23

This number is a prime.

(23, 29) is the only pair of consecutive double-digit primes p, q, such that the sum of digits of p^5 is equal to q and the sum of digits of q^5 is equal to p, (23^5=6436343 (whose sum of digits is 29) and 29^5=20511149 (whose sum of digits is 23). [Loungrides]