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

Terms

a(0) =-1a(1) =-1a(2) =3a(3) =29a(4) =159a(5) =749a(6) =3303a(7) =14069a(8) =58719a(9) =241949a(10) =988503a(11) =4015109a(12) =16241679a(13) =65506349a(14) =263636103a(15) =1059360149a(16) =4251855039a(17) =17050597949a(18) =68331794103a(19) =273715121189