To prove FLT, it suffices to prove it for any prime $n \ge 5$. I once read somewhere, (can't find link) that to prove Fermat's Last Theorem, assuming it has been proven for $n = 3, 4$, it suffices to prove it for every prime $n \ge 5$.  I have no idea why this is true.  Can somebody explain?

Suppose $n$ is not prime, then it can be written as $pq$ for some $p,q\in\mathbb{N}$, with $q$ prime, and $a^n$ can be written as $(a^p)^q$ so if there is a solution for $n$ there also is one for $q$ (which is smaller than $n$).
