The minimal periodicity for integral bases of pure number fields

Abstract

Fix $n\ge3$. For the pure field $K_a=\Bbb Q(\theta)$ with $\theta^n=a$, where $a\neq \pm1$ is $n$th-power-free, $x^n-a$ is irreducible over $\Bbb Q$, and Hypothesis~\textup{(\textbf{H})} holds, we encode an integral basis in the fixed coordinate $\{1,\theta,\dots,\theta^{n-1}\}$ by its \emph{shape}. We prove a sharp local-to-global principle: for each $p^e\!\parallel n$, the local shape at $p$ is determined by $a\bmod p^{\,e+1}$, and this precision is optimal. Moreover, the global shape is periodic with minimal modulus \[ M(n)=\prod_{p^e\parallel n}p^{\,e+1}=n\cdot\mathrm{rad}(n), \] providing many applications in the understanding integral bases of pure number fields.