257 (number)
257 is the natural number following 256 and preceding 258.257 is a prime number of the form 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.
It is also
a balanced prime,
an irregular prime,
a prime that is one more than a square,
and a Jacobsthal–Lucas number.
There are exactly 257 combinatorially distinct convex polyhedra with eight vertices.
