代数幾何学セミナー

過去の記録 ~07/24次回の予定今後の予定 07/25~

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

2010年11月16日(火)

16:30-18:00   数理科学研究科棟(駒場) 122号室
Viacheslav Nikulin 氏 (Univ Liverpool and Steklov Moscow)
Self-corresponences of K3 surfaces via moduli of sheaves (ENGLISH)
[ 講演概要 ]
In series of our papers with Carlo Madonna (2002--2008) we described self-correspondences via moduli of sheaves with primitive isotropic Mukai vectors for K3 surfaces with Picard number one or two. Here, we give a natural and functorial answer to the same problem for arbitrary Picard number of K3 surfaces. As an application, we characterize in terms of self-correspondences via moduli of sheaves K3 surfaces with reflective Picard lattices, that is when the automorphism group of the lattice is generated by reflections up to finite index. See some details in arXiv:0810.2945.

2010年11月15日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
吉冨 修平 氏 (東大数理)
Generators of tropical modules (JAPANESE)
[ 講演概要 ]
We study polytopes in a tropical projective space $X$. By Joswig and Kulas, a real convex polytope in $X$ is a tropical simplex, and therefore it is the tropically convex hull of at most $n+1$ points. We show a generalization of this result. It is given using tropical modules and its dual modules. The main interest is
the number of generators of a tropical module.

2010年11月01日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
伊藤 敦 氏 (東大数理)
How to estimate Seshadri constants (JAPANESE)
[ 講演概要 ]
Seshadri constant is an invariant which measures the positivities of ample line bundles. This relates with adjoint bundles, Nagata conjecture, slope stabilities, Gromov width (an invariant of symplectic manifolds) and so on. But it is very diffiult to compute or estimate Seshadri constants in general, especially in higher dimension.
In this talk, we first study Seshadri constants of toric varieties, and next consider about non-toric cases using toric degenerations. For example, good estimations are obtained for complete intersections in projective spaces.

2010年10月18日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
三内 顕義 氏 (東大数理)
ガロア拡大と局所コホモロジー間の写像について (JAPANESE)
[ 講演概要 ]
正則環に線型簡約群が作用するとき、その不変式環がコーエンマコーレー環になるという直和因子予想は正標数、等標数の場合にHochster, Hunekeらによってビッグコーエンマコーレー代数の存在定理を用いることで解決された。この存在定理の証明は大変複雑なものであったが2007年にHuneke, Lyubeznikらによって有限環拡大の局所コホモロジー間の射の計算に帰着された。
今回はその定理を強めた結果とそれによってできる新しいビッグコーエンマコーレー代数の存在について解説する。

2010年09月06日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Prof. Remke Kloosterman 氏 (Humboldt University, Berlin)
Non-reduced components of the Noether-Lefschetz locus (ENGLISH)
[ 講演概要 ]
Let $M_d$ be the moduli space of complex smooth degree $d$ surfaces in $\\mathbb{P}3$. Let $NL_d \\subset M_d$ be the subset corresponding to surfaces with Picard number at least 2. It is known that $NL_r$ is Zariski-constructable, and each irreducible component of $NL_r$ has a natural scheme structure. In this talk we describe the largest non-reduced components of $NL_r$. This extends work of Maclean and Otwinowska.
This is joint work with my PhD student Ananyo Dan.

2010年07月29日(木)

14:30-16:00   数理科学研究科棟(駒場) 126号室
いつもと曜日・時間帯が異なります。ご注意ください。
二木昌宏 氏 (東大数理)
Homological Mirror Symmetry for 2-dimensional toric Fano stacks (JAPANESE)
[ 講演概要 ]
Homological Mirror Symmetry (HMS for short) is a conjectural
duality between complex and symplectic geometry, originally proposed
for mirror pairs of Calabi-Yau manifolds and later extended to
Fano/Landau-Ginzburg mirrors (both due to Kontsevich, 1994 and 1998).

We explain how HMS is established in the case of 2-dimensional smooth
toric Fano stack X as an equivalence between the derived category of X
and the derived directed Fukaya category of its mirror Lefschetz
fibration W. This is related to Kontsevich-Soibelman's construction of
3d CY category from the quiver with potential.

