a(n) = (n-1)*a(n-2), a(0)=1, a(1)=0.

Terms

a(0) =1a(1) =0a(2) =1a(3) =0a(4) =3a(5) =0a(6) =15a(7) =0a(8) =105a(9) =0a(10) =945a(11) =0a(12) =10395a(13) =0a(14) =135135a(15) =0a(16) =2027025a(17) =0a(18) =34459425a(19) =0a(20) =654729075a(21) =0a(22) =13749310575a(23) =0a(24) =316234143225a(25) =0a(27) =0a(29) =0a(31) =0a(33) =0