a(n) = (n!+2)/2.

Terms

a(0) =2a(1) =4a(2) =13a(3) =61a(4) =361a(5) =2521a(6) =20161a(7) =181441a(8) =1814401a(9) =19958401a(10) =239500801a(11) =3113510401a(12) =43589145601a(13) =653837184001