催です。いつもと日時が異なります。
原和平 氏 (University of Glasgow)
Rank two weak Fano bundles on del Pezzo threefolds of degree 5 (日本語)
[ 講演概要 ]
射影化したとき反標準因子がネフかつ巨大になるようなベクトル束を弱Fanoベクトル束という.
本講演では,福岡氏,石川氏との共同研究で得られた,次数5の3次元del Pezzo多様体上の階数2のベクトル束の分類結果を紹介する.
今回は特に,導来圏の例外生成列を用いたベクトル束の分解を得る方法と,それを応用して得られるモジュライ空間についての諸結果に話題を絞って,証明を詳しく紹介したい.
[ 参考URL ]
Zoom

2021年06月09日(水)

15:00-16:00   数理科学研究科棟(駒場) 122号室
京大と共催です。
Andrea Fanelli 氏 (Bordeaux)
Rational simple connectedness and Fano threefolds (English)
[ 講演概要 ]
The notion of rational simple connectedness can be seen as an algebro-geometric analogue of simple connectedness in topology. The work of de Jong, He and Starr has already produced several recent studies to understand this notion.
In this talk I will discuss the joint project with Laurent Gruson and Nicolas Perrin to study rational simple connectedness for Fano threefolds via explicit methods from birational geometry.
[ 参考URL ]
Zoom

2021年06月02日(水)

15:00-16:00   数理科学研究科棟(駒場) 号室
京大と共催です。
青木孔 氏 (東大数理)
Quasiexcellence implies strong generation (日本語)
[ 講演概要 ]
BondalとVan den Berghは(小さい)三角圏からの反変関手がいつ表現可能かという問題の考察の中で、対象が三角圏を強生成(strongly generate)することの定義を導入した。強生成する対象が存在するときは良い表現可能性定理が成立する。
どのような有限次元Noetherスキームに対してその連接層の導来圏が強生成であるかについてはBondal–Van den Bergh以降Rouquier, Keller–Van den Bergh, Aihara–Takahashi, Iyengar–Takahashiなどにより多くの結果が得られていたが、最近Neemanは別の手法を用いてそれをalterationが適用できる分離Noetherスキームに対して示した。それを講演者はGabberのweak local uniformizationを用いることでさらに分離的準優秀スキームにまで拡張した。講演ではこの結果およびその証明の手法を紹介する。
[ 参考URL ]
Zoom

2021年05月26日(水)

15:00-16:00   数理科学研究科棟(駒場) zoom号室
京大と共催
山口樹 氏 (東大数理)
Multiplier ideals via ultraproducts (日本語)
[ 講演概要 ]
正標数の可換環と複素数体上の可換環の性質を比較する方法の一つにultraproductを用いた手法がある. このultraproductは超準解析において超実数の構成などに用いられているものである. これを可換環論へ応用する研究としてSchoutensによるnon-standard hullがある. この手法は等標数0の局所環に対するbig Cohen-Macaulay 代数の構成などにも応用がある. 彼の研究の一つに川又対数端末特異点のultraproductを用いた特徴付けがある. 本講演では, この結果の一般化として乗数イデアルがultraproductを用いて記述できることを説明する.

2021年05月17日(月)

17:00-18:00   数理科学研究科棟(駒場) zoom号室
いつもと日時が異なります。京大と共催
Ivan Cheltsov 氏 (エジンバラ)
Calabi problem for smooth Fano threefolds (English)
[ 講演概要 ]
In this talk I will explain which three-dimensional complex Fano manifolds admit Kahler-Einstein metrics.

2021年05月13日(木)

9:00-10:00   数理科学研究科棟(駒場) zoom号室
村山匠 氏 (プリンストン大学)
Relative vanishing theorems for schemes of equal characteristic zero (Englishg)
[ 講演概要 ]
In 1953, Kodaira proved the Kodaira vanishing theorem, which states that if L is an ample divisor on a complex projective manifold X, then H^i(X,-L) = 0 for all i < dim(X). Since then, Kodaira's theorem and its generalizations have become indispensable tools in algebraic geometry over fields of characteristic zero. Even in this context, however, it is often necessary to work with schemes of finite type over power series rings, and a fundamental problem has been the lack of vanishing theorems in this setting.
We prove the analogue of the Kawamata-Viehweg vanishing theorem for proper morphisms of schemes of equal characteristic zero, which implies Kodaira's vanishing theorem in this context. This result resolves conjectures of Boutot and Kawakita, and is an important ingredient toward establishing the minimal model program for excellent schemes of equal characteristic zero.

