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arXiv 2008.05669math.CO
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Dyck路径中的对称峰和对称谷

Symmetric peaks and symmetric valleys in Dyck paths

Sergi Elizalde

摘要:

Dyck路径中对称峰和非对称峰的概念是由Flórez和Rodr 'ıguez提出的,他们计算了给定长度的所有Dyck路径上此类峰的总数。在本文中,我们推广他们的结果,给出多元生成函数,跟踪对称峰的数量和非对称峰的数量,以及这些峰的宽度。我们恢复了Denise和Simion的公式作为我们结果的一个特例。我们也考虑类似的,但更复杂的概念对称谷。我们发现一个连分数的表达的生成函数的Dyck路径的对称山谷的数量和它们的宽度的总和,这提供了一个意想不到的连接对称山谷和统计有序有根树。最后,我们列举了戴克路径的峰或谷的高度满足一定的单调性和单峰性条件,使用一个共同的框架来恢复一些已知的结果,并与我们的问题,某些列凸多项式类的枚举。

英文摘要:

The notion of symmetric and asymmetric peaks in Dyck paths was introduced by Flórez and Rodr\'ıguez, who counted the total number of such peaks over all Dyck paths of a given length. In this paper we generalize their results by giving multivariate generating functions that keep track of the number of symmetric peaks and the number of asymmetric peaks, as well as the widths of these peaks. We recover a formula of Denise and Simion as a special case of our results. We also consider the analogous but more intricate notion of symmetric valleys. We find a continued fraction expression for the generating function of Dyck paths with respect to the number of symmetric valleys and the sum of their widths, which provides an unexpected connection between symmetric valleys and statistics on ordered rooted trees. Finally, we enumerate Dyck paths whose peak or valley heights satisfy certain monotonicity and unimodality conditions, using a common framework to recover some known results, and relating our questions to the enumeration of certain classes of column-convex polyominoes.

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