a(n) = 6*(8^n - 1)/7.

Terms

a(0) =0a(1) =6a(2) =54a(3) =438a(4) =3510a(5) =28086a(6) =224694a(7) =1797558a(8) =14380470a(9) =115043766a(10) =920350134a(11) =7362801078a(12) =58902408630a(13) =471219269046