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

Terms

a(0) =1a(1) =2a(2) =3a(3) =12a(4) =40a(5) =240a(6) =1260a(7) =10080a(8) =72576a(9) =725760a(10) =6652800a(11) =79833600a(12) =889574400a(13) =12454041600a(14) =163459296000