Journal of Integer Sequences, Vol. 10 (2007), Article 07.7.6

Semiorders and Riordan Numbers


Barry Balof and Jacob Menashe
Department of Mathematics
Whitman College
Walla Walla, WA 99362
USA

Abstract:

In this paper, we define a class of semiorders (or unit interval orders) that arose in the context of polyhedral combinatorics. In the first section of the paper, we will present a pure counting argument equating the number of these interesting (connected and irredundant) semiorders on n+1 elements with the nth Riordan number. In the second section, we will make explicit the relationship between the interesting semiorders and a special class of Motzkin paths, namely, those Motzkin paths without horizontal steps of height 0, which are known to be counted by the Riordan numbers.


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(Concerned with sequences A000108 and A005043 .)

Received February 14 2007; revised version received July 18 2007. Published in Journal of Integer Sequences, July 23 2007.


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