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

Terms

a(0) =0a(1) =1a(2) =7a(3) =37a(4) =175a(5) =781a(6) =3367a(7) =14197a(8) =58975a(9) =242461a(10) =989527a(11) =4017157a(12) =16245775a(13) =65514541a(14) =263652487a(15) =1059392917a(16) =4251920575a(17) =17050729021a(18) =68332056247a(19) =273715645477