617

This number is a prime.

617 is the only three-digit prime such that, if 2 is subtracted from its 4th power, the resulting number is prime and if 2 is subtracted from the 4th power of this number, the result is still prime. I.e., 617, 617^4-2, and (617^4-2)^4-2 are all primes. [Opao]