Nonvanishing of a nonprincipal Dirichlet L-function at one (source code)

= Nonvanishing of a nonprincipal Dirichlet L-function at one
{c}

For every nonprincipal <Dirichlet character> $\chi$,
$$
L(1,\chi)\ne0.
$$
Multiply the <Dirichlet L-functions> over every character modulo $q$. The resulting Euler product has nonnegative coefficients. A zero at one would cancel the principal factor's pole and contradict the <Landau theorem for a Dirichlet series with nonnegative coefficients>.