If n mod 2 = 0 then 3*2^(n-1)+n-1 else 3*2^(n-1)+n.

Terms

a(0) =4a(1) =7a(2) =15a(3) =27a(4) =53a(5) =101a(6) =199a(7) =391a(8) =777a(9) =1545a(10) =3083a(11) =6155a(12) =12301a(13) =24589a(14) =49167a(15) =98319a(16) =196625a(17) =393233a(18) =786451a(19) =1572883a(20) =3145749a(21) =6291477a(22) =12582935a(23) =25165847a(24) =50331673a(25) =100663321a(26) =201326619a(27) =402653211a(28) =805306397a(29) =1610612765