Algebraic Geometry Seminar

Seminar information archive ~10/26Next seminarFuture seminars 10/27~

Date, time & place Tuesday 15:30 - 17:00 122Room #122 (Graduate School of Math. Sci. Bldg.)

Seminar information archive

2017/02/10

14:00-15:30   Room #002 (Graduate School of Math. Sci. Bldg.)
Chenyang Xu (Beijing International Center of Mathematics Research)
Stability theory of a klt singularity II (English)
[ Abstract ]
In higher dimensional geometry, it has been known that from many perspectives a log terminal singularity is a local analogue of Fano varieties. Many statements of Fano varieties have a counterpart for log terminal singularities. One central topic on the geometry of a Fano variety is its stability which in particular reflects whether the Fano variety carries a canonical metric. In the talks, we will discuss a series of recent works started by Chi Li, and then by Harold Blum, Yuchen Liu and myself, in which we want to establish an algebro-geometric stability theory of a fixed log terminal singularity. Inspired by the study from differential geometry, (e.g. metric tangent cone, Sasakian-Einstein metric), for any log terminal singularity, we investigate the valuation which has the minimal normalized volume. Our goal is to prove various properties of this valuation which enable us to degenerate the singularity to a K-semistable T-singularity (with a torus action) in the Sasakian-Einstein sense.

2017/02/07

15:30-17:00   Room #117 (Graduate School of Math. Sci. Bldg.)
Chenyang Xu ( Beijing International Center of Mathematics Research)
Stability theory of a klt singularity I (English)
[ Abstract ]
In higher dimensional geometry, it has been known that from many perspectives a log terminal singularity is a local analogue of Fano varieties. Many statements of Fano varieties have a counterpart for log terminal singularities. One central topic on the geometry of a Fano variety is its stability which in particular reflects whether the Fano variety carries a canonical metric. In the talks, we will discuss a series of recent works started by Chi Li, and then by Harold Blum, Yuchen Liu and myself, in which we want to establish an algebro-geometric stability theory of a fixed log terminal singularity. Inspired by the study from differential geometry, (e.g. metric tangent cone, Sasakian-Einstein metric), for any log terminal singularity, we investigate the valuation which has the minimal normalized volume. Our goal is to prove various properties of this valuation which enable us to degenerate the singularity to a K-semistable T-singularity (with a torus action) in the Sasakian-Einstein sense.

2017/01/27

14:00-17:30   Room #122 (Graduate School of Math. Sci. Bldg.)
Luca Tasin (Roma Tre University) 14:00-15:30
On the number and boundedness of minimal models of general type (English)
[ Abstract ]
In this talk I will explain that the number of minimal models yields a constructible function on the base of any family of varieties of general type. From this it follows that the number of minimal models of a variety of general type can be bounded in terms of its volume. I will also show that in any dimension minimal models of general type and bounded volume form a bounded family. This is based on a joint work with D. Martinelli and S. Schreieder.
Roberto Svaldi (University of Cambridge/SISSA) 16:00-17:30
Adjoint dimension of foliations (English)
[ Abstract ]
The classification of foliated surfaces by Brunella, McQuillan and Mendes carries many similarities with Enriques-Kodaira classification of surfaces but also many important differences. I will discuss an alternative classification scheme where the role of differential forms along the leaves is replaced by differential forms along the leaves with values in fractional powers of the conormal bundle of the foliation. In this alternative setup one obtains a classification of foliated surfaces closer to the usual Enriques-Kodaira classification. If time permits, I will show how to apply this alternative classification to describe the Zariski closure of the set foliations which admit rational first integral of bounded genus in families of foliated surfaces. Joint work with Jorge Vitorio Pereira.
[ Reference URL ]
https://www.dpmms.cam.ac.uk/~rs872/

2016/11/29

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Karl Schwede (University of Utah)
Etale fundamental groups of F-regular schemes (English)
[ Abstract ]
I will discuss recent work studying etale fundamental groups of the regular locus of F-regular schemes. I will describe how to use F-signature to bound the size of the fundamental group of an F-regular scheme, similar to a result of Xu. I will then discuss a recent extension showing that every F-regular scheme X has a finite cover Y, etale over the regular lcous of X, so that the etale fundamental groups of Y and the regular locus of Y agree. This is analogous to results of Greb-Kebekus-Peternell.
All the work discussed is joint with Carvajal-Rojas and Tucker or with with Bhatt, Carvajal-Rojas, Graf and Tucker.

