Location: 

Mathematical Institute (Aobayama campus) https://maps.app.goo.gl/zcUcVf2zutWY3Zr89 ,or 

Science Complex A (Aobayama campus) https://maps.app.goo.gl/U8mZQsw3L7NhrrmA6

日時：12月4日（木）Thursday 4 Dec 2025

15:00-16:30

場所：数学系研究棟209室 (Room 209, Mathematical Institute)

Speaker: Tanabe Naomi (Bowdoin College)

Title: Large Sums of Fourier Coefficients of Modular Forms 

 Abstract: Understanding the global behavior of the Fourier coefficients of modular forms remains challenging, despite substantial progress in their local analysis. Summatory functions provide a powerful tool for capturing this global behavior. In this talk, we study large sums of coefficients and of their products, and describe the arithmetic information encoded in these large sums. We further show how the asymptotics of these sums are intertwined with the zeros of the corresponding L-functions. 

2025年12月11日(木) = Thu 11 Dec 2025

2 talks!

12:30からもみじダイニングにて、講演者を囲んでのランチを予定しております。参加される方は、12:30にもみじダイニング入口付近にお集まりください。

Lunch: Those who join meet at 12:30 in front of the cafeteria opposite to the math building.

講演者1：加藤裕基氏（久留米高専）

日時：12月11日（木）

13:30--15:00

場所：数学系研究棟517室 (Room 517, Mathematical Institute)

タイトル: Smithイデアル理論とAlmost mathematics

アブストラクト: 

Smithイデアル理論はHoveyとJeffrey H. Smithにより導入された対称モノイダル・モデル圏で展開されるイデアル理論である.　Smithイデアル理論は環準同型の核あるいは環の剰余環で定式化される代数の理論をモデル圏へ展開する可能性を含んでおり, 本講演ではSmithイデアル理論とalmost mathematicsの概説, Smithイデアル理論を用いたalmost mathematicsのモデル圏への展開と得られた結果を紹介する. 時間が許せば他の応用も紹介したい.　

Speaker 2: Mainak Gosh (Indian Institute of Science, Bangalore)

15:15-16:45

Place: Room 209, Mathematical Institute

Title: Bertini type theorems over fields and discrete valuation rings

Abstract:
The classical Bertini theorem states that, given a smooth projective variety X over an algebraically closed field, almost all hypersurface sections of X are also smooth. Given this, some natural questions come to mind. Does the base field have to be algebraically closed? Can one replace the base field by a more general base scheme? Can we replace smoothness with some other properties? In this talk, we will address some of these questions.

We will explore various extensions of Bertini-type statements over both finite and infinite fields. Over finite fields, we discuss extensions of classical Bertini theorems using Poonen’s closed-point sieve, which offers a probabilistic framework for establishing the existence of hypersurfaces with prescribed properties. We will also present results concerning Bertini-type theorems over discrete valuation rings. This talk is based on joint work with Prof. Amalendu Krishna.

Dinner: Contact Wataru Kai kaiw(at)tohoku.ac.jp by Monday 8 Dec. 

2025年12月23日(火) = Tuesday 23 Dec 2025

13:30--15:00

Place: 合同A棟508 (to be confirmed) = 508, Science Complex A (to be confirmed)

Speaker: 石塚康介 氏 (東北大学)

Title: Non-spherically complete valued field 上の有限次元ノルム空間の等長分類

Abstract:

Non-spherically complete valued field 上の 2 次元ノルム空間は本質的に二つの型を持つことが知られている. 一方で 3 次元以上に対しては, 統一的に調べる手法が存在せず手つかずであった. 本講演では, 次元によらない有限次元ノルム空間の分類手法をいくつか紹介し, それを応用して 3, 4 次元のノルム空間を分類したことを報告する. またこの分類を応用して, Schikhof が提唱した未解決問題である, 有限次元の strictly epicompact set の分類に対してアプローチする.

