Dirichlet/Dedekind L-functions

L(s,χ) = Σ χ(n) n^{-s} for Dirichlet character; Dedekind ζ_K(s) = Σ_a N(a)^{-s}. Encodes class number formula: res_{s=1} ζ_K(s) = (...)/|d_K|^{1/2}.