2016/11/08

15:30-17:00   Room #118 (Graduate School of Math. Sci. Bldg.)
Kenta Hashizume (Department of Mathematics, Kyoto University)
Minimal model theory for relatively trivial log canonical pairs
[ Abstract ]
The good minimal model conjecture is an important open problem in the birational geometry, and inductive arguments on the dimension of varieties are useful when we work on this conjecture. In fibration with a log canoinical pair having some good properties, it is expected that the above conjecture for the log canonical pair on total space can be proved by investigating the general fiber and base variety of the fibration. In this talk, I will explain an inductive argument in fibrations with relatively trivial log canonical pairs and introduce some applications of the inductive argument.

2016/10/25

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Yongnam Lee (KAIST/RIMS)
Q-Gorenstein deformation theory and it applications to algebraic surfaces (English)
[ Abstract ]
The notion of Q-Gorenstein variety is important for the minimal model theory and the compact moduli theory of algebraic varieties in characteristic 0. Also Q-Gorenstein deformation theory can be applied to construct (simply connected) surfaces of general type with geometric genus 0 over the field of any characteristic. In this talk, some applications of Q-Gorenstein deformation theory to algebraic surfaces and some interesting examples related to Q-Gorenstein morphisms will be presented.

2016/10/11

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Sho Ejiri (University of Tokyo)
On varieties with splittings of relative Frobenius morphisms of Albanese maps
[ Abstract ]
Varieties with splittings of Frobenius morphisms are called F-split varieties, which satisfy strong properties such as Kodaira vanishing. However, some important varieties are not F-split. For example, an abelian variety is F-split if and only if its p-rank is maximum. In this talk, we discuss the class of varieties with splittings of relative Frobenius morphisms of Albanese maps, which includes abelian varieties. As a consequence of Sannai and Tanaka's characterization of ordinary abelian varieties, we see that this class also includes F-split varieties. Furthermore, for varieties in this class, we show that the Kodaira vanishing theorem holds, and that Albanese maps are algebraic fiber spaces.

2016/10/04

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Taku Suzuki (Waseda University)
Higher order minimal families of rational curves and Fano manifolds with nef Chern characters (Japanese. Writing in English. )
[ Abstract ]
In this talk, we introduce higher order minimal families $H_i$ of rational curves
associated to Fano manifolds $X$. We prove that $H_i$ is also a Fano manifold
if the Chern characters of $X$ satisfy some positivity conditions. We also provide
a sufficient condition for Fano manifolds to be covered by higher rational manifolds.

2016/07/25

13:30-15:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Fumiaki Suzuki (Tokyo)
Birational rigidity of complete intersections (English)
[ Abstract ]
A complete intersection defined by s hypersurfaces of degree d_1, ... ,d_s in a projective space P^N is Q-Fano, i.e. normal, Q-factorial, terminal and having an ample anti-canonical divisor, if d_1 + ... + d_s is at most N and it has only mild singularities. Then it is rationally-connected by the results of Kollar-Miyaoka-Mori, Zhang and Hacon-Mckernan. A natural question is to determine its rationality. If its dimension or degree is at most 2, then it is rational. How about the remaining cases?

When d_1 + ... + d_s = N, birational rigidity give one of the most effective ways to tackle this problem. We recall that a Q-Fano variety is birationally superrigid if any birational map to the source of another Mori fiber space is isomorphism. It implies that X is non-rational and Bir(X) = Aut(X). After the works of Iskovskih-Manin, Pukhlikov, Chelt'so and de Fernex-Ein-Mustata, de Fernex proved that every smooth hypersurface of degree N in P^N is birationally superrigid for N at least 4. He also proved birational superrigidity of a large class of singular hypersurfaces of this type.

In this talk, we would like to extend de Fernex's results to complete intersections. As a key step, we generalize Pukhlikov's multiplicity bounds of cycles in hypersurfaces to complete intersections.

2016/07/25

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Sho Tanimoto (University of Copenhagen)
On the geometry of thin exceptional sets in Manin’s conjecture
[ Abstract ]
Manin’s conjecture predicts the asymptotic formula for the counting function of rational points on a Fano variety X after removing the exceptional sets. The original conjecture, which removes a proper closed subset, is wrong due to covering families of subvarieties violating the compatibility of Manin’s conjecture, and its refinement, suggested by Emmanuel Peyre, removes a thin set instead of a closed set. In this talk, first I would like to explain that subvarieties which conjecturally have more points than X only form a thin set using the minimal model program and the boundedness of log Fano varieties. After that, I would like to discuss our conjecture on the birational boundedness of covers violating the compatibility of Manin’s conjecture, and present some results in dimension 2 and 3. This is joint work with Brian Lehmann.

