28999999999

This number is a prime.

                                                                2 8999999999

The smallest prime for which the sum of the squares of its digits is also a prime throughout 5 iterations (6 "generations"): 28999999999 -> 797 -> 179 -> 131 -> 11 -> 2. Terry Trotter called this type of prime a "father" prime, and the subsequent sums (797, 179, 131, 11, 2) "child" primes. Is there a 7-generation father prime to be found? [Ganson]