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

Terms

a(0) =0a(1) =5a(2) =5a(3) =5a(4) =5a(5) =31a(6) =5a(7) =7a(8) =5a(9) =19531a(10) =97a(11) =281a(12) =2441a(13) =12207031a(14) =103a(15) =305175781a(16) =58789a(17) =94531a(18) =3814697a(19) =63281a(20) =95367431a(21) =6837158203a(22) =5791a(24) =60464477539a(25) =953a(26) =901161193a(27) =450580596923