a(n) = (2n)!/(n!*a(n-2)), with a(0) = 1, a(1) = 1.

Terms

a(0) =1a(1) =1a(2) =12a(3) =120a(4) =140a(5) =252a(6) =4752a(7) =68640a(8) =109200a(9) =257040a(10) =6139584a(11) =109549440a(12) =210974400a(13) =591192000a(14) =16576876800a(15) =343108846080a(16) =758663942400