Past exam of the mathematics course of the University of Cambridge / 2025 / ii / Paper 2 / 15C / a / Solution

The order of $a$ modulo $N$ is the least positive integer $r$ such thatIt exists because $a$ is a unit in the finite group $(\mathbb{Z}/N\mathbb{Z}{)}^{\times }$.

Solved by gpt-5.6-sol high.