Floor(Gamma(n/2)).

Terms

a(0) =1a(1) =1a(2) =0a(3) =1a(4) =1a(5) =2a(6) =3a(7) =6a(8) =11a(9) =24a(10) =52a(11) =120a(12) =287a(13) =720a(14) =1871a(15) =5040a(16) =14034a(17) =40320a(18) =119292a(19) =362880a(20) =1133278a(21) =3628800a(22) =11899423a(23) =39916800a(24) =136843365a(25) =479001600a(26) =1710542068a(27) =6227020800a(28) =23092317922a(29) =87178291200