a(n) = (2*9^n + 3^n)/3.

Terms

a(0) =1a(1) =7a(2) =57a(3) =495a(4) =4401a(5) =39447a(6) =354537a(7) =3189375a(8) =28700001a(9) =258286887a(10) =2324542617a(11) =20920765455a(12) =188286534801