Galois module structure of algebraic integers of the simplest cubic field

Abstract

Let $L_n$ be a simplest cubic field with Galois group $G=\rm{Gal} (L_n/\mathbb Q)$. The associated order is denoted as ${\cal A}_{L_n/\mathbb Q}:= \{ x\in {\mathbb Q} [G] \, |\, x \cdot \cal{O}_{L_n} \subset {\cal O}_{L_n } \}$, where ${\cal O}_{L_n}$ is the ring of integers of $L_n$. Leopoldt showed that $\cal{O}_{L_n} \simeq {\cal A}_{L_n/\mathbb Q}$ as ${\cal A}_{L_n/\mathbb Q}$-modules. In this paper, we give a generator of the ${\cal A}_{L_n/\mathbb Q}$-module ${\cal O}_{L_n}$ explicitly using the roots of Shanks' cubic polynomial. If $L_n/\mathbb Q$ is tamely ramified, then we have ${\cal A}_{L_n/\mathbb Q}=\mathbb Z [G]$, and the conjugates form a normal integral basis, which has been obtained explicitly in the previous work of Hashimoto and the second author.