On the sequence $\mathrm{gcd}(a^n-1,b^n-1)$

Abstract

For integers $a,b\ge 2$, put \[ g_n:=\gcd(a^n-1,b^n-1)\qquad(n\ge 1). \] We prove that $(g_n)$ satisfies a constant-coefficient linear recurrence if and only if $a$ and $b$ are multiplicatively dependent; the same conclusion holds if only a tail of $(g_n)$ is assumed to satisfy such a recurrence. More generally, if $a$ and $b$ are multiplicatively independent, then for every fixed shift $N\ge 0$, every integer sequence $(W_n)_{n\ge 1}$ that satisfies a constant-coefficient linear recurrence and the pointwise divisibilities \[ W_n\mid a^{n+N}-1 \qquad\text{and}\qquad W_n\mid b^{n+N}-1 \qquad(n\ge 1) \] is eventually periodic. If $(W_n)$ satisfies a recurrence with nonzero trailing coefficient, then it is periodic from the first term. In particular, every common integer linear divisibility-sequence factor of $a^n-1$ and $b^n-1$, in the convention adopted here, is periodic. The proof combines the Bugeaud--Corvaja--Zannier bound with the structure of subexponential linear recurrences and a local valuation argument.