a(n) = 6*2^n - 3*n - 5.

Terms

a(0) =1a(1) =4a(2) =13a(3) =34a(4) =79a(5) =172a(6) =361a(7) =742a(8) =1507a(9) =3040a(10) =6109a(11) =12250a(12) =24535a(13) =49108a(14) =98257a(15) =196558a(16) =393163a(17) =786376a(18) =1572805a(19) =3145666a(20) =6291391a(21) =12582844a(22) =25165753a(23) =50331574a(24) =100663219a(25) =201326512a(26) =402653101a(27) =805306282a(28) =1610612647a(29) =3221225380