Largest prime factor of 4^n - 1.

Terms

a(0) =3a(1) =5a(2) =7a(3) =17a(4) =31a(5) =13a(6) =127a(7) =257a(8) =73a(9) =41a(10) =683a(11) =241a(12) =8191a(13) =127a(14) =331a(15) =65537a(16) =131071a(17) =109a(18) =524287a(19) =61681a(20) =5419a(21) =2113a(22) =2796203a(23) =673a(24) =4051a(25) =8191a(26) =262657a(27) =15790321a(28) =3033169a(29) =1321