代数学コロキウム

過去の記録 ~12/03次回の予定今後の予定 12/04~

開催情報 水曜日 17:00~18:00 数理科学研究科棟(駒場) 056号室
担当者 今井 直毅, 三枝 洋一

過去の記録

2021年11月24日(水)

17:00-18:00   オンライン開催
小原 和馬 氏 (東京大学大学院数理科学研究科)
On the formal degree conjecture for non-singular supercuspidal representations (Japanese)
[ 講演概要 ]
We prove the formal degree conjecture for non-singular supercuspidal representations based on Schwein's work proving the formal degree conjecture for regular supercuspidal representations. The main difference between our work and Schwein's work is that in non-singular case, the Deligne--Lusztig representations can be reducible, and the $S$-groups are not necessarily abelian. Therefore, we have to compare the dimensions of irreducible constituents of the Deligne--Lusztig representations and the dimensions of irreducible representations of $S$-groups.

2021年10月20日(水)

17:00-18:00   オンライン開催
Alex Youcis 氏 (東京大学大学院数理科学研究科)
Geometric arc fundamental group (English)
[ 講演概要 ]
Unlike algebraic geometry, the correct notion for a ‘covering space’ of a rigid analytic variety is non-obvious to define. In particular, the class of finite etale covering spaces doesn’t encompass many real world examples of ‘covering space’-like maps (e.g. Tate’s uniformization of elliptic curves, or period mappings of Rapoport—Zink spaces). In de Jong’s seminal work on the topic he made great strides forward by studying a notion of covering space, suggested by work of Berkovich, which includes many previous ‘covering spaces’ which are not finite etale and is rich enough to support a theory of a fundamental group.

Unfortunately, de Jong’s notion of covering space lacks many of the natural properties one would expect from the notion of a ‘covering space’. In this talk we discuss recent work of Achinger, Lara, and myself which proposes a larger class of ‘covering spaces’ than those considered by de Jong which enjoys the geometric properties missing from de Jong’s picture. In addition, we mention how this larger category is related to work of Scholze on pro-etale local systems as well as work of Bhatt and Scholze on the pro-etale fundamental group of a scheme.

2021年07月07日(水)

17:00-18:00   オンライン開催
吉田 匠 氏 (慶應義塾大学)
On the BSD conjecture for the quadratic twists of the elliptic curve $X_0(49)$ (Japanese)
[ 講演概要 ]
The full BSD conjecture (the full Birch-Swinnerton-Dyer conjecture) is the important conjecture, which connects the algebraic invariants and analytic invariants of elliptic curves. When the elliptic curve is defined over $\mathbb{Q}$, these invariants are known to be rational numbers. Now, even when the elliptic curve is defined over $\mathbb{Q}$ and the $L$-function is not $0$ at $s=1$, it is not shown that the $2$-orders of these invariants are equal. Coates, Kim, Liang and Zhao proved the full BSD conjecture for some quadratic twists of $X_0(49)$, by proving that these $2$-orders are same. We extends this result, and prove the full BSD conjecture for more twists.

2021年06月30日(水)

17:00-18:00   オンライン開催
李 公彦 氏 (東京大学大学院数理科学研究科)
Prismatic and q-crystalline sites of higher level (Japanese)
[ 講演概要 ]
Two new p-adic cohomology theories, called prismatic cohomology and q-crystalline cohomology, were defined for generalizing crystalline cohomology and they recover most known integral p-adic cohomology theories. On the other hand, higher level crystalline cohomology was defined for constructing p-adic cohomology theory over a ramified base. In this talk, for a positive integer m, we will give a construction of the level m primastic and q-crystalline sites and prove a certain equivalence between the category of crystals on the m-prismatic site or the m-q-crystalline site and that on the usual prismatic site or the usual q-crystalline site, which can be regarded as the prismatic analogue of the Frobenius descent. We will also prove the equivalence between the category of crystals on the m-prismatic site and that on the (m-1)-q-crystalline site.

2021年06月23日(水)

17:00-18:00   オンライン開催
今井 湖都 氏 (東京大学大学院数理科学研究科)
Ramification groups of some finite Galois extensions of maximal nilpotency class over local fields of positive characteristic (Japanese)
[ 講演概要 ]
Galois extensions of local fields is one of the most important subjects in the field of number theory. A ramification filtration is a filtration of a Galois group used to investigate the ramification of the extension. It is particularly useful when the extension is wildly ramified. In this talk, we examine the ramification groups of finite Galois extensions over complete discrete valuation fields of characteristic $p>0$. Brylinski calculated the ramification groups in the case where the Galois groups are abelian. We extend the results of Brylinski to some non-abelian cases where the Galois groups are of order $\leq p^{p+1}$ and of maximal nilpotency class.

