2999

This number is a prime.

Start with any number greater than 1, and write down all its divisors, including 1 and itself. Now take the sum of the individual digits of these divisors. After repeating the process, you'll eventually get the product of the first two odd primes (fifteen) or what Dr. Michael W. Ecker describes as a "mathemagical black hole" with respect to this particular iteration. Note that Jason Earls of Blackwell, Oklahoma, found that 2999 is the least prime that takes exactly fifteen steps to reach the fifteen black hole.