Korselt Rational Bases of Prime Powers

Abstract

Let $N$ be a positive integer, $\mathbb{A}$ be a subset of $\mathbb{Q}$ and $\alpha =\frac{{\alpha }_1}{{\alpha }_2}\in \mathbb{A}\setminus \{0,N\}$. $N$ is called an $\alpha$-Korselt number (equivalently $\alpha$ is said an $N$-Korselt base) if ${\alpha }_{2}p-{\alpha }_1$ divides ${\alpha }_{2}N-{\alpha }_1$ for every prime divisor $p$ of $N$. By the Korselt set of $N$ over $\mathbb{A}$, we mean the set $\mathbb{A}$-$\mathcal{K}\mathcal{S}(N)$ of all $\alpha \in \mathbb{A}\setminus \{0,N\}$ such that $N$ is an $\alpha$-Korselt number. In this paper we determine explicitly for a given prime number $q$ and an integer $l\in \mathbb{N}\setminus \{0,1\}$, the set $\mathbb{Q}$-$\mathcal{K}\mathcal{S}(q^l)$ and we establish some connections between the $q^l$-Korselt bases in $\mathbb{Q}$ and others in $\mathbb{Z}$. The case of $\mathbb{A}=\mathbb{Q}\cap [-1,1[$ is studied where we prove that $(\mathbb{Q}\cap [-1,1[)$-$\mathcal{K}\mathcal{S}(q^l)$ is empty if and only if $l=2$. Moreover, we show that each nonzero rational $\alpha$ is an $N$-Korselt base for infinitely many numbers $N=q^l$ where $q$ is a prime number and $l\in \mathbb{N}$.