a(0)=1, a(1)=0; a(n) = floor(n/2)*(a(n-1) + a(n-2)).

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =4a(5) =10a(6) =42a(7) =156a(8) =792a(9) =3792a(10) =22920a(11) =133560a(12) =938880a(13) =6434640a(14) =51614640a(15) =406344960a(16) =3663676800a(17) =32560174080a(18) =326014657920