a(n) = 2*binomial(7*n+2,n)/(7*n+2).

Terms

a(0) =1a(1) =2a(2) =15a(3) =154a(4) =1827a(5) =23562a(6) =320866a(7) =4540200a(8) =66096459a(9) =983592304a(10) =14894775896a(11) =228784720710