a(n) = 7^(n-2) * C(n,2).

Terms

a(0) =1a(1) =21a(2) =294a(3) =3430a(4) =36015a(5) =352947a(6) =3294172a(7) =29647548a(8) =259416045a(9) =2219448385a(10) =18643366434a(11) =154231485954