257

257 (two hundred [and] fifty-seven) is the natural number following 256 and preceding 258. 257 is a prime number of the form 2 2 n + 1 , {\displaystyle 2^{2^{n}}+1,} specifically with n = 3, and therefore a Fermat prime. Thus, a regular polygon with 257 sides is constructible with compass and unmarked straightedge. It is currently the second largest known Fermat prime.