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

Terms

a(0) =2a(1) =1a(2) =1a(3) =1a(4) =2a(5) =2a(6) =4a(7) =6a(8) =12a(9) =24a(10) =53a(11) =120a(12) =288a(13) =720a(14) =1872a(15) =5040a(16) =14035a(17) =40320a(18) =119293a(19) =362880a(20) =1133279a(21) =3628800a(22) =11899424a(23) =39916800a(24) =136843366a(25) =479001600a(26) =1710542069a(27) =6227020800a(28) =23092317923a(29) =87178291200