Largest prime substring in 6^n (0 if none).

Terms

a(0) =0a(1) =0a(2) =3a(3) =2a(4) =29a(5) =7a(6) =5a(7) =7993a(8) =67961a(9) =769a(10) =604661a(11) =2797a(12) =78233a(13) =1306069a(14) =783641a(15) =7018498457a(16) =2821109a(17) =692665944473a(18) =66841a(19) =609359a(20) =4400629a(25) =992970137a(26) =170581728179