This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A236756 #17 Dec 07 2019 12:18:27 %S A236756 1,1,2,4,9,24,73,261,1103 %N A236756 Number of (unlabeled) partially ordered sets on n elements which are Cohen-Macaulay over the integers. %D A236756 R. P. Stanley, Enumerative Combinatorics, Volume 1, Second edition, Cambridge, page 273. %e A236756 For n=1, there is one poset up to isomorphism which corresponds to the simplicial complex with one facet of a single element. The homology groups over Z vanish for this, and it is therefore Cohen-Macaulay, thus a(1) = 1. %e A236756 For n=2, there are two posets up to isomorphism. The first of which, the sum of two singletons, also has order complex with vanishing homology groups over Z. The second has an order complex with a single facet, and any interval has a contractible order complex, which therefore has vanishing homology groups. They are both Cohen-Macaulay thus a(2) = 2. %o A236756 (Sage) %o A236756 for n in range(66): %o A236756 j=0; %o A236756 for p in Posets(n): %o A236756 if p.order_complex().is_cohen_macaulay(): %o A236756 j = j+1; %o A236756 print(j) %Y A236756 Cf. A000112. %K A236756 nonn,hard,more %O A236756 0,3 %A A236756 _Sam DeHority_, Jan 30 2014