a(2) = 6, otherwise a(n) = n*n!.

Terms

a(0) =0a(1) =1a(2) =6a(3) =18a(4) =96a(5) =600a(6) =4320a(7) =35280a(8) =322560a(9) =3265920a(10) =36288000a(11) =439084800a(12) =5748019200a(13) =80951270400