A326973 Number of unlabeled set-systems covering n vertices whose dual is a weak antichain.

1, 1, 3, 19, 1243

Offset

0,3

Comments

A set-system is a finite set of finite nonempty sets. The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. A weak antichain is a multiset of sets, none of which is a proper subset of any other.

Links

Table of n, a(n) for n=0..4.

Example

Non-isomorphic representatives of the a(0) = 1 through a(3) = 19 set-systems:
  {}  {{1}}  {{1,2}}          {{1,2,3}}
             {{1},{2}}        {{1},{2,3}}
             {{1},{2},{1,2}}  {{1},{2},{3}}
                              {{1,2},{1,3},{2,3}}
                              {{1},{2,3},{1,2,3}}
                              {{1},{2},{3},{2,3}}
                              {{1},{2},{1,3},{2,3}}
                              {{1},{2},{3},{1,2,3}}
                              {{3},{1,2},{1,3},{2,3}}
                              {{1},{2},{3},{1,3},{2,3}}
                              {{1,2},{1,3},{2,3},{1,2,3}}
                              {{1},{2},{3},{2,3},{1,2,3}}
                              {{2},{3},{1,2},{1,3},{2,3}}
                              {{1},{2},{1,3},{2,3},{1,2,3}}
                              {{1},{2},{3},{1,2},{1,3},{2,3}}
                              {{3},{1,2},{1,3},{2,3},{1,2,3}}
                              {{1},{2},{3},{1,3},{2,3},{1,2,3}}
                              {{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
                              {{1},{2},{3},{1,2},{1,3},{2,3},{1,2,3}}

Crossrefs

Unlabeled covering set-systems are A055621.

The labeled version is A326970.

The non-covering case is A326971 (partial sums).

The case that is also T_0 is the T_1 case A326974.

Cf. A000612, A059523, A319637, A326966, A326968, A326972, A326975, A326978.

Sequence in context: A114301 A258669 A192340 * A248704 A098796 A365579

Adjacent sequences: A326970 A326971 A326972 * A326974 A326975 A326976

Keyword

nonn,more

Author

Gus Wiseman, Aug 11 2019

Status

approved