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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =12a(5) =20a(6) =60a(7) =252a(8) =672a(9) =1440a(10) =5400a(11) =27720a(12) =88704a(13) =224640a(14) =982800a(15) =5821200a(16) =21288960a(17) =61102080a(18) =300736800a(19) =1990850400a(20) =8089804800a(21) =25662873600a(22) =138940401600