512

This number is a composite.

512 is the smallest (and only known) cube such that 512*2^512-1 is prime. [Gupta]

512 = 2^9. Both 2^9 +/- 9^2 are prime. This is the smallest number of the form 2^n such that both 2^n +/- n^2 are prime. [Astle]