a(n) = 2^n + 2^(n - 1) - n - 8.

Terms

a(0) =1a(1) =12a(2) =35a(3) =82a(4) =177a(5) =368a(6) =751a(7) =1518a(8) =3053a(9) =6124a(10) =12267a(11) =24554a(12) =49129a(13) =98280a(14) =196583a(15) =393190a(16) =786405a(17) =1572836a(18) =3145699a(19) =6291426a(20) =12582881a(21) =25165792a(22) =50331615a(23) =100663262a(24) =201326557a(25) =402653148a(26) =805306331a(27) =1610612698a(28) =3221225433a(29) =6442450904