a(n) = A(n) if n is even else a(n) = A(n)*(n-1)/(n+1) with A(n) = ((n-1)!/ floor((n-1)/2)!^2).

Terms

a(0) =0a(1) =1a(2) =1a(3) =6a(4) =4a(5) =30a(6) =15a(7) =140a(8) =56a(9) =630a(10) =210a(11) =2772a(12) =792a(13) =12012a(14) =3003a(15) =51480a(16) =11440a(17) =218790a(18) =43758a(19) =923780a(20) =167960a(21) =3879876a(22) =646646a(23) =16224936a(24) =2496144a(25) =67603900a(26) =9657700a(27) =280816200a(28) =37442160a(29) =1163381400