Good Integers: A Concise Completion of the Non-Coprime Case

Abstract

For coprime nonzero integers $a$ and $b$, a positive integer $\ell$ is said to be {\em good} with respect to $a$ and $b$ if there exists a positive integer $k$ such that $\ell |(a^{k}+b^{k})$. Since the early 1990s, such classical good integers have been studied intensively for their number theoretic structures and for applications, notably in coding theory. This work completes the study by relaxing the coprimality hypothesis and treating the non-coprime case $\gcd(a,b)\neq1$ in a concise and self-contained way. The results are presented in terms of the classical coprime criterion and $p$-adic valuations of $\ell$. As a consequence, whenever $\ell$ is good, all admissible exponents form a single arithmetic progression with an explicit starting point and period. Some special cases are discussed in the non-coprime setting. A practical decision procedure is developed that decides the goodness of a given integer and explicitly enumerates the full set of admissible exponents. Several illustrative examples are presented.