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%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