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

Terms

a(0) =0a(1) =-1a(2) =-1a(3) =1a(4) =7a(5) =21a(6) =51a(7) =113a(8) =239a(9) =493a(10) =1003a(11) =2025a(12) =4071a(13) =8165a(14) =16355a(15) =32737a(16) =65503a(17) =131037a(18) =262107a(19) =524249a(20) =1048535a(21) =2097109a(22) =4194259a(23) =8388561a(24) =16777167a(25) =33554381a(26) =67108811a(27) =134217673a(28) =268435399a(29) =536870853