a(n) = 4^n-3^n-2^n-1.

Terms

a(0) =-2a(1) =-2a(2) =2a(3) =28a(4) =158a(5) =748a(6) =3302a(7) =14068a(8) =58718a(9) =241948a(10) =988502a(11) =4015108a(12) =16241678a(13) =65506348a(14) =263636102a(15) =1059360148a(16) =4251855038a(17) =17050597948a(18) =68331794102a(19) =273715121188