A046271 Largest prime substring in 5^n (0 if none).

0, 5, 5, 5, 5, 31, 5, 7, 5, 19531, 97, 281, 2441, 12207031, 103, 305175781, 58789, 94531, 3814697, 63281, 95367431, 6837158203, 5791, 9209289550781, 60464477539, 953, 901161193, 450580596923, 2529029846191