E.g.f. x*(1-x)/(1-2*x-x^2+x^3).

Terms

a(0) =0a(1) =1a(2) =2a(3) =18a(4) =144a(5) =1680a(6) =22320a(7) =352800a(8) =6330240a(9) =128096640a(10) =2877638400a(11) =71131737600