Definition:Dirichlet Character

Definition
Let $q \in \Z_{>1}$.

Let $\paren {\Z / q \Z}$ denote the ring of integers modulo $q$.

Let $G = \paren {\Z / q \Z}^\times$ be the group of units of $\paren {\Z / q \Z}$.

Let $\C^\times$ be the group of units of $\C$.

A Dirichlet character modulo $q$ is a group homomorphism:
 * $\chi: G \to \C^\times$

By Reduced Residue System under Multiplication forms Abelian Group, $a + q \Z \in G$ $\map \gcd {a, q} = 1$.

It is standard practice to extend $\chi$ to a function on $\Z$ by setting:


 * $\map \chi a = \begin{cases}

\map \chi {a + q \Z} & : \map \gcd {a, q} = 1 \\ 0 & : \text{otherwise} \end{cases}$

Also see

 * Definition:Character (Number Theory)