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A202182 Number of n-element unlabeled N-free posets. 4

%I #25 Jul 01 2022 09:39:49

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

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

%C 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

%H Soheir M. Khamis, <a href="https://doi.org/10.1016/S0012-365X(03)00106-7">Height counting of unlabeled interval and N-free posets</a>, Discrete Math. 275 (2004), no. 1-3, 165-175.

%H Salah Uddin Mohammad, Md. Shah Noor, and Md. Rashed Talukder, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL25/Uddin/uddin6.html">An Exact Enumeration of the Unlabeled Disconnected Posets</a>, J. Int. Seq., Vol. 25 (2022), Article 22.5.4.

%e From _Salah Uddin Mohammad_, Mar 20 2021: (Start)

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

%e o o

%e | \ |

%e | o |

%e | \ |

%e o o

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

%e (End)

%Y Row sums of A202181.

%Y Cf. A003430.

%K nonn,more

%O 1,2

%A _N. J. A. Sloane_, Dec 13 2011

%E Missing a(12) inserted by _Salah Uddin Mohammad_, Mar 20 2021

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Last modified March 28 20:05 EDT 2024. Contains 371254 sequences. (Running on oeis4.)