Geometry
Published in Dan Zwillinger, CRC Standard Mathematical Tables and Formulas, 2018
Dan Zwillinger
A regular n‐gon inscribed in the unit circle can be constructed by straightedge and compass alone if and only if n has the form n = 2ℓp1p2…pk, where ℓ is a nonnegative integer and {pi} are distinct Fermat primes (primes of the form 22m+1
$ 2^{{2^{m} }} + 1 $
). The only known Fermat primes are for 3, 5, 17, 257, and 65537, corresponding to m = 1, 2, 3, 4. Thus, regular n‐gons can be constructed with this many sides n=3,4,5,6,8,10,12,15,16,17,20,24,…,257,…
$ n = 3,4, \, 5, \, 6, \, 8, \, 10, \, 12, \, 15, \, 16, \, 17, \, 20, \, 24, \ldots , \, 257, \ldots $