a(n) = 3*(2^n-1) - 2*n.

Terms

a(0) =0a(1) =1a(2) =5a(3) =15a(4) =37a(5) =83a(6) =177a(7) =367a(8) =749a(9) =1515a(10) =3049a(11) =6119a(12) =12261a(13) =24547a(14) =49121a(15) =98271a(16) =196573a(17) =393179a(18) =786393a(19) =1572823a(20) =3145685a(21) =6291411a(22) =12582865a(23) =25165775a(24) =50331597a(25) =100663243a(26) =201326537a(27) =402653127a(28) =805306309a(29) =1610612675