2021年04月28日(水)

15:00-16:00   数理科学研究科棟(駒場) Zoom号室
京大と共催です。
金城翼 氏 (東大数理/IPMU)
Dimensional reduction in cohomological Donaldson-Thomas theory (日本語)
[ 講演概要 ]
三次元Calabi-Yau多様体のコホモロジー的Donaldson-Thomas不変量(CoDT不変量)とは、Joyceらによって導入されたDonaldson-Thomas不変量の圏化であり、
Kontsevich-Soibelmanによって導入されたポテンシャル付き箙のCoDT不変量の大域化とみなすことができるものである。
ポテンシャル付き箙のCoDT理論は表現論とのつながりなどの深い理論が知られているのに対し、
三次元Calabi-Yau多様体のCoDT理論は定義以外のことがほとんど知られていないのが現状である。
本講演では滑らかな代数曲面の標準束の全空間として与えられる三次元Calabi-Yau多様体のCoDT不変量と元の曲面の連接層のモジュライのBorel-Mooreホモロジーを関連付ける次元還元定理について説明を行う。
また、次元還元定理をトム同型の一般化とみなしオイラー類の構成を適用することで、仮想基本類の新しい構成が与えられることを説明する。

2021年04月21日(水)

15:00-16:00   数理科学研究科棟(駒場) ZOOM号室
京大と共催です。
服部真史 氏 (京大数学教室)
A decomposition formula for J-stability and its applications (日本語)
[ 講演概要 ]
J-stability is an analog of K-stability and plays an important role in K-stability for general polarized varieties (not only for Kahler-Einstein metrics). Strikingly, G.Chen proved uniform J-stability and slope uniform J-stability are equivalent, analogous to Ross-Thomas slope theory and Mumford-Takemoto slope theory for vector bundles, by differential geometric arguments recently. However, this fact has not been proved in algebro-geometric way before. In this talk, I would like to explain a decomposition formula of non-Archimedean J-functional, the (n+1)-dimensional intersection number into n-dimensional intersection numbers and its applications to prove the fact for surfaces and to construct a K-stable but not uniformly K-stable lc pair. Based on arXiv:2103.04603

2021年04月14日(水)

15:00-16:00   数理科学研究科棟(駒場) Zoom号室
京大と共催です。
佐藤謙太 氏 (九州大学)
Arithmetic deformation of F-singularities (日本語)
[ 講演概要 ]
F正則特異点は,Frobenius写像の言葉で定義される正標数の特異点のクラスであるが,標数0のklt特異点と強い関係があることが知られている.例えば,標数0の特異点がkltであることと,無限個のpに関する正標数還元がF正則になることは同値である.近年Ma-Schwedeは,この関係の精密化として,total spaceがQ-Gorensteinという条件のもとで,一つのpでの正標数還元が強F正則ならば,もとの標数0の特異点はkltであることを証明した.
本講演では,total spaceがQ-Gorensteinでない場合にこの結果を一般化する.また,Liedtke-Martin-Matsumotoによる線形簡約商特異点に関する予想への本結果の応用や,F正則/klt特異点よりも少し広いクラスである,F純/lc特異点に関する同様の結果についても紹介する.これらの結果は,高木俊輔氏との共同研究である.

2020年03月02日(月)

15:30-17:00   数理科学研究科棟(駒場) 002号室
諸事情により中止を決定しました。Cancelled.
Evgeny Shinder 氏 (The University of Sheffield)
Semiorthogonal decompositions for singular varieties (English)
[ 講演概要 ]
I will define the semiorthogonal decomposition for derived categories of singular projective varieties due to Professor Kawamata, into finite-dimensional algebras, generalizing the concept of an exceptional collection in the smooth case. I will present known constructions of these for nodal curves (Burban), torsion-free toric surfaces (Karmazyn-Kuznetsov-Shinder) and two nodal threefolds (Kawamata). I will also explain obstructions coming from the K_{-1} group, and how it translates to maximal nonfactoriality in the nodal threefold case. This is joint work with M.Kalck and N.Pavic.

2020年02月21日(金)

13:30-15:00   数理科学研究科棟(駒場) 370号室
いつもと曜日・時間・部屋が異なります
Jakub Witaszek 氏 (Michigan)
Keel's theorem and quotients in mixed characteristic (English)
[ 講演概要 ]
In trying to understand characteristic zero varieties one can apply a wide range of techniques coming from analytic methods such as vanishing theorems. More complicated though they are, positive characteristic varieties come naturally with Frobenius action which sometimes allows for imitating analytic proofs or even showing results which are false over complex numbers. Of all the three classes, the mixed characteristic varieties are the most difficult to understand as they represent the worst of both worlds: one lacks the analytic methods as well the Frobenius action.

