

A327011


Number of unlabeled sets of subsets covering n vertices where every vertex is the unique common element of some subset of the edges, also called unlabeled covering T_1 sets of subsets.


1




OFFSET

0,1


COMMENTS

Alternatively, these are unlabeled sets of subsets covering n vertices whose dual is a (strict) antichain. 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. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. An antichain is a set of subsets where no edge is a subset of any other.


LINKS

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


FORMULA

a(n) = A326974(n) / 2.
a(n > 0) = A326951(n)  A326951(n  1).


EXAMPLE

Nonisomorphic representatives of the a(0) = 1 through a(2) = 4 sets of subsets:
{} {{1}} {{1},{2}}
{{}} {{},{1}} {{},{1},{2}}
{{1},{2},{1,2}}
{{},{1},{2},{1,2}}


CROSSREFS

Unlabeled covering sets of subsets are A003181.
The same with T_0 instead of T_1 is A326942.
The noncovering version is A326951 (partial sums).
The labeled version is A326960.
The case without empty edges is A326974.
Cf. A001146, A055621, A059523, A319637, A326961, A326972, A326973, A326976, A326977, A326979.
Sequence in context: A032082 A257616 A296048 * A300361 A257617 A309344
Adjacent sequences: A327008 A327009 A327010 * A327012 A327013 A327014


KEYWORD

nonn,more


AUTHOR

Gus Wiseman, Aug 13 2019


STATUS

approved



