Joint universality and generalized strong recurrence with rational parameter

Abstract

We prove that, for every rational $d\ne 0,\pm 1$ and every compact set $K\subset\{s\in\mathbb{C}:1/2<\Re(s)<1\}$ with connected complement, any analytic non-vanishing functions $f_1,f_2$ on $K$ can be approximated, uniformly on $K$, by the shifts $ζ(s+iτ)$ and $ζ(s+idτ)$, respectively. As a consequence we deduce that the set of $τ$ satisfying $|ζ(s+iτ)-ζ(s+idτ)|<\varepsilon$ uniformly on $K$ has a positive lower density for every $d\ne 0$.