題目如下：
Suppose that (G,‧) is a group and G = <a>. Prove that G = <a^k> if and
only if gcd(k, |G|) = 1, where k is a positive integer.
請問有人會證嗎囧/
這貌似是Lagrange定理的引理，可是我實在是不會證XD"
請大家幫幫忙了！