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

Terms

a(0) =1a(1) =2a(2) =8a(3) =40a(4) =192a(5) =864a(6) =3712a(7) =15488a(8) =63488a(9) =257536a(10) =1038336a(11) =4171776a(12) =16728064a(13) =67002368a(14) =268206080a(15) =1073250304a(16) =4293918720a(17) =17177640960a(18) =68714758144a(19) =274867945472