a(n) = 9^n - 2^n.

Terms

a(0) =0a(1) =7a(2) =77a(3) =721a(4) =6545a(5) =59017a(6) =531377a(7) =4782841a(8) =43046465a(9) =387419977a(10) =3486783377a(11) =31381057561a(12) =282429532385