a(n) = (n+1)*a(n-2) with a(0) = a(1) = 1.

Terms

a(0) =1a(1) =1a(2) =3a(3) =4a(4) =15a(5) =24a(6) =105a(7) =192a(8) =945a(9) =1920a(10) =10395a(11) =23040a(12) =135135a(13) =322560a(14) =2027025a(15) =5160960a(16) =34459425a(17) =92897280a(18) =654729075a(19) =1857945600a(20) =13749310575a(21) =40874803200a(22) =316234143225a(23) =980995276800