Sums of S-units in X-coordinates of Pell equations

Abstract

Let $S$ be a fixed set of primes and let $(X_l{)}_{l\ge 1}$ be the $X$-coordinates of the positive integer solutions $(X,Y)$ of the Pell equation $X^2-d{Y}^2=1$ corresponding to a non-square integer $d>1$. We show that there are only a finite number of non-square integers $d>1$ such that there are at least two different elements of the sequence $(X_l{)}_{l\ge 1}$ that can be represented as a sum of $S$-units with a fixed number of terms. Furthermore, we solve explicitly a particular case in which two of the $X$-coordinates are product of power of two and power of three.