We also obtain a local mirror extension following Seidel's suspension
theorem, that is, the local HMS for the canonical bundle K_X and the
double suspension W+uv. This talk is joint with Kazushi Ueda (Osaka
U.).

2010年07月12日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
大川 領 氏 (東京工業大学)
Flips of moduli of stable torsion free sheaves with $c_1=1$ on
$\\\\mathbb{P}^2$ (JAPANESE)
[ 講演概要 ]
We study flips of moduli schemes of stable torsion free sheaves
on the projective plane via wall-crossing phenomena of Bridgeland stability.
They are described as stratified Grassmann bundles by variation of
stability of modules over certain finite dimensional algebra.

2010年07月05日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
古川 勝久 氏 (早稲田大学)
Rational curves on hypersurfaces (JAPANESE)
[ 講演概要 ]
Our purpose is to study the family of smooth rational curves of degree $e$ lying on a hypersurface of degree $d$ in $\\mathbb{P}^n$, and to investigate properties of this family (e.g., dimension, smoothness, connectedness).
Our starting point is the research about the family of lines (i.e., $e = 1$), which was studied by W. Barth and A. Van de Ven over $\\mathbb{C}$, and by J. Koll\\'{a}r over an algebraically closed field of arbitrary characteristic.
For the degree $e > 1$, the family of rational curves was studied by J. Harris, M. Roth, and J. Starr over $\\mathbb{C}$ in the case of $d < (n+1)/2$.
In this talk, we study the family of rational curves in arbitrary characteristic under the assumption $e = 2,3$ and $d > 1$, or $e > 3$ and $d > 2e-4$.

2010年06月21日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
月岡 透 氏 (大阪府立大学)
ファノ多様体の擬指数と端射線の長さの最小値 (JAPANESE)
[ 講演概要 ]
ファノ多様体上の有理曲線と反標準因子の交点数
の最小値は擬指数(pseudo-index)と呼ばれる。ファノ多様体
の構造は端射線によって制御されるという観点から、
端射線上にある有理曲線を考えるのが特に重要である。
本講演では、双有理射を持つ4次元ファノ多様体について
端射線の長さの最小値が擬指数と一致することを示す。

2010年06月14日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Yongnam Lee 氏 (Sogang University)
Slope of smooth rational curves in an anticanonically polarized Fano manifold (ENGLISH)
[ 講演概要 ]
Ross and Thomas introduce the concept of slope stability to study K-stability, which has conjectural relation with the existence of constant scalar curvature metric. Since K-stability implies slope stability, slope stability gives an algebraic obstruction to theexistence of constant scalar curvature. This talk presents a systematic study of slope stability of anticanonically polarized Fano manifolds with respect to smooth rational curves. Especially, we prove that an anticanonically polarized Fano maniold is slope semistable with respect to any free smooth rational curves, and that an anticanonically polarized Fano threefold X with Picard number 1 is slope stable with respect to any smooth rational curves unless X is the project space. It is a joint work with Jun-Muk Hwang and Hosung Kim.

2010年06月07日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Xavier Roulleau 氏 (東大数理)
Genus 2 curve configurations on Fano surfaces (ENGLISH)

2010年05月31日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
金沢 篤 氏 (東大数理)
On Pfaffian Calabi-Yau Varieties and Mirror Symmetry (JAPANESE)
[ 講演概要 ]
We construct new smooth CY 3-folds with 1-dimensional Kaehler moduli and
determine their fundamental topological invariants. The existence of CY
3-folds with the computed invariants was previously conjectured. We then
report mirror symmetry for these non-complete intersection CY 3-folds.
We explicitly build their mirror partners, some of which have 2 LCSLs,
and carry out instanton computations for g=0,1.

2010年05月24日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
上原 北斗 氏 (首都大学東京)
A counterexample of the birational Torelli problem via Fourier--Mukai transforms (JAPANESE)
[ 講演概要 ]
We study the Fourier--Mukai numbers of rational elliptic surfaces. As
its application, we give an example of a pair of minimal 3-folds $X$
with Kodaira dimensions 1, $h^1(O_X)=h^2(O_X)=0$ such that they are
mutually derived equivalent, deformation equivalent, but not
birationally equivalent. It also supplies a counterexample of the
birational Torelli problem.