2021年06月16日(水)

17:00-18:00   オンライン開催
寺門 康裕 氏 (National Center for Theoretical Sciences)
Hecke eigensystems of automorphic forms (mod p) of Hodge type and algebraic modular forms (Japanese)
[ 講演概要 ]
In a 1987 letter to Tate, Serre showed that the prime-to-p Hecke eigensystems arising in the space of mod p modular forms are the same as those appearing in the space of automorphic forms on a quaternion algebra. This result is regarded as a mod p analogue of the Jacquet-Langlands correspondence. In this talk, we give a generalization of Serre's result to the Hecke eigensystems of mod p automorphic forms on a Shimura variety of Hodge type with good reduction at p. This is joint work with Chia-Fu Yu.

2021年05月26日(水)

17:00-18:00   オンライン開催
島田 了輔 氏 (東京大学大学院数理科学研究科)
Geometric Structure of Affine Deligne-Lusztig Varieties for $\mathrm{GL}_3$ (Japanese)
[ 講演概要 ]
The Langlands correspondence, which contains class field theory as a special case, is one of the most important topics in number theory. Shimura varieties have been used, with great success, towards applications in the realm of the Langlands program. In this context, geometric and homological properties of affine Deligne-Lusztig varieties have been used to examine Shimura varieties and the local Langlands correspondence.
In this talk we study the geometric structure of affine Deligne-Lusztig varieties $X_{\lambda}(b)$ for $\mathrm{GL}_3$ and $b$ basic.
We completely determine the irreducible components of the affine Deligne-Lusztig variety. In particular, we classify the cases where all of the irreducible components are classical Deligne-Lusztig varieties times finite-dimensional affine spaces. If this is the case, then the irreducible components are pairwise disjoint.

2021年03月10日(水)

17:00-18:00   オンライン開催
板東 克之 氏 (東京大学数理科学研究科)
Geometric Satake equivalence in mixed characteristic and Springer correspondence (Japanese)
[ 講演概要 ]
The geometric Satake correspondence is an equivalence between the category of equivariant perverse sheaves on the affine Grassmannian and the category of representations of the Langlands dual group. It is known that there is a mixed characteristic version of the geometric Satake correspondence. The Springer correspondence is a correspondence between the category of equivariant perverse sheaves on the nilpotent cone and the category of representation of the Weyl group. In this talk, we will explain some relation between these two correspondences, including the mixed characteristic case.

2021年01月20日(水)

17:00-18:00   オンライン開催
齋藤 勇太 氏 (東京大学数理科学研究科)
Overconvergent Lubin-Tate $(\varphi, \Gamma)$-modules for different uniformizers (Japanese)
[ 講演概要 ]
$(\varphi, \Gamma)$-modules are used for investigating p-adic Galois representations, which has an important role in constructing the p-adic local Langlands correspondence for GL_2(Q_p). When we try to construct the p-adic local correspondence for GL_2(F) for a general local field F, we want more useful and more suitable $(\varphi, \Gamma)$-modules. Lubin-Tate $(\varphi, \Gamma)$-modules are the candidates for such $(\varphi, \Gamma)$-modules. Lubin-Tate extensions are used for defining Lubin-Tate $(\varphi, \Gamma)$-modules. However, these extensions depend on the choice of uniformizers and the behavior of Lubin-Tate $(\varphi, \Gamma)$-modules for different uniformizers has not been discussed so much. We focus on overconvergency and discuss the coincidence for 2-dimensional triangulable $(\varphi, \Gamma)$-modules for different uniformizers.

2020年12月16日(水)

17:00-18:00   オンライン開催
山田 一紀 氏 (慶應義塾大学)
Rigid analytic Hyodo--Kato theory with syntomic coefficients (Japanese)
[ 講演概要 ]
The Hyodo—Kato theory is the study of comparison between Hyodo—Kato cohomology and de Rham cohomology associated to semistable schemes over complete discrete valuation rings of mixed characteristic $(0,p)$.
In this talk, we will give a rigid analytic reconstruction of Hyodo—Kato theory and study coefficients of cohomology.
Our construction is useful for explicit computation and treatment of base extension, because it gives us a natural interpretation of the dependence of Hyodo—Kato theory on the choice of a branch of the $p$-adic logarithm.
The results of this talk are based on a joint work with Veronika Ertl, which deals with the case of trivial coefficient.

