6899

This number is a prime.

The smallest prime whose sum of digits is a fifth power. [Gupta]

The 2nd composite-digit super prime. It is the 887th prime in the list of primes, with 887 being prime. It is the first composite-digit super prime p with the property “the index of p followed by p,” i.e., 8876899 "is also prime." [Loungrides]