A327808 - OEIS

A327808

Number of unlabeled, disconnected, nonempty antichains of subsets of {1..n}.

0, 0, 1, 3, 9, 32, 233, 16578

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OFFSET

0,4

COMMENTS

An antichain is a set of nonempty sets, none of which is a subset of any other. A singleton is considered to be connected.

LINKS

FORMULA

a(n) = A327424(n) - 1.

EXAMPLE

Non-isomorphic representatives of the a(2) = 1 through a(4) = 9 antichains:

{{1},{2}} {{1},{2}} {{1},{2}}

{{1},{2,3}} {{1},{2,3}}

{{1},{2},{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}}

{{4},{1,2},{1,3},{2,3}}

CROSSREFS

The labeled version is A327354 - 1.

The covering case is A327426.

Unlabeled antichains that are either not connected or not covering are A327437.

The version with empty antichains allowed is A327424.

KEYWORD

nonn,more

AUTHOR

Gus Wiseman, Sep 26 2019

STATUS

approved