a(n) = 6^n - 4^n + 1.

Terms

a(0) =1a(1) =3a(2) =21a(3) =153a(4) =1041a(5) =6753a(6) =42561a(7) =263553a(8) =1614081a(9) =9815553a(10) =59417601a(11) =358602753a(12) =2160005121a(13) =12993585153a(14) =78095728641a(15) =469111242753