2020年11月18日(水)

17:00-18:00   オンライン開催
竹内 大智 氏 (東京大学数理科学研究科)
Symmetric bilinear forms and local epsilon factors of isolated singularities in positive characteristic (Japanese)
[ 講演概要 ]
For a function on a smooth variety with an isolated singular point, we have two invariants. One is a non-degenerate symmetric bilinear form (de Rham), and the other is the vanishing cycles complex (\'etale). The latter is a Galois representation of a local field measuring a complexity of the singularity.
In this talk, I will give a formula which expresses the local epsilon factor of the vanishing cycles complex in terms of the bilinear form. In particular, the sign of the local epsilon factor is determined by the discriminant of the bilinear form. This can be regarded as a refinement of Milnor formula in SGA 7, which compares the rank of the bilinear form and the total dimension of the vanishing cycles.
In characteristic 2, we find a generalization of Arf invariant, which can be regarded as an invariant for a non-degenerate quadratic singularity, to a general isolated singularity.

2020年06月17日(水)

17:30-18:30   オンライン開催
Christophe Breuil 氏 (CNRS, Université Paris-Sud)
On modular representations of GL_2(L) for unramified L (ENGLISH)
[ 講演概要 ]
Let p be a prime number and L a finite unramified extension of Q_p. We give a survey of past and new results on smooth admissible representations of GL_2(L) that appear in mod p cohomology. This is joint work with Florian Herzig, Yongquan Hu, Stefano Morra and Benjamin Schraen.
[ 参考URL ]
https://www.ms.u-tokyo.ac.jp/~t-saito/todai_IHES.html

2020年05月27日(水)

17:30-18:30   オンライン開催
坂内健一 氏 (慶応大学)
Shintani generating class and the p-adic polylogarithm for totally real fields (ENGLISH)
[ 講演概要 ]
In this talk, we will give a new interpretation of Shintani's work concerning the generating function of nonpositive values of Hecke $L$-functions for totally real fields. In particular, we will construct a canonical class, which we call the Shintani generating class, in the cohomology of a certain quotient stack of an infinite direct sum of algebraic tori associated with a fixed totally real field. Using our observation that cohomology classes, not functions, play an important role in the higher dimensional case, we proceed to newly define the p-adic polylogarithm function in this case, and investigate its relation to the special value of p-adic Hecke $L$-functions. Some observations concerning the quotient stack will also be discussed. This is a joint work with Kei Hagihara, Kazuki Yamada, and Shuji Yamamoto.
[ 参考URL ]
https://www.ms.u-tokyo.ac.jp/~t-saito/todai_IHES.html

2020年05月13日(水)

17:30-18:30   オンライン開催
Yifeng Liu 氏 (Yale University)
On the Beilinson-Bloch-Kato conjecture for Rankin-Selberg motives (ENGLISH)
[ 講演概要 ]
In this talk, we will explain the final outcome on the Beilinson-Bloch-Kato conjecture for motives coming from certain automorphic representations of GL(n) x GL(n+1), of our recent project with Yichao Tian, Liang Xiao, Wei Zhang, and Xinwen Zhu. In particular, we show that the nonvanishing of the central L-value of the motive implies the vanishing of the corresponding Bloch-Kato Selmer group. We will also explain the main ideas and ingredients of the proof.
[ 参考URL ]
https://www.ms.u-tokyo.ac.jp/~t-saito/todai_IHES.html

2020年04月22日(水)

17:30-18:30   オンライン開催
Arthur-César Le Bras 氏 (CNRS & Université Paris 13)
Prismatic Dieudonné theory (ENGLISH)
[ 講演概要 ]
I would like to explain a classification result for p-divisible groups, which unifies many of the existing results in the literature. The main tool is the theory of prisms and prismatic cohomology recently developed by Bhatt and Scholze. This is joint work with Johannes Anschütz.

本講演は「東京北京パリ数論幾何セミナー」として,インターネットによる
東大数理,Morningside Center of Mathematics と IHES の双方向同時中継で行います.
今回はパリからの中継です.

2019年12月04日(水)

17:00-18:00   数理科学研究科棟(駒場) 117号室
竹内 大智 氏 (東京大学数理科学研究科)
Characteristic epsilon cycles of l-adic sheaves on varieties (ENGLISH)
[ 講演概要 ]
For l-adic sheaves on varieties over finite fields, the constant terms of the functional equations of the L-functions, called global epsilon factors, are important arithmetic invariants. When the varieties are curves, Deligne and Laumon show that they admit product formulae in terms of local epsilon factors.
In this talk, I will explain that, attaching some coefficients to irreducible components of singular supports, we can define refinements of characteristic cycles. We will see that, after taking modulo roots of unity, they give product formulae of global epsilon factors for higher dimensional varieties.
I will also explain that these results can be generalized to arbitrary perfect fields of any characteristic.

