On a Variant of Pillai's problem involving convergent denominators of quadratic irrationals

TL;DR

This paper investigates the finiteness and explicit solutions of equations involving differences of convergent denominators of quadratic irrationals, extending Pillai's problem in a specific algebraic context.

Contribution

It establishes finiteness results for solutions to difference equations of convergent denominators of quadratic irrationals and provides explicit solutions in particular cases.

Findings

Finitely many integers c satisfy the difference equation with multiple solutions.

Complete solutions are explicitly listed for specific cases.

The results extend Pillai's problem to quadratic irrational contexts.

Abstract

Let $(q_{\alpha, n})_{n \geq 0}$ be the sequence of convergent denominators to the simple continued fraction expansion of $\alpha$. For certain specific choices of $\alpha$, this sequence is a Lehmer sequence. In this paper, we show that there are only finitely many integers $c$ such that the equation $q_{\alpha, n} - q_{\beta, m} = c$ has at least two distinct solutions $(n,m)$, where $\alpha,\beta$ are quadratic irrationals with $\mathbb{Q}(\alpha)\neq \mathbb{Q}(\beta)$. In specific instances, we solve the equation $q_{\alpha, n} - q_{\beta, m} = c$ completely and explicitly list all solutions.