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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =6a(5) =17a(6) =43a(7) =100a(8) =220a(9) =467a(10) =969a(11) =1982a(12) =4018a(13) =8101a(14) =16279a(15) =32648a(16) =65400a(17) =130919a(18) =261973a(19) =524098a(20) =1048366a(21) =2096921a(22) =4194051a(23) =8388332a(24) =16776916a(25) =33554107a(26) =67108513a(27) =134217350a(28) =268435050a(29) =536870477