A046274 Largest prime substring in 8^n (0 if none).

0, 0, 0, 5, 409, 7, 2621, 971, 1677721, 1342177, 107, 89, 6871947673, 54975581, 4398046511, 883, 28147497671, 2251799813, 180143985094819, 1518807585587, 46068469, 3685477, 629483, 9029581035870565171, 472236648286964521369