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A202182
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Number of n-element unlabeled N-free posets.
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4
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1, 2, 5, 15, 49, 180, 715, 3081, 14217, 69905, 363926, 1996922, 1150036, 69269925
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OFFSET
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1,2
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COMMENTS
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An N-free poset is a poset that does not contain any four elements x, y, z, w such that x is covered by z, y is covered by z, y is covered by w, x||y, x||w, and z||w. - Salah Uddin Mohammad, Mar 20 2021
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LINKS
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EXAMPLE
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The following poset on 5 nodes is considered to be N-free in this sequence.
o o
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o o
An alternative definition of N-free used by A003430 excludes this.
(End)
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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