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

Terms

a(0) =0a(1) =0a(2) =2a(3) =18a(4) =96a(5) =960a(6) =11520a(7) =161280a(8) =2580480a(9) =46448640a(10) =928972800a(11) =20437401600a(12) =490497638400