Assume $a^{(p−1)(q−1)} \equiv 1 \pmod N$, and for any positive integer k where $0 < k < (p−1)(q−1)$, $a^k \neq 1 \pmod N$.
Show that for any integer s which is relatively prime to N, s can be written as $a^k \pmod\ N$ for some integer $0 \leq k < (p−1)(q−1)$.
Any hints on this?