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arXiv · 1408.1859

Context-free Grammars for Permutations and Increasing Trees

William Y. C. Chen·Amy M. Fu
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Abstract

In this paper, we introduce the notion of a grammatical labeling to describe a recursive process of generating combinatorial objects based on a context-free grammar. For example, by labeling the ascents and descents of a Stirling permutation, we obtain a grammar for the second-order Eulerian polynomials. By using the grammar for $0$-$1$-$2$ increasing trees given by Dumont, we obtain a grammatical derivation of the generating function of the Andr\'e polynomials obtained by Foata and Sch\"utzenberger, without solving a differential equation. We also find a grammar for the number $T(n,k)$ of permutations of $[n]=\{1,2,\ldots, n\}$ with $k$ exterior peaks, which was independently discovered by Ma. We demonstrate that Gessel's formula for the generating function of $T(n,k)$ can be deduced from this grammar. Moreover, by using grammars we show that the number of the permutations of $[n]$ with $k$ exterior peaks equals the number of increasing trees on $[n]$ with $2k+1$ vertices of even degree. A combinatorial proof of this fact is also presented.

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William Y. C. Chen, Amy M. Fu. 2014-08-08. Context-free Grammars for Permutations and Increasing Trees. https://arxiv.org/abs/1408.1859

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