On the rational approximation to $p$-adic Thue--Morse numbers

Abstract

Let $p$ be a prime number and $\xi$ an irrational $p$-adic number. Its multiplicative irrationality exponent ${{\mu^{\times}}} (\xi)$ is the supremum of the real numbers ${{\mu^{\times}}}$ for which the inequality $$ |b \xi - a|_{p} \leq | a b |^{- {{\mu^{\times}}} / 2} $$ has infinitely many solutions in nonzero integers $a, b$. We show that ${{\mu^{\times}}} (\xi)$ can be expressed in terms of a new exponent of approximation attached to a sequence of rational numbers defined in terms of $\xi$. We establish that ${{\mu^{\times}}} ({{\xi_{{\bf t}, p}}}) = 3$, where ${{\xi_{{\bf t}, p}}}$ is the $p$-adic number $1 - p - p^2 + p^3 - p^4 + \ldots$, whose sequence of digits is given by the Thue--Morse sequence over $\{-1, 1\}$.