a(n) = 3*8^n-1.

Terms

a(0) =2a(1) =23a(2) =191a(3) =1535a(4) =12287a(5) =98303a(6) =786431a(7) =6291455a(8) =50331647a(9) =402653183a(10) =3221225471a(11) =25769803775a(12) =206158430207