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

Terms

a(0) =0a(1) =3a(2) =10a(3) =25a(4) =56a(5) =119a(6) =246a(7) =501a(8) =1012a(9) =2035a(10) =4082a(11) =8177a(12) =16368a(13) =32751a(14) =65518a(15) =131053a(16) =262124a(17) =524267a(18) =1048554a(19) =2097129a(20) =4194280a(21) =8388583a(22) =16777190a(23) =33554405a(24) =67108836a(25) =134217699a(26) =268435426a(27) =536870881a(28) =1073741792a(29) =2147483615