2016/07/12

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Kenji Matsuki (Purdue/RIMS)
Hypersurfaces of maximal contact and jumping phenomenon in the problem of resolution of singularities in positive characteristic (English)
[ Abstract ]
According to our approach for resolution of singularities in positive characteristic (called the Idealistic Filtration Program, alias the I.F.P.) the algorithm is divided into the following two steps:

Step 1. Reduction of the general case to the monomial case.

Step 2. Solution in the monomial case.

While we have established Step 1 in abritrary dimension, Step 2 becomes very subtle and difficult in positive characteristic. This is in clear contrast to the classical setting in characteristic zero, where the solution in the monomial case is quite easy.

The talk consists of the two parts.

・Part I [13:30--15:00]: This part is mainly for the students, who are not familiar with the classical results in characteristic zero. Through Hironaka's reformulation of the problem of resolution of singularities, we will see how the notion of a hypersurface of maximal contact provides an inductive structure on dimension to the problem, and hence leading to a solution. Since our I.F.P. is closely modelled upon the classical algorithm in characteristic zero, this part should also give some background material and motivation for our approach in positive characteristic.

・Part II [15:30--17:00]: This is the main body of my talk. I will proceed according to the following menu.

{\bf Framewrok of the I.F.P.}: First I will explain the framewrok of the I.F.P., which further extends Hironaka's refomulation. The biggest obstacle to establish Step 1 is the fact that, in positive characteristic, a smooth hypersurface of maximal contact does not exist in general. In order to overcome this obstacle, we introduce the notion of the Leading Generator System, which is the collection of multiple singular hypersurfaces of maximal contcat.

{\bf Monomial Case}: As metioned above, then the problem is reduced to the one in the monomial case.

・ {\bf Inductive scheme on the invariant \boldmath$\tau$}: We firstly observe that, by the inductive scheme on the invariant $\tau$, we have only to consider the case with $\tau = 1$, i.e., the case where there is only one single singular hypersurface of maximal contact.

・ {\bf Tight Monomail Case}: We secondly observe that, if we reach the so-called Tight Monomial Case, then we can easily solve the problem.

・ {\bf Introduction of the invariant `` \boldmath$\mathrm{inv}_{\mathrm{MON},real}$''}: Thus our final task is, after arriving at the monimial case with $\tau = 1$, to reach the Tight Monomial Case, which is characterized by $\mathrm{inv}_{\mathrm{MON},real} = 0$.

・ {\bf Moh-Hauser Jumping phenomenon}: The invariant $\mathrm{inv}_{\mathrm{MON},real}$ usually behaves well, i.e., decreases after each blow up. But under some circustances, it strictly increases. I will explain this well-known Moh-Jumping phenomenon by giving a simple example.

・ {\bf Eventual decrease of the jumping peaks}: At last, the problem boils down to analyzing and overcoming the Moh-Hauser Jumping phenomenon. For this purpose, we will present the conjecture of ``Eventual decrease of the jumping peaks'', which is affirmatively solved in dimension 3, and is the current focus of our research in dimension 4.
[ Reference URL ]
https://www.math.purdue.edu/people/bio/kmatsuki/home

2016/07/05

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Dulip Piyaratne (IPMU)
Generalized Bogomolov-Gieseker type inequality for Fano 3-folds (English)
[ Abstract ]
Construction of Bridgeland stability conditions on a given smooth projective 3-fold is an important problem. A conjectural construction for any 3-fold was introduced by Bayer, Macri and Toda, and the problem is reduced to proving so-called Bogomolov-Gieseker type inequality holds for certain stable objects in the derived category. It has been shown to hold for Fano 3-folds of Picard rank one due to the works of Macri, Schmidt and Li. However, Schmidt gave a counter-example for a Fano 3-fold of higher Picard rank. In this talk, I will explain how to modify the original conjectural inequality for general Fano 3-folds and why it holds.

