Multiplicative Order
Definition
Given $n \in \mathbb N$ and $a \in \mathbb N$ coprime with $n$, the multiplicative order of $a$ modulo $n$ is, equivalently:
The order of $a$ in the group $((\mathbb Z / n)^\times, \cdot)$
The minimal $k \in \mathbb N$ such that $a^k \equiv 1\ (\text{mod } n)$
Also see

Referenced by: