a(n) = 2^(floor((n-1)/2)) - n*(n-1)/2.

Terms

a(0) =1a(1) =0a(2) =-1a(3) =-4a(4) =-6a(5) =-11a(6) =-13a(7) =-20a(8) =-20a(9) =-29a(10) =-23a(11) =-34a(12) =-14a(13) =-27a(14) =23a(15) =8a(16) =120a(17) =103a(18) =341a(19) =322a(20) =814a(21) =793a(22) =1795a(23) =1772a(24) =3796a(25) =3771a(26) =7841a(27) =7814a(28) =15978a(29) =15949