a(n) = 1 + (4*9^n - 9*4^n) / 5.

Terms

a(0) =1a(1) =37a(2) =469a(3) =4789a(4) =45397a(5) =417781a(6) =3796885a(7) =34319413a(8) =309464533a(9) =2787540085a(10) =25097297941a(11) =225913430197