a(n) = n! - binomial(n,3).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =20a(5) =110a(6) =700a(7) =5005a(8) =40264a(9) =362796a(10) =3628680a(11) =39916635a(12) =479001380a(13) =6227020514a(14) =87178290836