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

Terms

a(0) =0a(1) =0a(2) =4a(3) =24a(4) =48a(5) =240a(6) =1440a(7) =10080a(8) =80640a(9) =725760a(10) =7257600a(11) =79833600a(12) =958003200a(13) =12454041600a(14) =174356582400