講演者：藤野修氏（京都大学）

日時：1月8日（木）13時30分から15時00分まで

場所：数理科学記念館（川井ホール）

タイトル：小平次元の不等式について

アブストラクト：非特異複素射影多様体の間の全射$f \colon X \to Y$を考える。ファイバーは連結であると仮定する。このとき、一般ファイバーを$F$として$\kappa(X) \ge \kappa(Y) + \kappa(F)$が成り立つと期待されている。これは飯高予想として知られている。一方、Popaは射$f$が滑らかな場合には、逆向きの不等式$\kappa(X) \le \kappa(Y) + \kappa(F)$も成立するのではないかと予想した。この予想は専門家にとっても意外なものであった。これらの予想は、より一般に対数的小平次元の場合にも成立すると期待されている。本講演では、これらの劣加法性予想および優加法性予想のいずれもが、一般化されたアバンダンス予想から導かれることを説明する。アバンダンス予想は、極小モデル理論における最も難しい未解決問題の一つとして知られている。本講演の内容は、藤澤太郎氏との共同研究に基づくものである。

2025年11月27日(木) Thursday 27 Nov 2025

13:00-14:30

Place: 合同A棟508 (= Room 508, Science Complex A)

Speaker: 角野 裕太 氏 (東北大学) [Yuta Kadono (Tohoku University)]

Title: 無限積を用いた（$q$-）超幾何関数の特殊値公式の新たな導出方法について

Abstract: 

超幾何関数は、ある線型微分方程式によって特徴付けられる特殊関数であり、多くの数学分野や数理物理において重要な役割を果たしてきた。超幾何関数に対する重要な問題の一つは、パラメータを特殊化したときの $z=1$ における特殊値をガンマ関数などの「よく知られた関数」で表す明示式（和公式）がどれだけ存在し、それらをどの程度統一的に導出・記述できるかを調べることである。特定のパラメータ条件を課した場合や、パラメータが整数シフトしている場合については体系的な理解が進んでいる一方で、こうした状況から外れる超幾何関数の系列に対する和公式はあまり知られていない。本講演では、多重ゼータ値の母関数にその起源をもつある無限積に着目し、既存の枠組みでは捉えることができない超幾何関数の和公式を導出する新たな手法について解説する。さらに、本手法の本質的な構成に注目することにより、得られた和公式をその $q$-類似や多重base版、逆数版などへ自然に拡張できることを、$q$-超幾何関数の和公式を具体例として示す。

Lunch: Espace Ouvert (Science Complex C) 11:45

2025年11月20日(木) Thursday 20 Nov 2025

13:00-14:30

Place: Room 508, Science Complex A (= 合同A棟508)

Speaker: 根本 裕介 氏 (千葉大学 / 早稲田大学本庄高等学院) [Yusuke Nemoto (Chiba University / Honjo Senior High School, Waseda University)]

Title: Non-triviality of the modified diagonal cycles of hypergeometric curves

Abstract: 

The modified diagonal cycles are important examples of algebraic cycles that are generically homologically trivial but algebraically non-trivial. However, it is difficult to show the non-triviality for the modified diagonal cycles of specific curves. In this talk, we consider hypergeometric curves (which are introduced by Asakura-Otsubo) that are closely related to Gaussian hypergeometric functions and Fermat curves. We prove that the modified diagonal cycles of hypergeometric curves are non-trivial modulo rational equivalence under some assumptions. This is a joint work with P. Eskandari.

Dinner: Contact Takuya Yamauchi by Monday 17 Nov. (= 夕方の懇親会に参加される方は11月17日(月)までに山内卓也さんにご連絡ください。)

2025年9月30日(火) Tuesday 30 Sep 2025

13:30-15:00

Place: Room 209, Mathematical Institute (= 数学棟209)

Speaker: Wansu Kim (KAIST, South Korea)

Title: On degeneration of $D$-shtukas over ramified legs

Abstract: 

Let $D$ be a central division algebra over a global function field $F$, and choose a parahoric integral model $G$ of $D^\times$ over the smooth projective model of $F$. We obtain a sufficient condition for properness of moduli stacks of $r$-legged $G$-shtukas bounded by some Beilinson--Drinfeld Schubert variety allowing legs, allowing legs to meet the ramification locus of $D$. In particular, we show that the moduli stack is proper if $D$ is ``sufficiently ramified'' relative to the number of legs $r$ and the bound. Our sufficient condition is more stringent than the condition for the moduli stack to be proper over the unramified locus for $G$, obtained by Eike Lau. This is joint work with Yong-Gyu Choi and Junyeong Park.

Lunch: 12:30 at the entrance of cafeteria opposite to Mathematical Institute.

2 sessions on different days!!

