a(n) = (n!!)^2 - n!.

Terms

a(0) =2a(1) =3a(2) =40a(3) =105a(4) =1584a(5) =5985a(6) =107136a(7) =530145a(8) =11116800a(9) =68139225a(10) =1644364800a(11) =12034447425a(12) =329001523200