a(n) = 3*11^n - 1.

Terms

a(0) =2a(1) =32a(2) =362a(3) =3992a(4) =43922a(5) =483152a(6) =5314682a(7) =58461512a(8) =643076642a(9) =7073843072a(10) =77812273802a(11) =855935011832