a(n) = n!/(m*(n-m)) with m = floor(n/2).

Terms

a(0) =2a(1) =3a(2) =6a(3) =20a(4) =80a(5) =420a(6) =2520a(7) =18144a(8) =145152a(9) =1330560a(10) =13305600a(11) =148262400a(12) =1779148800a(13) =23351328000a(14) =326918592000