Distributions of statistics on separable permutations

Chen, Joanna N. and Kitaev, Sergey and Zhang, Philip B. (2024) Distributions of statistics on separable permutations. Discrete Applied Mathematics, 355. pp. 169-179. ISSN 0166-218X (https://doi.org/10.1016/j.dam.2024.05.004)

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We derive functional equations for distributions of six classi- cal statistics (ascents, descents, left-to-right maxima, right-to-left maxima, left-to-right minima, and right-to-left minima) on separable and irreducible separable permutations. The equations are used to find a third degree equation for joint distribution of ascents and descents on separable permuta- tions that generalizes the respective known result for the descent distribution. Moreover, our general functional equations allow us to derive explicitly (joint) distribution of any subset of maxima and minima statistics on irreducible, reducible and all separable permutations. In particular, there are two equivalence classes of distributions of a pair of maxima or minima statistics. Finally, we present three unimodality conjectures about distributions of statistics on separable permutations.