2016/06/27

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Christopher Hacon (University of Utah)
Generic vanishing and birational geometry in char p>0 (ENGLISH)
[ Abstract ]
Many precise results on the birational geometry of irregular varieties have been obtained by combining the generic vanishing theorems of Green and Lazarsfeld with the Fourier-Mukai transform. In this talk we will discuss the failure of the generic vanishing theorems of Green and Lazarsfeld in positive characteristic. We will then explain a different approach to generic vanishing based on the theory of F-singularities that leads to concrete applications in birational geometry in positive characteristics
[ Reference URL ]
http://www.math.utah.edu/~hacon/

2016/06/20

16:30-18:00   Room #123 (Graduate School of Math. Sci. Bldg.)
De-Qi Zhang (National University of Singapore)
BUILDING BLOCKS OF POLARIZED ENDOMORPHISMS OF NORMAL PROJECTIVE VARIETIES (English)
[ Abstract ]
An endomorphism f of a normal projective variety X is polarized if f∗H ∼ qH for some ample Cartier divisor H and integer q > 1.

We first assert that a suitable maximal rationally connected fibration (MRC) can be made f-equivariant using a construction of N. Nakayama, that f descends to a polarized endomorphism of the base Y of this MRC and that this Y is a Q-abelian variety (quasi- ́etale quotient of an abelian variety). Next we show that we can run the minimal model program (MMP) f-equivariantly for mildly singular X and reach either a Q-abelian variety or a Fano variety of Picard number one.

As a consequence, the building blocks of polarized endomorphisms are those of Q- abelian varieties and those of Fano varieties of Picard number one.

Along the way, we show that f always descends to a polarized endomorphism of the Albanese variety Alb(X) of X, and that a power of f acts as a scalar on the Neron-Severi group of X (modulo torsion) when X is smooth and rationally connected.

Partial answers about X being of Calabi-Yau type or Fano type are also given with an extra primitivity assumption on f which seems necessary by an example.
This is a joint work with S. Meng.
[ Reference URL ]
http://www.math.nus.edu.sg/~matzdq/

2016/06/20

14:45-16:15   Room #123 (Graduate School of Math. Sci. Bldg.)
Zhixian Zhu (KIAS)
Fujita's freeness conjecture for 5-fold (English)
[ Abstract ]
Let X be a smooth projective variety of dimension n and L any ample line bundle. Fujita conjectured that the adjoint line bundle O(K_X+mL) is globally generated for any m greater or equal to dimX+1. By studying the singularity of pairs, we prove Fujita's freeness conjecture for smooth 5-folds.

2016/05/31

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Kiwamu Watanabe (Saitama University)
A Characterization of Symplectic Grassmannians (JAPANESE)
[ Abstract ]
In the series of their works, J. M. Hwang and N. Mok have been developing the theory of Varieties of Minimal Rational Tangents (VMRT for short). In this direction, the results of Mok and J. Hong-Hwang allow us to recognize a homogeneous Fano manifold X of Picard number one by looking at its VMRT at a general point. This characterization works for all rational homogeneous manifolds of Picard number one whenever the VMRT is rational homogeneous, which is always the case except for the short root cases; namely for symplectic Grassmannians, and for two varieties of type F*4*.

In this talk we show that, if we impose that the VMRT is the expected one at every point of the variety, we may still characterize symplectic Grassmannians. This is a joint work with G. Occhetta and L. E. Sola Conde (arXiv:1604.06867).

2016/05/24

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Katsutoshi Yamanoi (Osaka University)
ON PSEUDO KOBAYASHI HYPERBOLICITY OF SUBVARIETIES OF ABELIAN VARIETIES
(tba)
[ Abstract ]
We prove that the Kobayashi pseudo distance of a closed subvariety X of an abelian variety A is a true distance outside the special set Sp(X) of X, where Sp(X) is the union of all positive dimensional translated abelian subvarieties of A which are contained in X. More strongly, we prove that a closed subvariety X of an abelian variety is taut modulo Sp(X); Every sequence fn : D → X of holomorphic mappings from the unit disc D admits a subsequence which converges locally uniformly, unless the image fn(K) of a fixed compact set K of D eventually gets arbitrarily close to Sp(X) as n gets larger. These generalize a classical theorem on algebraic degeneracy of entire holomorphic curves in irregular varieties.

2016/05/17

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Atsushi Ito (Dep. of Math. Kyoto Univ. )
On dual defects of toric varieties (TBA)
[ Abstract ]
For a projective variety embedded in a projective space,
we can define the dual variety in the dual projective space.
By dimension count, the codimension of the dual variety is expected to be one,
but it can be greater than one for some varieties.

