2^(n-1) - n*(n+1)/2.

Terms

a(0) =0a(1) =-1a(2) =-2a(3) =-2a(4) =1a(5) =11a(6) =36a(7) =92a(8) =211a(9) =457a(10) =958a(11) =1970a(12) =4005a(13) =8087a(14) =16264a(15) =32632a(16) =65383a(17) =130901a(18) =261954a(19) =524078a(20) =1048345a(21) =2096899a(22) =4194028a(23) =8388308a(24) =16776891a(25) =33554081a(26) =67108486a(27) =134217322a(28) =268435021a(29) =536870447