G.f.: Sum_{n>=0} (1 + n*x)^n * x^n / (1 + x + n*x^2)^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =6a(6) =12a(7) =24a(8) =60a(9) =120a(10) =360a(11) =720a(12) =2520a(13) =5040a(14) =20160a(15) =40320a(16) =181440a(17) =362880a(18) =1814400a(19) =3628800a(20) =19958400a(21) =39916800a(22) =239500800a(23) =479001600a(24) =3113510400a(25) =6227020800a(26) =43589145600a(27) =87178291200a(28) =653837184000