a(n) = (7*6^n - 2)/5.

Terms

a(0) =1a(1) =8a(2) =50a(3) =302a(4) =1814a(5) =10886a(6) =65318a(7) =391910a(8) =2351462a(9) =14108774a(10) =84652646a(11) =507915878a(12) =3047495270a(13) =18284971622a(14) =109709829734a(15) =658258978406