arXiv subjects

Takuya Asayama

Publications and source records attributed to Takuya Asayama.

5 recordsLinked to original sources

arXiv2026

Kummer-faithful fields with finitely generated absolute Galois group

This paper studies the structure of the Mordell--Weil groups of semiabelian varieties over algebraic extensions of number fields whose absolute Galois group is finitely generated, with particular emphasis on that generated by a single element. A probabilistic argument using the Haar measure on the absolute Galois group of a number field shows that almost all such fields are Kummer-faithful, i.e., the Mordell--Weil group of any semiabelian variety over any finite extension of such a field has trivial divisible part. This result implies that there exists a Kummer-faithful field algebraic over a number field whose absolute Galois group is abelian.

math.NT↗

arXiv2024

Ramification of Tate modules for rank $2$ Drinfeld modules

In this paper, we study the ramification of extensions of a function field generated by division points of rank 2 Drinfeld modules. Also conductors of certain rank 2 Drinfeld modules are defined as analogues of those for elliptic curves. A calculation of these conductors allows us to show an analogue of Szpiro's conjecture under a certain limited situation.

math.NT↗

arXiv2024

Mordell--Weil groups over large algebraic extensions of fields of characteristic zero

We study the structure of the Mordell--Weil groups of semiabelian varieties over large algebraic extensions of a finitely generated field of characteristic zero. We consider two types of algebraic extensions in this paper; one is of extensions obtained by adjoining the coordinates of certain points of various semiabelian varieties; the other is of extensions obtained as the fixed subfield in an algebraically closed field by a finite number of automorphisms. Some of such fields turn out to be new examples of Kummer-faithful fields which are not sub-$p$-adic. Among them, we find both examples of Kummer-faithful fields over which the Mordell--Weil group modulo torsion can be free of infinite rank and not free.

math.NT↗

arXiv2023

Kummer-faithfulness for function fields

A perfect field $K$ is said to be Kummer-faithful if the Mordell-Weil group of every semi-abelian variety over every finite extension of $K$ has no nonzero divisible element. The class of Kummer-faithful fields contains that of sub-$p$-adic fields and is thought to be suitable for developing anabelian geometry. In this paper, we investigate a function field analogue of the notion of Kummer-faithful fields. We introduce a notion of Drinfeld-Kummer-faithful (DKF) fields using Drinfeld modules. A sufficient condition for a Galois extension of a function field to be DKF is provided in terms of ramification theory. More precisely, a Galois extension with finite maximal ramification break outside the infinite prime $(1 / t)$ over a finite extension of the rational function field $\mathbb{F}_q(t)$ over the finite field $\mathbb{F}_q$ of $q$ elements is DKF. Some examples of DKF fields are also given. The construction of these examples is inspired by Ozeki and Taguchi's examples of highly Kummer-faithful fields.

math.NT↗

arXiv2020

Torsion points of Drinfeld modules over large algebraic extensions of finitely generated function fields

Geyer and Jarden proved several results for torsion points of elliptic curves defined over the fixed field by finitely many elements in the absolute Galois group of a finitely generated field over the prime field in its algebraic closure. As an analogue of these results, this paper studies torsion points of Drinfeld modules defined over the fixed field by finitely many elements in the absolute Galois group of a finitely generated function field in its algebraic closure. We prove some results which are similar to those of Geyer and Jarden.

math.NT↗