Speakers: Piotr Achinger (Warsaw), Ko Aoki (Bonn), Alberto Merici (Heidelberg)

2025年9月24日（水） Wednesday 24 September 

Place: Room 202, Science Complex A (合同A棟202)

13:30--15:00 Piotr Achinger (Warsaw/Kyiv)

Title: Tame fundamental groups of rigid spaces 

Abstract:
The fundamental group of a complex variety is finitely presented. The talk will survey algebraic variants (in fact, distant corollaries) of this fact, in the context of variants of the etale fundamental group. We will then zoom in on "tame" etale fundamental groups of p-adic analytic spaces. Our main result is that it is (topologically) finitely generated (for a quasi-compact and quasi-separated rigid space over an algebraically closed field).  The proof uses logarithmic geometry beyond its usual scope of finitely generated monoids to (eventually) reduce the problem to the more classical one of finite generation of tame fundamental groups of algebraic varieties over the residue field. This is joint work with Katharina Hübner, Marcin Lara, and Jakob Stix.

2025年9月25日（木）Thursday 25 September 

Place: Room 305, Mathematical Institute (数学棟305)

13:30--15:00 Alberto Merici (Heidelberg)

Title: Descent for logarithmic invariants and applications

Abstract:
Following a suggestion by Mathew, I will explain how to identify some arithmetic invariants of logarithmic schemes with (non-log) invariants of the associated infinity root stack of Talpo and Vistoli. A key ingredient is a form of descent for logarithmic invariants that we call "saturated descent”. As an application, we deduce comparison theorems of logarithmic invariants from classical (non-logarithmic) comparisons and construct variants of Beilinson/Bloch--Esnault--Kerz fiber squares for semi-stable varieties over a local field.
This is a joint work with F. Binda, T. Lundemo and D. Park.

15:15--16:45 Ko Aoki (Bonn)

Title: Higher motives

Abstract:
I talk about my thesis work in progress. I first explain how to categorify the idea of motives of algebraic varieties. Now we parametrize six operations rather than cohomology theories, which I call “2-motives.” To formulate this, we need the theory of presentable (∞, 2)-categories, which was initiated by Stefanich and has been recently advanced by me. I then explain Scholze’s idea for relating this to the transmutation philosophy of Drinfeld and Bhatt–Lurie. I present my theorem precisely realizing this idea. Moreover, I describe the analytic version of this theorem, which is useful for constructing various realizations, such as the p-adic de Rham realization and the Habiro realization.

Lunch before each session

Dinner: Thu 25 Sep

2025年8月8日（金）Friday 8 August (2 talks!)

Place: Room 202, Science Complex A, Aobayama Campus. Not the mathematics building!

= 合同A棟202室（数学棟ではありません！）

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Talk 1

Time: 13:45--15:15

Speaker: Christopher Frei (TU Graz)

Title: Asymptotics for solubility of unit equations over real quadratic fields

Abstract: For a real quadratic field K and positive integer r, we prove an asymptotic formula for the number of rational integers of bounded absolute value that can be written as a sum of r units of the ring of integers of K. This is joint work with M. Widmer and V. Ziegler and answers partially a question posed by M. Jarden and W. Narkiewicz. 

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Talk 2

Time: 15:30--17:00

Speaker: Bruno Chiarellotto (University of Padua, Italy)

Title: "The life of  Alexander Grothendieck"

Abstract: In the 20th century we speak about the mathematics before and after Alexander Grothendieck. Starting from his interesting life we will try to introduce his mathematics.

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Lunch: Those who join meet at 12:45 at the entrance of the cafeteria in front of Mathematical Institute.

2025年７月28日（月）Monday 28 July 

Time: 13:00--14:30

Room: 数学棟201 Lecture room 201, Mathematical Institute 

Speaker: 星明考 氏（新潟大学）Akinari Hoshi (Niigata University)

Title: 

Rationality problem for fields of invariants, norm one tori and Hasse norm principle

Abstract: 

The aim of this talk is to give a survey of recent developments in the rationality problem for fields of invariants, e.g. Noether's problem, rationality problem for algebraic tori, norm one tori and quasi-monomial actions.

I will give some explicit examples of (non-)stably (resp. retract) rational fields

which are weaker concepts than the rationality.

Negative results may be obtained by using (stable) birational invariants, e.g. flabby class, unramified Brauer (cohomology) group which is an avatar of the Artin-Mumford invariant.

