OFFSET
0,3
COMMENTS
An antichain is a set of sets, none of which is a subset of any other. It is covering if there are no isolated vertices.
The spanning edge-connectivity of a set-system is the minimum number of edges that must be removed (without removing incident vertices) to obtain a set-system that is disconnected or covers fewer vertices.
EXAMPLE
Non-isomorphic representatives of the a(1) = 1 through a(4) = 15 antichains:
{} {} {} {}
{{1}} {{1}} {{1}}
{{1},{2}} {{1,2}} {{1,2}}
{{1},{2}} {{1},{2}}
{{1},{2,3}} {{1,2,3}}
{{1},{2},{3}} {{1},{2,3}}
{{1,2},{1,3}}
{{1},{2},{3}}
{{1},{2,3,4}}
{{1,2},{3,4}}
{{1},{2},{3,4}}
{{1},{2},{3},{4}}
{{2},{1,3},{1,4}}
{{1,2},{1,3},{2,3}}
{{4},{1,2},{1,3},{2,3}}
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Sep 11 2019
STATUS
approved