a(n) = (n!*m)/(m!*(m+1)!) where m = floor(n/2).

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =4a(5) =20a(6) =15a(7) =105a(8) =56a(9) =504a(10) =210a(11) =2310a(12) =792a(13) =10296a(14) =3003a(15) =45045a(16) =11440a(17) =194480a(18) =43758a(19) =831402a(20) =167960a(21) =3527160a(22) =646646a(23) =14872858a(24) =2496144a(25) =62403600a(26) =9657700a(27) =260757900a(28) =37442160a(29) =1085822640