arXiv subjects

Ryoji Shimizu

Publications and source records attributed to Ryoji Shimizu.

5 recordsLinked to original sources

arXiv2026

On branched coverings of the projective line over the integers

We investigate the \'etale fundamental group of the complement of a horizontal divisor on $\mathbb{P}^{1}_{\mathbb{Z}}$. We prove that this group has no nontrivial finite solvable quotient if and only if the divisor is normal crossings at the prime~$2$. Moreover, if the divisor is normal crossings at the prime~$2$ and either has three irreducible components or is normal crossings at the prime~$3$, we show that no quotient isomorphic to $\mathrm{PSL}_{2}(q)$ can occur for certain prime powers~$q$.

math.AG↗

arXiv2025

\'Etale fundamental groups of smooth arithmetic surfaces and the Grothendieck conjecture

We study the structure of the \'etale fundamental groups of smooth curves over certain arithmetic schemes, and investigate the relative version of Grothendieck's anabelian conjecture in this setting. Consequently, every hyperbolic curve over the ring of S-integers of a number field in which a rational prime is inverted is anabelian, i.e., its schematic structure is completely determined by its \'etale fundamental group. Moreover, we obtain a partial result toward the semi-absolute version of Grothendieck's anabelian conjecture in this context.

math.NT↗

arXiv2023

The pro-$\mathcal{C}$ anabelian geometry of number fields

Let $K$ be a number field and $\mathcal{C}$ a full class of finite groups. We write $K^{\mathcal{C}}/K$ for the maximal pro-$\mathcal{C}$ Galois extension of $K$, and $G_K^{\mathcal{C}}$ for its Galois group. In this paper, we deal with the following question: ``For $i=1,2$, let $K_i$ be a number field, $\mathcal{C}_i$ a nontrivial full class of finite groups, and $σ:G_{K_1}^{\mathcal{C}_1}\overset \sim \rightarrow G_{K_2}^{\mathcal{C}_2}$ an isomorphism. Is $σ$ induced by a unique isomorphism between $K_2^{\mathcal{C}_2}/K_2$ and $K_1^{\mathcal{C}_1}/K_1$?'' In one of the main results, we answer this question affirmatively only assuming that the upper Dirichlet density of the set of prime numbers concerning $\mathcal{C}_i$ is not zero for at least one $i$. Moreover, we obtain some results which are still valid even when $\mathcal{C}_1$, $\mathcal{C}_2$ consist of all finite $p$-groups for a prime number $p$, that is, $G_{K_1}^{\mathcal{C}_1}$, $G_{K_2}^{\mathcal{C}_2}$ are the maximal pro-$p$ quotients of the absolute Galois groups.

math.NT↗

arXiv2021

The Neukirch-Uchida theorem with restricted ramification

Let $K$ be a number field and $S$ a set of primes of $K$. We write $K_S/K$ for the maximal extension of $K$ unramified outside $S$ and $G_{K,S}$ for its Galois group. In this paper, we prove the following generalization of the Neukirch-Uchida theorem under some assumptions: "For $i=1,2$, let $K_i$ be a number field and $S_i$ a set of primes of $K_i$. If $G_{K_1,S_1}$ and $G_{K_2,S_2}$ are isomorphic, then $K_1$ and $K_2$ are isomorphic." Here the main assumption is that the Dirichlet density of $S_i$ is not zero for at least one $i$. A key step of the proof is to recover group-theoretically the $l$-adic cyclotomic character of an open subgroup of $G_{K,S}$ for some prime number $l$.

math.NT↗

arXiv2021

Isomorphisms of Galois groups of number fields with restricted ramification

Let $K$ be a number field and $S$ a set of primes of $K$. We write $K_S/K$ for the maximal extension of $K$ unramified outside $S$ and $G_{K,S}$ for its Galois group. In this paper, we answer the following question under some assumptions: "For $i=1,2$, let $K_i$ be a number field, $S_i$ a (sufficiently large) set of primes of $K_i$ and $σ:G_{K_1,S_1}\to G_{K_2,S_2}$ an isomorphism. Is $σ$ induced by a unique isomorphism between $K_{1,S_1}/K_1$ and $K_{2,S_2}/K_2$?" Here the main assumption is about the Dirichlet density of $S_i$.

math.NT↗