By Colliot-Thelene and Voisin's result (2012, Duke Math. J.), an explicit example of

non-vanishing of the unramified cohomology group of degree three provides an explicit counter-example of the integral Hodge conjecture.

A counter-example of Noether's problem for N\rtimes A_6 over C is also given

where A_6 is the alternating group of degree 6 and N=(Z/10Z)^9.

As an application, we present recent results about Hasse norm principle for K/k where K/k is a finite separable field extension of a global field k.

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Dinner: beer 

2025年7月17日（木）Thursday 17 July 

Time: 13:30--15:00

Room: 数学系研究棟305 = Lecture Room 305, Mathematical Institute

Speaker: 三井健太郎 氏（琉球大学）Kentaro Mitsui (University of the Ryukyus)

Title: 

Actions of degenerating abelian varieties on their Mumford models

Abstract: 

Mumfordは，完備環上で分裂トーラス還元を持つアーベル多様体について，相対的コンパクトモデルを構成した．このモデルはMumfordモデルと呼ばれ，一般には一意的でないが有用である．中村郁氏との共同研究において，完備離散付値環上のアーベル多様体のネロンモデルについて，ネロンモデルが作用するような対称性の高い相対的コンパクトモデルをMumfordモデルに基づき構成した．この対称性は，連結成分群と単位成分群に関する二種類の対称性からなる．本講演では，一般の完備環上のMumfordモデルについて，後者の対称性を解説する．

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Lunch: 12:30

2025年6月23日（月）Monday 23 June 

Time: 10:30--12:00

Room: 数学棟201 Lecture room 201, Mathematical Institute 

Speaker: 阿部健 氏（同志社大学）Takeshi Abe (Doshisha University)

タイトル：一般形超曲面の部分多様体に関するいくつかの結果

アブストラクト：非特異な一般型の複素射影多様体$X$にはZariski閉真部分集合$Z$（例外集合と呼ばれる）が存在し，Zを含まないXの部分多様体は一般型になる，との予想がある．この講演では，$X$が$n$次元射影空間$\mathbb{P}^{n}$内のvery generalな次数$d$の一般型超曲面の場合に，上記の予想に関するEin,Voisin,Clemens-Ranによる先行結果を紹介してから，講演者による僅かな改良結果をのべる．また，$n$次元射影空間$\mathbb{P}^{n}$とその中のvery generalな次数$d$の一般型超曲面$D$の組$(\mathbb{P}^{n},D)$に対して，Clemens-Ranの結果の対数版が成り立つことを紹介する．

 2025年5月8日（木）Thursday 8 May 

13:30--15:00

数学棟305 [Lecture Room 305, Mathematical Institute]

Speaker: Paul Helminck (Tohoku University)

Title: 

Gluing coamoebas of hyperplane arrangements to obtain pair-of-pants decompositions of complex varieties

Abstract: 

In this talk, I will discuss a general technique for expressing the topological structure of a complex algebraic variety in terms of glued hyperplane complements in projective space. In the case of a Riemann surface, this recovers well-known pair-of-pants decompositions. Here the hyperplane complement consists of the projective line minus three points. 

At the beginning of the talk, I will give a short elementary introduction to various concepts in algebraic geometry and algebraic topology to explain the relevant framework for the remainder of the talk. I will then move over to the main theorem of the talk, which says that the topology of a hyperplane complement can be recovered in terms of the associated angle set, or coamoeba, of the hyperplanes. Loosely speaking, this shows that the essential topological data associated to a hyperplane complement lies in the possible angles resulting from the hyperplane equations, so that the radii can be forgotten. 

To prove this result, I will introduce the more general notion of a quasi-polyhedral fibration and show that these define homotopy equivalences. This will require a general semi-algebraic form of the Smale-Vietoris theorem combined with a limiting argument. The angle map for a hyperplane complement is a particular example of a quasi-polyhedral fibration, so this automatically shows that the angles of the hyperplanes recover the topology of the complement. We then extend this theorem to the Kato-Nakayama space of a suitable smooth toric compactification of the complement and show that the extended angle set (or coamoeba) is homotopy equivalent to the Kato-Nakayama space. This allows us to glue extended angle sets along boundary divisors to obtain generalized pair-of-pants decompositions. This is part of a project with Yassine El Maazouz.

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Official lunch (= an ordinary casual lunch): We meet at 12:30 at the entrance of the cafeteria in front of Mathematical Institute.