Question 16
(ROM 1) IMO2 Let $n>6$ and $a_1\lt a_2\lt \cdots\lt a_k$ be all natural numbers that are less than $n$ and relatively prime to $n$. Show that if $a_1,a_2,\ldots,a_k$ is an arithmetic progression, then $n$ is a prime number or a natural power of two.