Large Gaps between Primes in Arithmetic Progressions

Abstract: For $(M,a)=1$, put \begin{equation*} G(X;M,a)=\sup_{p^\prime_n\leq X}(p^\prime_{n+1}-p^\prime_n), \end{equation*} where $p^\prime_n$ denotes the $n$-th prime that is congruent to $a\pmod{M}$. We show that for any positive $C$, provided $X$ is large enough in terms of $C$, there holds \begin{equation*} G(MX;M,a)\geq(C+o(1))\varphi(M)\frac{\log X\log_2 X\log_4 X} {{(\log_3 X)}^2}, \end{equation*} uniformly for all $M\leq\kappa{(\log X)}^{1/5}$ that satisfy \begin{equation*} \omega(M)\leq \exp\biggl(\frac{\log_2 M\log_4 M}{\log_3 M}\biggr). \end{equation*}