(2*7^n - 6*3^n + 4)/6.

Terms

a(0) =0a(1) =0a(2) =8a(3) =88a(4) =720a(5) =5360a(6) =38488a(7) =272328a(8) =1915040a(9) =13431520a(10) =94099368a(11) =658931768a(12) =4613230960a(13) =32294742480a(14) =226069574648