A202182 - OEIS

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A202182

Number of n-element unlabeled N-free posets.

1, 2, 5, 15, 49, 180, 715, 3081, 14217, 69905, 363926, 1996922, 1150036, 69269925

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OFFSET

1,2

COMMENTS

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

LINKS

Table of n, a(n) for n=1..14.

Soheir M. Khamis, Height counting of unlabeled interval and N-free posets, Discrete Math. 275 (2004), no. 1-3, 165-175.

Salah Uddin Mohammad, Md. Shah Noor, and Md. Rashed Talukder, An Exact Enumeration of the Unlabeled Disconnected Posets, J. Int. Seq., Vol. 25 (2022), Article 22.5.4.

EXAMPLE

From Salah Uddin Mohammad, Mar 20 2021: (Start)

The following poset on 5 nodes is considered to be N-free in this sequence.

o o

| \ |

| o |

| \ |

o o

An alternative definition of N-free used by A003430 excludes this.

(End)

CROSSREFS

Row sums of A202181.