a(n) = 6^n + 3^n - 1.

Terms

a(0) =1a(1) =8a(2) =44a(3) =242a(4) =1376a(5) =8018a(6) =47384a(7) =282122a(8) =1686176a(9) =10097378a(10) =60525224a(11) =362974202a(12) =2177313776a(13) =13062288338a(14) =78368947064a(15) =470199333482