= Gauss-Wantzel theorem
{c}
{title2=$N=2^a p_1\cdots p_r$}
For $N\geq3$, a <regular polygon> with $N$ sides is a <constructible regular polygon> exactly when $N=2^a p_1\cdots p_r$, where $a$ is a <nonnegative integer> and the $p_j$ are distinct <Fermat primes>; the product may be empty. Thus seventeen and fifteen qualify, while nine does not. <Carl Friedrich Gauss> established sufficiency, and <Pierre Wantzel> proved necessity in 1837.<Primality testing: then and now by Carl Pomerance>[slide 18]{ref}
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