Dirichlet's theorem on Diophantine approximation: For any real number $x$, for integer $N>0$, there exists integers $a$ and $b>0$ with $(a,b)=1$ such that $b\leq N$ and $$|x-\frac a b|<\frac{1}{b(N+1)}.$$ Let $c$ be a fixed positive integer. Is there any version of Dirichlet's theorem to make the following restriction? 1) $c|b$ 2) $(b,c)=1$