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

Terms

a(0) =0a(1) =2a(2) =32a(3) =386a(4) =4160a(5) =42242a(6) =413792a(7) =3959426a(8) =37281920a(9) =347066882a(10) =3204309152a(11) =29403732866a(12) =268588249280