  Dirichlet's proof of infinitely many primes in arithmetic progressions was
published in 1837, introduced L-series for the first time, and it is said to
have started rigorous analytic number theory. Dirichlet uses Euler's earlier
work on the zeta function and the distribution of primes. He first proves a
simpler case before going to full generality. The paper was translated from
German by R. Stephan and given a reference section.
