# Siegel–Walfisz theorem

In analytic number theory, the Siegel–Walfisz theorem was obtained by Arnold Walfisz as an application of a theorem by Carl Ludwig Siegel to primes in arithmetic progressions. It is a refinement both of the prime number theorem and of Dirichlet's theorem on primes in arithmetic progressions.

## Statement

Define

$\psi (x;q,a)=\sum _{n\leq x \atop n\equiv a{\pmod {q}}}\Lambda (n),$ where $\Lambda$ denotes the von Mangoldt function and define φ to be Euler's totient function.

Then the theorem states that given any real number N there exists a positive constant CN depending only on N such that

$\psi (x;q,a)={\frac {x}{\varphi (q)}}+O\left(x\exp \left(-C_{N}(\log x)^{\frac {1}{2}}\right)\right),$ whenever (a, q) = 1 and

$q\leq (\log x)^{N}.$ ## Remarks

The constant CN is not effectively computable because Siegel's theorem is ineffective.

From the theorem we can deduce the following bound regarding the prime number theorem for arithmetic progressions: If, for (a,q)=1, by $\pi (x;q,a)$ we denote the number of primes less than or equal to x which are congruent to a mod q, then

$\pi (x;q,a)={\frac {{\rm {Li}}(x)}{\varphi (q)}}+O\left(x\exp \left(-{\frac {C_{N}}{2}}(\log x)^{\frac {1}{2}}\right)\right),$ where N, a, q, CN and φ are as in the theorem, and Li denotes the logarithmic integral.