E.g.f. x^2*(1-x)/(1-x-x^2).

Terms

a(0) =0a(1) =0a(2) =2a(3) =0a(4) =24a(5) =120a(6) =1440a(7) =15120a(8) =201600a(9) =2903040a(10) =47174400a(11) =838252800a(12) =16286054400a(13) =342486144000