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

Terms

a(0) =0a(1) =0a(2) =2a(3) =20a(4) =128a(5) =672a(6) =3168a(7) =14016a(8) =59648a(9) =247808a(10) =1014272a(11) =4113408a(12) =16588800a(13) =66674688a(14) =267444224a(15) =1071497216a(16) =4289921024a(17) =17168596992a(18) =68694441984a(19) =274822594560