a(n) = n!/24.

A001720

a(n) = n!/24.

Terms

    a(0) =1a(1) =5a(2) =30a(3) =210a(4) =1680a(5) =15120a(6) =151200a(7) =1663200a(8) =19958400a(9) =259459200a(10) =3632428800a(11) =54486432000a(12) =871782912000

External references