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

Terms

a(0) =0a(1) =5a(2) =34a(3) =171a(4) =776a(5) =3361a(6) =14190a(7) =58967a(8) =242452a(9) =989517a(10) =4017146a(11) =16245763a(12) =65514528a(13) =263652473a(14) =1059392902a(15) =4251920559a(16) =17050729004a(17) =68332056229a(18) =273715645458