What is key for many applications of Frobenius in positive characteristic (to birational geometry, moduli theory, constructing quotients, etc.) is the fact that every universal homeomorphism of algebraic varieties factors through a power of Frobenius. In this talk I will discuss an analogue of this fact (and applications thereof) in mixed characteristic.
[ 参考URL ]
http://www-personal.umich.edu/~jakubw/

2020年01月21日(火)

15:30-17:00   数理科学研究科棟(駒場) 118号室
普段と部屋が異なりますのでご注意ください。The room is different from our usual.
Matthias Schütt 氏 (Universität Hannover)
(Few) rational curves on K3 surfaces (English)
[ 講演概要 ]
Rational curves play a fundamental role for the structure of a K3 surface. I will first review the general theory before focussing on the case of low degree curves where joint work with S. Rams (Krakow) extends bounds of Miyaoka and Degtyarev. Time permitting, I will also discuss the special case of smooth rational curves as well as applications to Enriques surfaces.

2019年12月03日(火)

14:30-16:00   数理科学研究科棟(駒場) 056号室
普段と曜日・時間・部屋が異なりますのでご注意ください。The room and time are different from our usual.
Gavril Farkas 氏 (Humboldt Univ. Berlin)
Moduli of K3 surfaces via cubic 4-folds (English)
[ 講演概要 ]
In a celebrated series of papers, Mukai established structure theorems for polarized K3 surfaces of all genera g<21, with the exception of the case g=14. Using the identification between certain moduli spaces of polarized K3 surfaces of genera 14 and the moduli space of special cubic fourfolds of given discriminant, we discuss a novel approach to moduli spaces of K3 surfaces. As an application, we establish the rationality of the universal K3 surface of these genus 14,22. This is joint work with A. Verra.

2019年10月30日(水)

15:30-17:00   数理科学研究科棟(駒場) 122号室
Andrew Macpherson 氏 (IPMU)
A Tannakian perspective on rigid analytic geometry (English)
[ 講演概要 ]
Raynaud's conception of analytic geometry contends that the category of analytic spaces over a non-Archimedean field is a (suitably "geometric") localisation of the category of formal schemes over the ring of integers at a class of modifications "along the central fibre". Unfortunately, as with all existing presentations of non-Archimedean geometry, this viewpoint is confounded by a proliferation of technical difficulties if one does not impose absolute finiteness conditions on the formal schemes under consideration.

I will argue that by combining Raynaud's idea with a Tannakian perspective which prioritises the module category, we can obtain a reasonable framework for rigid analytic geometry with no absolute finiteness hypotheses whatsoever, but which has descent for finitely presented modules.

2019年10月16日(水)

15:30-17:00   数理科学研究科棟(駒場) 122号室
佐藤 悠介 氏 (東大数理/ IPMU)
Multidimensional continued fraction for Gorenstein cyclic quotient singularity
[ 講演概要 ]
Let G be a finite cyclic subgroup of GL(n,C). Then Cn/G is a cyclic quotient singularity. In the case n = 2, Cn/G possess the unique minimal resolution, and it is obtained by Hirzubruch-Jung continued fraction. In this talk, we show a sufficient condition of existence of crepant desingularization for Gorenstein abelian quotient singularities in all dimensions by using Ashikaga’s continuous fractions. Moreover, as a corollary, we prove that all three dimensional Gorenstein abelian quotient singularities possess a crepant desingularization.

2019年07月09日(火)

13:00-14:30   数理科学研究科棟(駒場) 122号室
いつもと曜日・時間・部屋が異なります。
佐野 太郎 氏 (神戸大学)
Construction of non-Kähler Calabi-Yau 3-folds by smoothing normal crossing varieties (TBA)
[ 講演概要 ]
It is an open problem whether there are only finitely many diffeomorphism types of projective Calabi-Yau 3-folds. Kawamata--Namikawa developed log deformation theory of normal crossing Calabi-Yau varieties. As an application of their result, one can construct examples of Calabi-Yau manifolds by smoothing SNC varieties. In this talk, I will explain how to construct examples of non-Kähler Calabi-Yau 3-folds with arbitrarily large 2nd Betti numbers. If time permits, I will also explain an example of involutions on a family of K3 surfaces which do not lift biregularly to the total space. This is based on joint work with Kenji Hashimoto.

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