## Divisibility sequences and powers of algebraic integers

### Summary

Let $\alpha$ be an algebraic integer and define a sequence of rational integers $d_n(\alpha)$ by the condition $$d_n(\alpha) = \max\{d\in\Bbb Z : \alpha^n \equiv 1 {MOD}{d} \}.$$ We show that $d_n(\alpha)$ is a strong divisibility sequence and that it satisfies $\log d_n(\alpha)=o(n)$ provided that no power of $\alpha$ is in $\Bbb Z$ and no power of $\alpha$ is a unit in a quadratic field. We completely analyze some of the exceptional cases by showing that $d_n(\alpha)$ splits into subsequences satisfying second order linear recurrences. Finally, we provide numerical evidence for the conjecture that aside from the exceptional cases, $d_n(\alpha)=d_1(\alpha)$ for infinitely many $n$, and we ask whether the set of such $n$ has positive (lower) density.

### Mathematics Subject Classification

11R04, 11A05, 11D61

### Keywords/Phrases

divisibility sequence, multiplicative group