2010年05月17日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
尾高 悠志 氏 (京都大学数理解析研究所(RIMS))
On the GIT stability of Polarized Varieties (JAPANESE)
[ 講演概要 ]
Background:
Original GIT-stability notion for polarized variety is
"asymptotic stability", studied by Mumford, Gieseker etc around 1970s.
Recently a version appeared, so-called "K-stability", introduced by
Tian(1997) and reformulated by Donaldson(2002), by the way of seeking
the analogue of Kobayashi-Hitchin correspondence, which gives
"differential geometric" interpretation of "stability". These two have
subtle but interesting differences in dimension higher than 1.

Contents:
(1*) Any semistable (in any sense) polarized variety should have only
"semi-log-canonical" singularities. (Partly observed around 1970s)
(2) On the other hand, we proved some stabilities, which corresponds to
"Calabi conjecture", also with admitting mild singularities.

As applications these yield
(3*) Compact moduli spaces with GIT interpretations.
(4) Many counterexamples (as orbifolds) to folklore conjecture:
"K-stability implies asymptotic stability".

(*: Some technical points are yet to be settled.
Some parts for (1)(2) are available on arXiv:0910.1794.)

2010年05月10日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
三浦 真人 氏 (東大数理)
Grassmann多様体のトーリック退化とミラー対称性 (JAPANESE)
[ 講演概要 ]
Grassmann多様体のトーリック退化と、
それを用いたGrassmann多様体の完全交叉カラビ・ヤウ多様体に対するミラー構
成について説明する。
とくに、項順序によるトーリック退化に着目すれば、
座標環が一様斉次なsagbi基底を持つように項順序を選ぶ必要がある。
A型Grassmann多様体やスピノル多様体などの例に関してこの条件を考察する。

2010年04月26日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
馬 昭平 氏 (東大数理)
The unirationality of the moduli spaces of 2-elementary K3
surfaces (JAPANESE)
[ 講演概要 ]
We prove the unirationality of the moduli spaces of K3 surfaces
with non-symplectic involution. As a by-product, we describe the
configuration spaces of 5, 6, 7, 8 points in the projective plane as
arithmetic quotients of type IV.

2010年04月19日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
松村 慎一 氏 (東大数理)
制限型体積と因子的ザリスキー分解
[ 講演概要 ]
豊富な因子の部分多様体に沿った自己交点数は, 基本的かつ重要である.
(部分多様体に沿った)自己交点数の巨大な因子への一般化である制限型体積は,
多くの状況で出現する重要な概念である.
様々な部分多様体に沿った制限型体積の振る舞いと
巨大な因子のザリスキー分解可能性の関係について考察したい.
また, 時間が許せば, 元々の問題意識であった制限型体積の複素解析的な側面に
ついても触れたい.

2010年04月05日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Alexandru Dimca 氏 (Université Nice-Sophia Antipolis)
From Lang's Conjecture to finiteness properties of Torelli groups
[ 講演概要 ]
First we recall one of Lang's conjectures in diophantine geometry
on the interplay between subvarieties and translated subgroups in a
commutative algebraic group
(proved by M. Laurent in the case of affine tori in 1984).

Then we present the technique of resonance and characteristic varieties,
a powerful tool in the study of fundamental groups of algebraic varieties.


Finally, using the two ingredients above, we show that the Torelli
groups $T_g$
have some surprising finiteness properties for $g>3$.
In particular, we show that for any subgroup $N$ in $T_g$ containing
the Johnson kernel $K_g$, the complex vector space $N_{ab} \\otimes C$
is finite dimensional.

All the details are available in our joint preprint with S. Papadima
arXiv:1002.0673.

2010年02月01日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
大川 新之介 氏 (東大数理)
Extensions of two Chow stability criteria to positive characteristics
[ 講演概要 ]
I will talk about two results on Chow (semi-)stability of cycles in positive characteristics, which were originally known in characteristic 0. One is on the stability of non-singular projective hypersurfaces of degree greater than 2, and the other is the criterion by Y. Lee in terms of the log canonical threshold of Chow divisor. A couple of examples will be discussed in detail.

