a(n) = 4^n*(n^3 - 3*n^2 + 2*n + 384)/384.

Terms

a(0) =1a(1) =4a(2) =16a(3) =65a(4) =272a(5) =1184a(6) =5376a(7) =25344a(8) =122880a(9) =606208a(10) =3014656a(11) =15007744a(12) =74448896a(13) =367001600a(14) =1795162112a(15) =8707375104a(16) =41875931136a(17) =199715979264a(18) =944892805120