a(n) = largest prime factor of 7^n-1.

Terms

a(0) =3a(1) =3a(2) =19a(3) =5a(4) =2801a(5) =43a(6) =4733a(7) =1201a(8) =1063a(9) =2801a(10) =293459a(11) =181a(12) =16148168401a(13) =4733a(14) =159871a(15) =169553a(16) =2767631689a(17) =117307a(19) =4021a(20) =11898664849a(21) =10746341a(23) =1201a(25) =16148168401a(26) =2583253