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Proceedings of the American Mathematical Society

Published by the American Mathematical Society since 1950, Proceedings of the American Mathematical Society is devoted to shorter research articles in all areas of pure and applied mathematics.

ISSN 1088-6826 (online) ISSN 0002-9939 (print)

The 2020 MCQ for Proceedings of the American Mathematical Society is 0.85.

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$n!$ matchings, $n!$ posets
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by Anders Claesson and Svante Linusson PDF
Proc. Amer. Math. Soc. 139 (2011), 435-449


We show that there are $n!$ matchings on $2n$ points without so-called left (neighbor) nestings. We also define a set of naturally labeled $(\mathbf {2}+\mathbf {2})$-free posets and show that there are $n!$ such posets on $n$ elements. Our work was inspired by Bousquet-Mélou, Claesson, Dukes and Kitaev [J. Combin. Theory Ser. A. 117 (2010) 884–909]. They gave bijections between four classes of combinatorial objects: matchings with no neighbor nestings (due to Stoimenow), unlabeled $(\mathbf {2}+\mathbf {2})$-free posets, permutations avoiding a specific pattern, and so-called ascent sequences. We believe that certain statistics on our matchings and posets could generalize the work of Bousquet-Mélou et al., and we make a conjecture to that effect. We also identify natural subsets of matchings and posets that are equinumerous to the class of unlabeled $(\mathbf {2}+\mathbf {2})$-free posets.

We give bijections that show the equivalence of (neighbor) restrictions on nesting arcs with (neighbor) restrictions on crossing arcs. These bijections are thought to be of independent interest. One of the bijections factors through certain upper-triangular integer matrices that have recently been studied by Dukes and Parviainen [Electron. J. Combin. 17 (2010) #R53].

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Additional Information
  • Anders Claesson
  • Affiliation: School of Computer Science, Reykjavik University, Menntavegi 1, 101 Reykjavik, Iceland
  • Svante Linusson
  • Affiliation: Department of Mathematics, KTH-Royal Institute of Technology, SE-100 44 Stockholm, Sweden
  • MR Author ID: 360616
  • Received by editor(s): March 24, 2010
  • Published electronically: October 7, 2010
  • Additional Notes: The first author was supported by grant no. 090038011 from the Icelandic Research Fund.
    The second author is a Royal Swedish Academy of Sciences Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation.
  • Communicated by: Jim Haglund
  • © Copyright 2010 Anders Claesson and Svante Linusson
  • Journal: Proc. Amer. Math. Soc. 139 (2011), 435-449
  • MSC (2010): Primary 05A05, 05A15
  • DOI:
  • MathSciNet review: 2736327