Diophantine approximation with primes in an arithmetic progression

Abstract: Let $α\in \mathbb{R} \setminus \mathbb{Q}$, $β\in \R$, $N \in \mathbb{R}<em>{\ge 1}$ and $Δ\in (0, 1/2)$. For any real $y$, let $|y|$ denote the distance from $y$ to the nearest integer. In the first part of this paper, we show that given two coprime integers $u, v \ge 1$, there are infinitely many primes $\ell \equiv u \bmod v$ such that $$|α\ell - β| \ll_v \ell^{-1/4} \log^{8} \ell.$$ In order to prove this result we first prove the following general theorem and then deduce the above as a corollary. Before stating the result, let us define a function on $f</em>Δ(θ)$ on $\R$ such that $f_Δ(θ)$ is $ 1 \text{ if } | θ| &lt; Δ$ and $ 0 $ otherwise. Further, suppose that $u, v \in \mathbb{Z}<em>{\ge 1}$ are coprime and $a$, $q \in \Z$ are coprime with $q&gt; N<sup>{1/4}$ and $|αv -a/q| \le 1/q<sup>2$. Then, for every $ε\in \mathbb{R}</em>{&gt;0}$ we have \begin{equation*} \sum_{\substack{n=1 \ n \equiv u \bmod v}}^N Λ(n) (f_Δ(αn - β) - 2Δ) \ll_v (Nq^{-1/2} + N^{3/4} + N^{5/6}Δ^{1/2} + (ΔNq)^{1/2} + N^εq Δ^{1-ε}) \mathcal{L}^8 \end{equation*} where $\mathcal{L}=\log (Nq/Δ)$. This generalises a well known result of Vaughan from 1977 proving a similar bound for the sum \begin{equation*} \sum_{\substack{n=1}}^N Λ(n) (f_Δ(αn - β) - 2Δ). \end{equation*} In the second part of this paper, we explicitly construct an uncountable set $S \subset \R\setminus \Q$ such that for every $γ\in S$ there are infinitely many primes $\ell \equiv u \bmod v$ satisfying $| γ\ell | &lt; \ell<sup>{-1}$. Further we prove unconditionally that not all the elements of $S$ are Liouville numbers. This addresses a question of Erd{ö}s and Mahler from 1939.