Distribution of statistics on separable permutations restricted by a flat POP

Finding distributions of statistics in pattern-avoiding permutations has attracted significant attention in the literature. In particular, Chen, Kitaev, and Zhang derived functional equations for the joint distributions of any subset of classical minima and maxima statistics, as well as for the joint distributions of ascents and descents in separable permutations. Meanwhile, partially ordered patterns (POPs) have also been extensively studied. Notably, so-called flat POPs played a key role, via the notion of shape-Wilf-equivalence, in proving a conjecture on pattern-avoiding permutations.

In this paper, we study flat POP-avoiding separable permutations, where the maximum element in a flat POP receives the largest label. Avoiding such a POP imposes restrictions on the position of the maximum element in a separable permutation, forcing it to be positioned to the left. We establish a system of functional equations describing the joint distribution of six classical statistics in the most general case, extending the work of Chen, Kitaev, and Zhang.

As a specialization, when the POP has length 3, we recover a joint distribution result of Han and Kitaev on permutations avoiding classical patterns of length 3. As another specialization, for the flat POP of length 4, we derive an explicit rational generating function that captures the distribution of six statistics, with a numerator containing 100 monomials and a denominator containing 19 monomials.