a(n) = 6*9^n-1.

Terms

a(0) =5a(1) =53a(2) =485a(3) =4373a(4) =39365a(5) =354293a(6) =3188645a(7) =28697813a(8) =258280325a(9) =2324522933a(10) =20920706405a(11) =188286357653