A327425 - OEIS

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A327425

Number of unlabeled antichains of nonempty sets covering n vertices where every two vertices appear together in some edge (cointersecting).

1, 1, 1, 2, 6, 54

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,4

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.

LINKS

Table of n, a(n) for n=0..5.

EXAMPLE

Non-isomorphic representatives of the a(1) = 1 through a(4) = 6 antichains:

{1} {12} {123} {1234}

{12}{13}{23} {12}{134}{234}

{124}{134}{234}

{12}{13}{14}{234}

{123}{124}{134}{234}

{12}{13}{14}{23}{24}{34}

CROSSREFS

The labeled version is A327020.

Unlabeled covering antichains are A261005.

The weighted version is A327060.

Cf. A006126, A014466, A055621, A293606, A293993, A305844, A307249, A319639, A326704, A327057, A327058, A327358, A327359.

Sequence in context: A156340 A337510 A259553 * A262046 A280982 A219692

Adjacent sequences: A327422 A327423 A327424 * A327426 A327427 A327428

KEYWORD

nonn,more

AUTHOR

Gus Wiseman, Sep 11 2019

STATUS

approved