Classical and uniform exponents of multiplicative $p$-adic approximation

Abstract

Let $p$ be a prime number and $\xi$ an irrational $p$-adic number. Its irrationality exponent $\mu (\xi)$ is the supremum of the real numbers $\mu$ for which the system of inequalities $$ 0 < \max\{|x|, |y|\} \le X, \quad |y \xi - x|_{p} \leq X^{-\hmu} $$ has a solution in integers $x, y$ for arbitrarily large real number $X$. Its multiplicative irrationality exponent $\tmu (\xi)$ (resp., uniform multiplicative irrationality exponent $\htmu (\xi)$) is the supremum of the real numbers $\hmu$ for which the system of inequalities $$ 0 < |x y|^{1/2} \le X, \quad |y \xi - x|_{p} \leq X^{-\hmu} $$ has a solution in integers $x, y$ for arbitrarily large (resp., for every sufficiently large) real number $X$. It is not difficult to show that $\mu (\xi) \le \tmu(\xi) \le 2 \mu (\xi)$ and $\htmu (\xi) \le 4$. We establish that the ratio between the multiplicative irrationality exponent $\tmu$ and the irrationality exponent $\mu$ can take any given value in $[1, 2]$. Furthermore, we prove that $\htmu (\xi) \le (5 + \sqrt{5})/2$ for every $p$-adic number $\xi$.