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

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =5a(5) =16a(6) =42a(7) =99a(8) =219a(9) =466a(10) =968a(11) =1981a(12) =4017a(13) =8100a(14) =16278a(15) =32647a(16) =65399a(17) =130918a(18) =261972a(19) =524097a(20) =1048365a(21) =2096920a(22) =4194050a(23) =8388331a(24) =16776915a(25) =33554106a(26) =67108512a(27) =134217349a(28) =268435049a(29) =536870476