%N Number of automorphism groups of partial orders on n points.
%H G. Pfeiffer, <a href="http://schmidt.nuigalway.ie/subgroups">Subgroups</a>.
%H G. Pfeiffer, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL7/Pfeiffer/pfeiffer6.html">Counting Transitive Relations</a>, Journal of Integer Sequences, Vol. 7 (2004), Article 04.3.2.
%e a(3)=3 because of the 5 partial orders on 3 points, 2 have trivial automorphism group, 2 have an automorphism of order 2 and one has the full symmetric group as its automorphism group; thus 3 different (conjugacy classes of) subgroups occur.
%Y Cf. A000638 (subgroups of the symmetric group), A000112 (partial orders).
%A Goetz Pfeiffer (goetz.pfeiffer(AT)nuigalway.ie), Jan 21 2004