a(n) = 4^n - 2^(n-6)*15*binomial(n,6).

Terms

a(0) =1a(1) =4a(2) =16a(3) =64a(4) =256a(5) =1024a(6) =4081a(7) =16174a(8) =63856a(9) =252064a(10) =998176a(11) =3972544a(12) =15890176a(13) =63814144a(14) =256903936a(15) =1035303424a(16) =4171964416a(17) =16799678464a(18) =67578904576a(19) =271543926784