17717171

This number is a prime.

Formed from 17 followed by its reversal repeated three times. Repeating the reversal before 17, we obtain another prime: 71717117. Taking the reversal of 17 and and using the same process, two new primes are yielded: 71171717, 17171771. Note that the reversal must be repeated at least three times with a number to give prime. [Silva]