Dirichlet L-Functions and Dirichlet's Theorem

This article provides a formalisation of Dirichlet characters and Dirichlet L-functions including proofs of their basic properties – most notably their analyticity, their areas of convergence, and their nonvanishing for R(s) ≥ 1. All of this is built in a very high-level style using Dirichlet series. The proof of the non-vanishing follows a very short and elegant proof by Newman [4], which we attempt to reproduce faithfully in a similar level of abstraction in Isabelle. This also leads to a relatively short proof of Dirichlet’s Theorem, which states that, if h and n are coprime, there are infinitely many primes p with p ≡ h (mod n).