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

Terms

a(0) =1a(1) =1a(2) =3a(3) =10a(4) =40a(5) =184a(6) =948a(7) =5384a(8) =33300a(9) =222192a(10) =1587512a(11) =12071776a(12) =97206544a(13) =825343600a(14) =7362067888a(15) =68772244640a(16) =670917511424