代数幾何学セミナー

過去の記録 ~10/20次回の予定今後の予定 10/21~

開催情報 火曜日 15:30~17:00 数理科学研究科棟(駒場) 122号室
担当者 權業 善範・中村 勇哉・田中公

過去の記録

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.

2019年07月05日(金)

10:30-12:00   数理科学研究科棟(駒場) 123号室
いつもと曜日・時間・部屋が異なります。
榎園 誠 氏 (東京理科大学)
Durfee-type inequality for complete intersection surface singularities
[ 講演概要 ]
Durfee's negativity conjecture says that the signature of the Milnor fiber of a 2-dimensional isolated complete intersection singularity is always negative. In this talk, I will explain that this conjecture is true (more precisely, the signature is bounded above by the negative number determined by the geometric genus, the embedding dimension and the number of irreducible components of the exceptional set of the minimal resolution) by using the theory of invariants of fibered surfaces. If time permits, I will explain the higher dimensional analogue of Durfee's conjecture for isolated complete intersection singularities.

2019年06月28日(金)

15:30-17:00   数理科学研究科棟(駒場) 118号室
いつもと曜日が異なります。
谷本 祥 氏 (熊本)
Rational curves on prime Fano 3-folds (TBA)
[ 講演概要 ]
One of important topics in algebraic geometry is the space of rational curves, e.g., the dimension and the number of components of the moduli spaces of rational curves on an algebraic variety X. One of interesting situations where this question is extensively studied is when X is a Fano variety since in this case X is rationally connected so that it does contain a lots of rational curves. In this talk I will talk about my joint work with Brian Lehmann which settles this problem for most Fano 3-folds of Picard rank 1, e.g., a general quartic 3-fold in P^4, and our approach is inspired by Manin’s conjecture which predicts the asymptotic formula for the counting function of rational points on a Fano variety. In particular we systematically use geometric invariants in Manin’s conjecture which have been studied by many mathematicians including Brian and me.

2019年06月19日(水)

15:30-17:00   数理科学研究科棟(駒場) 118号室
今学期は基本水曜日とします。部屋も去年度と異なります。
鈴木文顕 氏 (イリノイ州立シカゴ大学)
A pencil of Enriques surfaces with non-algebraic integral Hodge classes (TBA)
[ 講演概要 ]
The integral Hodge conjecture is the statement that the integral Hodge classes are algebraic on smooth complex projective varieties. It is known that the conjecture can fail in general. There are two types of counterexamples, ones with non-algebraic integral Hodge classes of torsion-type and of non-torsion type, the first of which were given by Atiyah-Hirzebruch and Kollar, respectively.

In this talk, we exhibit a pencil of Enriques surfaces defined over Q with non-algebraic integral Hodge classes of non-torsion type. This construction relates to certain questions concerning rational points of algebraic varieties.

This gives the first example of a threefold with the trivial Chow group of zero-cycles on which the integral Hodge conjecture fails. As an application, we construct a fourfold which gives the negative answer to a classical question on the universality of the Abel-Jacobi maps.

This is a joint work with John Christian Ottem.

2019年05月29日(水)

15:30-17:00   数理科学研究科棟(駒場) 118号室
今学期は基本水曜日とします。部屋も去年度と異なります。
江辰 氏 (Fudan/MSRI)
Minimal log discrepancies of 3-dimensional non-canonical singularities (English)
[ 講演概要 ]
Canonical and terminal singularities, introduced by Reid, appear naturally in minimal model program and play important roles in the birational classification of higher dimensional algebraic varieties. Such singularities are well-understood in dimension 3, while the property of non-canonical singularities is still mysterious. We investigate the difference between canonical and non-canonical singularities via minimal log discrepancies (MLD). We show that there is a gap between MLD of 3-dimensional non-canonical singularities and that of 3-dimensional canonical singularities, which is predicted by a conjecture of Shokurov.
This result on local singularities has applications to global geometry of Calabi–Yau 3-folds. We show that the set of all non-canonical klt Calabi–Yau 3-folds are bounded modulo flops, and the global indices of all klt Calabi–Yau 3-folds are bounded from above.

2019年05月22日(水)

15:30-17:00   数理科学研究科棟(駒場) 122号室
河上 龍郎 氏 (東大数理)
Bogomolov type vanishing on three-dimensional Mori fiber spaces in positive characteristic

[ 講演概要 ]
In characteristic zero, cotangent bundle of n(>1)-dimensional smooth projective varieties does not contain a big line bundle. This is a part of Bogomolov vanishing and this vanishing plays an important role in the proof of Miyaoka-Yau inequality. In positive characteristic, it is known that Bogomolov vanishing does not hold. There exists a general type surface whose cotangent bundle contains an ample line bundle. So, it is natural to ask when Bogomolov type vanishing holds in positive characteristic. In this talk, I discuss Bogomolov type vanishing on three-dimensional Mori fiber spaces in positive characteristic.

2019年05月15日(水)

15:30-17:00   数理科学研究科棟(駒場) 118号室
今学期は基本水曜日とします。部屋も去年度と異なります。
藤野修 氏 (大阪大学)
On quasi-log canonical pairs
(Japanese)
[ 講演概要 ]
The notion of quasi-log canonical pairs was introduced by Florin Ambro. It is a kind of generalizations of that of log canonical pairs. Now we know that quasi-log canonical pairs are ubiquitous in the theory of minimal models. In this talk, I will explain some basic properties and examples of quasi-log canonical pairs. I will also discuss some new developments around quasi-log canonical pairs. Some parts are joint works with Haidong Liu.

1234567891011 次へ >