a(n) = 2^n - n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =12a(5) =27a(6) =58a(7) =121a(8) =248a(9) =503a(10) =1014a(11) =2037a(12) =4084a(13) =8179a(14) =16370a(15) =32753a(16) =65520a(17) =131055a(18) =262126a(19) =524269a(20) =1048556a(21) =2097131a(22) =4194282a(23) =8388585a(24) =16777192a(25) =33554407a(26) =67108838a(27) =134217701a(28) =268435428a(29) =536870883