a(0) = 1, a(n) = 3*2^(n-1) - n for n>0.

Terms

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