2019年11月27日(水)

17:00-18:00   数理科学研究科棟(駒場) 117号室
Ahmed Abbes 氏 (CNRS & IHÉS)
The relative Hodge-Tate spectral sequence (ENGLISH)
[ 講演概要 ]
It is well known that the p-adic étale cohomology of a smooth and proper variety over a p-adic field admits a Hodge-Tate decomposition and that it is the abutment of a spectral sequence called Hodge-Tate; these two properties are incidentally equivalent. The Hodge-Tate decomposition was generalized in higher dimensions to Hodge-Tate local systems by Hyodo, and was studied by Faltings, Tsuji and others. But the generalization of the Hodge-Tate spectral sequence to a relative situation has not yet been considered (not even conjectured), with the exception of a special case of abelian schemes by Hyodo. This has now been done in a joint work with Michel Gros. The relative Hodge-Tate spectral sequence that we construct takes place in the Faltings topos, but its construction requires the introduction of a relative variant of this topos which is the main novelty of our work. The relative Hodge-Tate spectral sequence sheds new light on the fact that the relative p-adic étale cohomology is Hodge-Tate, but the two properties are not equivalent in general.

2019年11月20日(水)

18:00-19:00   数理科学研究科棟(駒場) 056号室
Vasudevan Srinivas 氏 (Tata Institute of Fundamental Research)
Algebraic versus topological entropy for surfaces over finite fields (ENGLISH)
[ 講演概要 ]
For an automorphism of an algebraic variety, we consider some properties of eigenvalues of the induced linear transformation on l-adic cohomology, motivated by some results from complex dynamics, related to the notion of entropy. This is a report on joint work with Hélène Esnault, and some subsequent work of K. Shuddhodan.

(本講演は「東京北京パリ数論幾何セミナー」として,インターネットによる東大数理,Morningside Center of Mathematics と IHES の双方向同時中継で行います.今回は東京からの中継です.)

2019年10月16日(水)

17:30-18:30   数理科学研究科棟(駒場) 056号室
Liang Xiao 氏 (BICMR, Peking University)
On slopes of modular forms (ENGLISH)
[ 講演概要 ]
In this talk, I will survey some recent progress towards understanding the slopes of modular forms, with or without level structures. This has direct application to the conjecture of Breuil-Buzzard-Emerton on the slopes of Kisin's crystabelline deformation spaces. In particular, we obtain certain refined version of the spectral halo conjecture, where we may identify explicitly the slopes at the boundary when given a reducible non-split generic residual local Galois representation. This is a joint work in progress with Ruochuan Liu, Nha Truong, and Bin Zhao.

(本講演は「東京北京パリ数論幾何セミナー」として,インターネットによる東大数理,Morningside Center of Mathematics と IHES の双方向同時中継で行います.今回は北京からの中継です.)

2019年10月09日(水)

17:00-18:00   数理科学研究科棟(駒場) 056号室
Yuanqing Cai 氏 (京都大学)
Twisted doubling integrals for classical groups (ENGLISH)
[ 講演概要 ]
In the 1980s, Piatetski-Shapiro and Rallis discovered a family of Rankin-Selberg integrals for the classical groups that did not rely on Whittaker models. This is the so-called doubling method. It grew out of Rallis' work on the inner products of theta lifts -- the Rallis inner product formula.
In this talk, we present a family of Rankin-Selberg integrals (the twisted doubling method, in joint work with Friedberg, Ginzburg, and Kaplan) for the tensor product L-function of a pair of automorphic cuspidal representations, one of a classical group, the other of a general linear group. This can be viewed as a generalization of the doubling integrals of Piatetski-Shapiro and Rallis. Time permitting, we will discuss the twisted doubling integrals for Brylinski-Deligne covers of classical groups.

