a(n) = (5*2^(n+1)-9-(-1)^n)/6-2*n.

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =17a(5) =42a(6) =93a(7) =198a(8) =409a(9) =834a(10) =1685a(11) =3390a(12) =6801a(13) =13626a(14) =27277a(15) =54582a(16) =109193a(17) =218418a(18) =436869a(19) =873774a(20) =1747585a(21) =3495210a(22) =6990461a(23) =13980966a(24) =27961977a(25) =55924002a(26) =111848053a(27) =223696158a(28) =447392369a(29) =894784794