a(n) = 2^n*n!/(floor(n/2)!)^2.

Terms

a(0) =1a(1) =2a(2) =8a(3) =48a(4) =96a(5) =960a(6) =1280a(7) =17920a(8) =17920a(9) =322560a(10) =258048a(11) =5677056a(12) =3784704a(13) =98402304a(14) =56229888a(15) =1686896640a(16) =843448320a(17) =28677242880a(18) =12745441280a(19) =484326768640a(20) =193730707456