a(n) = 2*7^n-5^n.

Terms

a(0) =1a(1) =9a(2) =73a(3) =561a(4) =4177a(5) =30489a(6) =219673a(7) =1568961a(8) =11138977a(9) =78754089a(10) =555184873a(11) =3905825361a(12) =27438433777a(13) =192557317689