A202182 Number of n-element unlabeled N-free posets.

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

Offset: 1

Author

N. J. A. Sloane, Dec 13 2011

Keywords

• nonn
• more

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

Examples

			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)
		

Links

• 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.

Crossrefs

Row sums of A202181.

Cf. A003430.

Extensions

Missing a(12) inserted by Salah Uddin Mohammad, Mar 20 2021