a(n) = 4^n - 3^n + 2^n.

Terms

a(0) =1a(1) =3a(2) =11a(3) =45a(4) =191a(5) =813a(6) =3431a(7) =14325a(8) =59231a(9) =242973a(10) =990551a(11) =4019205a(12) =16249871a(13) =65522733a(14) =263668871a(15) =1059425685a(16) =4251986111a(17) =17050860093a(18) =68332318391a(19) =273716169765