Asymptotic enumeration of partial orders on a finite set

By considering special cases, the number Pn of partially ordered sets on a set of n elements is shown to be (1 + O(lIn))Qn, where Qn is the number of partially ordered sets in one of the special classes. The number Qn can be estimated, and we ultimately obtain I (n n-i lnz(ni) 2i1 j(2f -1)n-i