a(n) = C( C(n,2), n).

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =15a(5) =252a(6) =5005a(7) =116280a(8) =3108105a(9) =94143280a(10) =3190187286a(11) =119653565850