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

Terms

a(0) =0a(1) =0a(2) =0a(3) =5a(4) =409a(5) =7a(6) =2621a(7) =971a(8) =1677721a(9) =1342177a(10) =107a(11) =89a(12) =6871947673a(13) =54975581a(14) =4398046511a(15) =883a(16) =28147497671a(17) =2251799813a(20) =46068469a(21) =3685477a(22) =629483