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

Terms

a(0) =1a(1) =15a(2) =129a(3) =1071a(4) =8961a(5) =75855a(6) =649089a(7) =5606511a(8) =48811521a(9) =427774095a(10) =3769259649a(11) =33358386351a(12) =296270823681