a(n) = 4^n - 3^n + 1.

Terms

a(0) =1a(1) =2a(2) =8a(3) =38a(4) =176a(5) =782a(6) =3368a(7) =14198a(8) =58976a(9) =242462a(10) =989528a(11) =4017158a(12) =16245776a(13) =65514542a(14) =263652488a(15) =1059392918a(16) =4251920576a(17) =17050729022a(18) =68332056248a(19) =273715645478