mathlib documentation


Selmer groups of fraction fields of Dedekind domains #

Let $K$ be the field of fractions of a Dedekind domain $R$. For any set $S$ of prime ideals in the height one spectrum of $R$, and for any natural number $n$, the Selmer group $K(S, n)$ is defined to be the subgroup of the unit group $K^\times$ modulo $n$-th powers where each element has $v$-adic valuation divisible by $n$ for all prime ideals $v$ away from $S$. In other words, this is precisely $$ K(S, n) := \{x(K^\times)^n \in K^\times / (K^\times)^n \ \mid \ \forall v \notin S, \ \mathrm{ord}_v(x) \equiv 0 \pmod n\}. $$

There is a fundamental short exact sequence $$ 1 \to R_S^\times / (R_S^\times)^n \to K(S, n) \to \mathrm{Cl}_S(R)[n] \to 0, $$ where $R_S^\times$ is the $S$-unit group of $R$ and $\mathrm{Cl}_S(R)$ is the $S$-class group of $R$. If the flanking groups are both finite, then $K(S, n)$ is finite by the first isomorphism theorem. Such is the case when $R$ is the ring of integers of a number field $K$, $S$ is finite, and $n$ is positive, in which case $R_S^\times$ is finitely generated by Dirichlet's unit theorem and $\mathrm{Cl}_S(R)$ is finite by the class number theorem.

This file defines the Selmer group $K(S, n)$ and some basic facts.

Main definitions #

Main statements #

Notations #

Implementation notes #

The Selmer group is typically defined as a subgroup of the Galois cohomology group $H^1(K, \mu_n)$ with certain local conditions defined by $v$-adic valuations, where $\mu_n$ is the group of $n$-th roots of unity over a separable closure of $K$. Here $H^1(K, \mu_n)$ is identified with $K^\times / (K^\times)^n$ by the long exact sequence from Kummer theory and Hilbert's theorem 90, and the fundamental short exact sequence becomes an easy consequence of the snake lemma. This file will define all the maps explicitly for computational purposes, but isomorphisms to the Galois cohomological definition will be provided when possible.

References #

Tags #

class group, selmer group, unit group

Valuations of non-zero elements #

Selmer groups #

The natural homomorphism from to K⟮∅, n⟯.