a(n) = (5^n - 3^n)/2.

Terms

a(0) =0a(1) =1a(2) =8a(3) =49a(4) =272a(5) =1441a(6) =7448a(7) =37969a(8) =192032a(9) =966721a(10) =4853288a(11) =24325489a(12) =121804592a(13) =609554401a(14) =3049366328a(15) =15251614609a(16) =76272421952a(17) =381405156481