|
|
A326951
|
|
Number of unlabeled sets of subsets of {1..n} where every covered vertex is the unique common element of some subset of the edges.
|
|
10
|
|
|
|
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.
|
|
LINKS
|
|
|
FORMULA
|
|
|
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
|
Unlabeled sets of subsets are A003180.
Unlabeled T_0 sets of subsets are A326949.
The case without empty edges is A326972.
The covering case is A327011 (first differences).
|
|
KEYWORD
|
nonn,more
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|