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

Terms

a(0) =0a(1) =0a(2) =2a(3) =18a(4) =96a(5) =600a(6) =4320a(7) =35280a(8) =322560a(9) =3265920a(10) =36288000a(11) =439084800a(12) =5748019200a(13) =80951270400