The art of computer programming, volume 3: (2nd ed.) sorting and searching
The art of computer programming, volume 3: (2nd ed.) sorting and searching
The combinatorics of associated Hermite polynomials
European Journal of Combinatorics
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How many matchings on the vertex set V = {1,2,...,2n} avoid a given configuration of three edges? Chen, Deng and Du have shown that the number of matchings that avoid three nesting edges is equal to the number of matchings avoiding three pairwise crossing edges. In this paper, we consider other forbidden configurations of size three. We present a bijection between matchings avoiding three crossing edges and matchings avoiding an edge nested below two crossing edges. This bijection uses non-crossing pairs of Dyck paths of length 2n as an intermediate step. Apart from that, we give a bijection that maps matchings avoiding two nested edges crossed by a third edge onto the matchings avoiding all configurations from an infinite family M, which contains the configuration consisting of three crossing edges. We use this bijection to show that for matchings of size n3, it is easier to avoid three crossing edges than to avoid two nested edges crossed by a third edge. Our results on pattern-avoiding matchings can be regarded as an extension of previous results on pattern-avoiding permutations.