# Analytic Continuation of Dirichlet L-Function

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## Theorem

Let $\chi : G := \paren {\Z / q \Z}^\times \to \C^\times$ be a Dirichlet character modulo $q$.

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Let $\map L {s, \chi}$ be the Dirichlet $L$-function for $\chi$.

Let $\chi$ be the trivial character.

Then $\map L {s, \chi}$ has an analytic continuation to $\C$ except for a simple pole at $s = 1$.

Let $\chi$ be non-trivial.

Then $\map L {s, \chi}$ is analytic on $\map \Re s > 0$.

## Proof

Let $\chi$ be the trivial character.

Then by Dirichlet L-Function from Trivial Character:

- $\ds \map L {s, \chi} = \map \zeta s \cdot \prod_{p \mathop \divides q} \paren {1 - p^{-s} }$

where $\divides$ denotes divisibility.

Also, by Poles of Riemann Zeta Function, $\zeta$ is analytic on $\C$ except for a simple pole at $s = 1$.

Since $\map L {s, \chi}$ is just $\zeta$ times some constant, the same holds for this function.

If $\chi$ is non-trivial, then by the Orthogonality Relations for Characters:

- $\ds \sum_{x \mathop \in G} \map \chi x = 0$

By definition, $\chi$ is $q$-periodic, and zero on integers not coprime to $q$.

So for any $M \in \N$:

- $\ds \sum_{n \mathop = M + 1}^{M + Q} \map \chi n = 0$

Let $M, N \in \N$ be arbitrary.

Let $d$ be such that $M + q d \le N \le M + q \paren {d + 1}$.

Then:

\(\ds \sum_{n \mathop = M}^N \map \chi n\) | \(=\) | \(\ds \sum_{k \mathop = 0}^{d - 1} \sum_{n \mathop = 0}^{q - 1} \map \chi {M + k q + n} + \sum_{n \mathop = M + q d}^N \map \chi n\) | ||||||||||||

\(\ds \) | \(=\) | \(\ds \sum_{n \mathop = M \mathop + q d}^N \map \chi n\) | because $\chi$ is $q$-periodic, and zero on integers not coprime to $q$ | |||||||||||

\(\ds \) | \(\le\) | \(\ds q\) | because $\size {N - M + q d} \le q$ |

So the coefficients $\map \chi n$ of $\map L {s, \chi}$ have bounded partial sums.

Therefore, by Convergence of Dirichlet Series with Bounded Partial Sums, $\map L {s, \chi}$ converges locally uniformly to an analytic function on $\map \Re s > 0$.

$\blacksquare$

This article is complete as far as it goes, but it could do with expansion.In particular: Should be extended to all of $\C$, put the above down to get to Dirichlet's theoremYou can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by adding this information.To discuss this page in more detail, feel free to use the talk page.When this work has been completed, you may remove this instance of `{{Expand}}` from the code.If you would welcome a second opinion as to whether your work is correct, add a call to `{{Proofread}}` the page. |