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

Terms

a(0) =0a(1) =2a(2) =6a(3) =14a(4) =28a(5) =52a(6) =94a(7) =170a(8) =312a(9) =584a(10) =1114a(11) =2158a(12) =4228a(13) =8348a(14) =16566a(15) =32978a(16) =65776a(17) =131344a(18) =262450a(19) =524630a(20) =1048956a(21) =2097572a(22) =4194766a(23) =8389114a(24) =16777768a(25) =33555032a(26) =67109514a(27) =134218430a(28) =268436212a(29) =536871724