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

Terms

a(0) =2a(1) =4a(2) =9a(3) =20a(4) =43a(5) =90a(6) =185a(7) =376a(8) =759a(9) =1526a(10) =3061a(11) =6132a(12) =12275a(13) =24562a(14) =49137a(15) =98288a(16) =196591a(17) =393198a(18) =786413a(19) =1572844a(20) =3145707a(21) =6291434a(22) =12582889a(23) =25165800a(24) =50331623a(25) =100663270a(26) =201326565a(27) =402653156a(28) =805306339a(29) =1610612706