2010年01月25日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
權業 善範 氏 (東大数理)
On weak Fano varieties with log canonical singularities
[ 講演概要 ]
We prove that the anti-canonical divisors of weak Fano
3-folds with log canonical singularities are semiample. Moreover, we consider
semiampleness of the anti-log canonical divisor of any weak log Fano pair
with log canonical singularities. We show semiampleness dose not hold in
general by constructing several examples. Based on those examples, we propose
sufficient conditions which seem to be the best possible and we prove
semiampleness under such conditions. In particular we derive semiampleness of the
anti-canonical divisors of log canonical weak Fano 4-folds whose lc centers
are at most 1-dimensional. We also investigate the Kleiman-Mori cones of
weak log Fano pairs with log canonical singularities.

2010年01月18日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Anne-Sophie Kaloghiros 氏 (RIMS)
The divisor class group of terminal Gorenstein Fano 3-folds and rationality questions
[ 講演概要 ]
Let Y be a quartic hypersurface in CP^4 with mild singularities, e.g. no worse than ordinary double points.
If Y contains a surface that is not a hyperplane section, Y is not Q-factorial and the divisor class group of Y, Cl Y, contains divisors that are not Cartier. However, the rank of Cl Y is bounded.

In this talk, I will show that in most cases, it is possible to describe explicitly the divisor class group Cl Y by running a Minimal Model Program (MMP) on X, a small Q-factorialisation of Y. In this case, the generators of Cl Y/ Pic Y are ``topological traces " of K-negative extremal contractions on X.
This has surprising consequences: it is possible to conclude that a number of families of non-factorial quartic 3-folds are rational.
In particular, I give some examples of rational quartic hypersurfaces Y_4\\subset CP^4 with rk Cl Y=2 and show that when the divisor class group of Y has sufficiently high rank, Y is always rational.

2009年12月21日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
源 泰幸 氏 (京都大学理学部数学教室)
Ampleness of two-sided tilting complexes
[ 講演概要 ]
From the view point of noncommutative algebraic geometry (NCAG),
a two-sided tilting complex is an analog of a line bundle.
In this talk we introduce the notion of ampleness for two-sided
tilting complexes over finite dimensional algebras.
From the view point of NCAG, the Serre functors are considered to be
shifted canonical bundles. We show by examples that the property
of shifted canonical bundle captures some representation theoretic
property of algebras.

2009年12月14日(月)

14:40-16:10   数理科学研究科棟(駒場) 126号室
いつもと時間帯が異なります。ご注意ください。
Sergey Galkin 氏 (IPMU)
Invariants of Fano varieties via quantum D-module
[ 講演概要 ]
We will introduce and compute Apery characteristic
class and Frobenius genera - invariants of Fano variety derived from
it's Gromov-Witten invariants. Then we will show how to compute them
and relate with other invariants.

2009年11月16日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Colin Ingalls 氏 (University of New Brunswick and RIMS)
Rationality of the Brauer-Severi Varieties of Skylanin algebras
[ 講演概要 ]
Iskovskih's conjecture states that a conic bundle over
a surface is rational if and only if the surface has a pencil of
rational curves which meet the discriminant in 3 or fewer points,
(with one exceptional case). We generalize Iskovskih's proof that
such conic bundles are rational, to the case of projective space
bundles of higher dimension. The proof involves maximal orders
and toric geometry. As a corollary we show that the Brauer-Severi
variety of a Sklyanin algebra is rational.

2009年11月02日(月)

16:40-18:10   数理科学研究科棟(駒場) 126号室
Gerard van der Geer 氏 (Universiteit van Amsterdam)
Cohomology of moduli spaces of curves and modular forms
[ 講演概要 ]
The Eichler-Shimura theorem expresses cohomology of local systems
on the moduli of elliptic curves in terms of modular forms. The
cohomology of local systems can be succesfully explored by counting
points over finite fields. We show how this can be applied to
obtain a lot of information about the cohomology of other moduli spaces
of low genera and also about Siegel modular forms of genus 2 and 3.
This is joint work with Jonas Bergstroem and Carel Faber.

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