2019年07月03日(水)

17:00-18:00   数理科学研究科棟(駒場) 056号室
佐藤謙 氏 (東京大学数理科学研究科)
Explicit calculation of values of the regulator maps on a certain type of Kummer surfaces (Japanese)
[ 講演概要 ]
複素数に埋め込まれた体K上定義された射影代数多様体Xに対して、レギュレーター写像というモチヴィックコホモロジーからDeligneコホモロジーへの写像がBeilinsonにより定義された。特にKが有理数体の時、Beilinsonによりレギュレーター写像の値はモチーフのL関数の特殊値の無理数部分と結びつくと予想されているが、予想が成り立つことが知られている例は少ない。しかしながら、レギュレーター写像の値を超幾何関数のような特殊関数を用いて表す研究は朝倉政典氏や大坪紀之氏の研究に見られるように近年盛んである。本講演では、楕円曲線の直積に付随するようなKummer曲面に対し、高次Chow群との同型を用いてモチヴィックコホモロジーの中に具体的に元を構成し、その元のレギュレーター写像による値を考察する。またその応用として、上記の曲面のモチヴィックコホモロジーのindecomposal partが複素数体上十分一般の場合に消えていないことを示す。

2019年06月12日(水)

17:00-18:00   数理科学研究科棟(駒場) 056号室
笠浦一海 氏 (東京大学数理科学研究科)
On extension of overconvergent log isocrystals on log smooth varieties (Japanese)
[ 講演概要 ]
Kを混標数の完備な非アルキメデス付値体とし,kをその剰余体とする.
Kedlayaおよび志甫の研究により,k上の滑らかな代数多様体Xとその上の単純正規交叉因子Zについて,X ¥setminus Z上の過収束アイソクリスタルのうちZの周りである種のモノドロミーを持つものは,XにZから定まる対数的構造を入れた対数的代数多様体上の収束対数的アイソクリスタルに延長できることが知られている.
本講演では,この結果の,適当な条件を満たす一般の対数的に滑らかな代数多様体と,その対数的構造から定められる部分スキーム上の過収束対数的アイソクリスタルへの拡張について議論する.

2019年06月05日(水)

17:30-18:30   数理科学研究科棟(駒場) 056号室
服部新 氏 (東京都市大学)
Duality of Drinfeld modules and P-adic properties of Drinfeld modular forms (English)
[ 講演概要 ]
Let p be a rational prime, q>1 a p-power and P a non-constant irreducible polynomial in F_q[t]. The notion of Drinfeld modular form is an analogue over F_q(t) of that of elliptic modular form. Numerical computations suggest that Drinfeld modular forms enjoy some P-adic structures comparable to the elliptic analogue, while at present their P-adic properties are less well understood than the p-adic elliptic case. In 1990s, Taguchi established duality theories for Drinfeld modules and also for a certain class of finite flat group schemes called finite v-modules. Using the duality for the latter, we can define a function field analogue of the Hodge-Tate map. In this talk, I will explain how the Taguchi's theory and our Hodge-Tate map yield results on Drinfeld modular forms which are classical to elliptic modular forms e.g. P-adic congruences of Fourier coefficients imply p-adic congruences of weights.

2019年05月29日(水)

17:00-18:00   数理科学研究科棟(駒場) 056号室
沖泰裕 氏 (東京大学数理科学研究科)
On supersingular loci of Shimura varieties for quaternion unitary groups of degree 2 (Japanese)
[ 講演概要 ]
PEL型志村多様体のp進整数環上の整モデルは, Abel多様体と付加構造のモジュライ空間として定義される. その幾何的特殊ファイバーのうち, 超特異Abel多様体に対応する点からなる閉部分スキームを超特異部分という. 超特異部分の構造の明示的な記述は, arithmetic intersectionをはじめとする整数論への応用をもつことが知られている.
本講演では, 2次四元数ユニタリ群に対する志村多様体の超特異部分の明示的記述に関して, 講演者が得た結果を紹介する. また, 関連するRapoport-Zink空間の底空間に対する同様の結果についても言及する.

2019年05月08日(水)

17:00-18:00   数理科学研究科棟(駒場) 056号室
山本祐輝 氏 (東京大学数理科学研究科)
On the types for supercuspidal representations of inner forms of GL_n (Japanese)
[ 講演概要 ]
Aを非アルキメデス的局所体F上の中心的単純環とし,Gをその乗法群とする.
Gのsmooth表現を考察する際に有用な理論の一つとしてtypeの理論が存在する.
type (J, ¥lambda) とはGのコンパクト部分群Jと J の既約部分表現 ¥lambda の組であって,Gの既約表現をある意味で分類することのできるものである.
S¥'echerre-Stevenにより,Gのtypeの族としてsimple typeという概念が構成されている.
本講演ではGのtypeについて説明した後,JをGの極大コンパクト部分群Kとして固定した場合にtypeがどれだけ存在するかについて議論する.
特に,Gのsupercuspidal表現 ¥pi に対し,¥pi がsimple typeとしてある種の不分岐的な条件を満たすようなものを含むときに,Kの表現で ¥pi に対応するtypeがGでの共役を除き一意であることを示す.

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