Dirichlet's work on L-functions (reviewed)

The first use of the letter L to denote these functions was by Dirichlet in 1837 (see Werke I [MR:249268] pages 313-342) who used L-functions to prove that there are infinitely many primes in any (primitive) arithmetic progression. He considered series of the form $$L(s,\chi)=\sum_{n=1}^\infty \frac{\chi(n)}{n^s}$$ where $\chi$ is a (Dirichlet) character, which is the extension to the integers of a character of the group $(\mathbb Z/q\mathbb Z)^*$. For example the arithmetic function $\chi$ defined by

\[\chi(n)=\begin{cases} 1 & \textrm{ if } n\equiv 1 \bmod 3\\ -1 & \textrm{ if } n\equiv 2\bmod 3\\ 0 & \textrm{ if }n\equiv 0\bmod 3\end{cases}\]

is a Dirichlet character modulo 3. The above L-function has an Euler product $$L(s,\chi)=\prod_{p }\left(1-\frac{\chi(p)}{p^s}\right)^{-1}$$ and satisfies a functional equation that $$\left(\frac{3}{\pi}\right)^{\frac{s}{2}}\Gamma\left(\frac{s+1}{2}\right) L(s,\chi)$$ is invariant under $s\to 1-s$. Also, note the special value $$L(1,\chi)= \frac{\pi}{3\sqrt{3}}; $$ Dirichlet needed to know that his L-functions did not vanish at 1 and he used special values to prove this fact. Dirichlet's original proof was for prime moduli; for composite moduli he required his class number formula which was proven in 1839 - 1840 (see his Werke I, pp. 411-496).