For a smooth toric variety, it is known that the codimension of the dual variety is greater than one
if and only if the toric variety is a suitable projective bundle over some toric variety.
In this talk, I will explain a generalization of this result to toric varieties without the assumption of singularities.
This is a joint work with Katsuhisa Furukawa.
[ Reference URL ]
https://sites.google.com/site/atsushiito221/

2016/04/26

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Yuji Odaka (Dept. of Math., Kyoto U.)
A gentle introduction to K-stability and its recent development (Japanese)
[ Abstract ]
K安定性とは複素代数多様体上の「標準的な」ケーラー計量の存在問題に端を発する,代数幾何的な概念です.二木先生や満渕先生等の先駆的な仕事に感化されて導入され,特に近年ホットに研究され始めている一方,未だその大半はより微分幾何的な研究者の方々や背景の中でなされているように講演者には感じられます.

代数幾何的にもどのように面白いか,どういった意義があるかに私見で軽く触れた上で,その基礎付けをより拡張した枠組みで説明しつつ,最先端でどのようなことが問題になっているかをいくらか(私の力量と時間の許す限り)解説しつつ,文献をご紹介できればと思っています
[ Reference URL ]
https://sites.google.com/site/yujiodaka2013/

2016/04/19

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Keiji Oguiso (University of Tokyo)
Isomorphic quartic K3 surfaces and Cremona transformations (JAPANESE)
[ Abstract ]
We show that

(i) there is a pair of smooth complex quartic K3 surfaces such that they are isomorphic as abstract varieties but not Cremona equivalent.

(ii) there is a pair of smooth complex quartic K3 surfaces such that they are Cemona equivalent but not projectively equivalent.

These two results are much inspired by e-mails from Professors Tuyen Truong and J\'anos Koll\'ar.

2016/04/11

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Piotr Pragacz (Institute of Mathematics, Polish Academy of Sciences )
Gysin maps, duality and Schubert classes (English)
[ Abstract ]
We establish a Gysin formula for Schubert bundles
and a strong version of the duality theorem in Schubert calculus
on Grassmann bundles. We then combine them to compute the fundamental
classes of Schubert bundles in Grassmann bundles, which yields a new
proof of the Giambelli formula for vector bundles. This is a joint
work with Lionel Darondeau.
[ Reference URL ]
https://www.impan.pl/~pragacz/main.htm

2015/12/17

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Dulip Piyaratne (IPMU)
Polarization and stability on a derived equivalent abelian variety (English)
[ Abstract ]
In this talk I will explain how one can define a polarization on a derived equivalent abelian variety by using Fourier-Mukai theory. Furthermore, we see how such a realisations is connected with stability conditions on their derived categories. Then I will discuss these ideas for abelian surfaces and abelian 3-folds in detail.
[ Reference URL ]
http://db.ipmu.jp/member/personal/3989en.html

2015/12/14

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Atsushi Kanazawa (Harvard)
Extending Hori-Vafa toric mirror symmetry via SYZ and modular forms (English)
[ Abstract ]
In this talk, I will introduce partial compactification for a class of toric Calabi-Yau manifolds. A fundamental question is how the Hori-Vafa toric mirror symmetry extends to this new class of Calabi-Yau manifolds. The answer leads us to a new connection between SYZ mirror symmetry and modular forms. If time permits, I will also discuss higher dimensional analogues of the Yau-Zaslow formula (for an elliptic K3 surface) in terms of Siegel modular forms. This talk is based on a joint work with Siu-Cheong Lau.

2015/12/07

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Alexey Bondal (IPMU)
Flops and spherical functors (English)
[ Abstract ]
I will describe various functors on derived categories of coherent sheaves
related to flops and relations between these functors. A categorical
version of deformation theory of systems of objects in abelian categories
will be outlined and its relation to flop spherical functors will be
presented.

2015/11/30

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Fabrizio Catanese (Universität Bayreuth)
Interesting surfaces which are coverings of a rational surface branched over few lines (English)
[ Abstract ]
Surfaces which are covers of the plane branched over 5 or 6 lines have provided answers to long standing questions, for instance the BCD surfaces for Fujita's question on semiampleness of VHS (Dettweiler-Cat); and examples of ball quotients (Hirzebruch), automorphisms acting trivially on integral cohomology (Cat-Gromadtzki), canonical maps with high degree or image-degree (Pardini, Bauer-Cat). I shall speak especially about the above Abelian coverings of the plane, the geometry of the del Pezzo surface of degree 5, the rigidity of BCD surfaces, and a criterion for a fibred surface to be a projective classifying space.

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