Chebyshev function in an arithmetic progression
= Chebyshev function in an arithmetic progression
{c}
{title2=$\psi(x;q,a)$}
For $(a,q)=1$,
$$
\psi(x;q,a)
=\sum_{\substack{n\leq x\\n\equiv a\pmod q}}\Lambda(n).
$$
The <Orthogonality of Dirichlet characters> expresses it in terms of character-twisted <Von Mangoldt functions>.