OFFSET
0,1
COMMENTS
Alternatively, these are unlabeled sets of subsets of {1..n} whose dual is a (strict) antichain, also called T_1 sets of subsets. The dual of a set of subsets has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. An antichain is a set of subsets where no edge is a subset of any other.
EXAMPLE
Non-isomorphic representatives of the a(0) = 2 through a(2) = 8 sets of subsets:
{} {} {}
{{}} {{}} {{}}
{{1}} {{1}}
{{},{1}} {{},{1}}
{{1},{2}}
{{},{1},{2}}
{{1},{2},{1,2}}
{{},{1},{2},{1,2}}
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Aug 13 2019
STATUS
approved