a(n) = n! * Sum_{k=0..floor(n/2)} k! / (2*k)!.

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =38a(5) =190a(6) =1146a(7) =8022a(8) =64200a(9) =577800a(10) =5778120a(11) =63559320a(12) =762712560a(13) =9915263280a(14) =138813690960