a(n) = 1^n + 2^n + 6^n.

Terms

a(0) =3a(1) =9a(2) =41a(3) =225a(4) =1313a(5) =7809a(6) =46721a(7) =280065a(8) =1679873a(9) =10078209a(10) =60467201a(11) =362799105a(12) =2176786433a(13) =13060702209a(14) =78364180481a(15) =470185017345