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

Terms

a(0) =2a(1) =0a(2) =0a(3) =4a(4) =16a(5) =44a(6) =104a(7) =228a(8) =480a(9) =988a(10) =2008a(11) =4052a(12) =8144a(13) =16332a(14) =32712a(15) =65476a(16) =131008a(17) =262076a(18) =524216a(19) =1048500a(20) =2097072a(21) =4194220a(22) =8388520a(23) =16777124a(24) =33554336a(25) =67108764a(26) =134217624a(27) =268435348a(28) =536870800a(29) =1073741708