a(n) = 9^n - 7^n + 1.

Terms

a(0) =1a(1) =3a(2) =33a(3) =387a(4) =4161a(5) =42243a(6) =413793a(7) =3959427a(8) =37281921a(9) =347066883a(10) =3204309153a(11) =29403732867a(12) =268588249281