a(n) = 7^n - 3^n.

Terms

a(0) =0a(1) =4a(2) =40a(3) =316a(4) =2320a(5) =16564a(6) =116920a(7) =821356a(8) =5758240a(9) =40333924a(10) =282416200a(11) =1977149596a(12) =13840755760a(13) =96887416084a(14) =678218289880