a(n) = n!/5!.

Terms

a(0) =1a(1) =6a(2) =42a(3) =336a(4) =3024a(5) =30240a(6) =332640a(7) =3991680a(8) =51891840a(9) =726485760a(10) =10897286400a(11) =174356582400