Slicings of parallelogram polyominoes: Catalan, Schr\"oder, Baxter, and other sequences

TL;DR

This paper introduces a new generating tree for Schr"oder numbers that bridges Catalan and Baxter numbers, defining related combinatorial objects called slicings and exploring their subclasses and generating functions.

Contribution

It presents a novel succession rule for Schr"oder numbers, defines Schr"oder and Baxter slicings, and analyzes their subclasses and generating functions using the kernel method.

Findings

New succession rule interpolates between Catalan and Baxter numbers.

Defined Schr"oder and Baxter slicings and subclasses.

Generated functions are computed in special cases, conjectured to be algebraic.

Abstract

We provide a new succession rule (i.e. generating tree) associated with Schr\"oder numbers, that interpolates between the known succession rules for Catalan and Baxter numbers. We define Schr\"oder and Baxter generalizations of parallelogram polyominoes, called slicings, which grow according to these succession rules. In passing, we also exhibit Schr\"oder subclasses of Baxter classes, namely a Schr\"oder subset of triples of non-intersecting lattice paths, a new Schr\"oder subset of Baxter permutations, and a new Schr\"oder subset of mosaic floorplans. Finally, we define two families of subclasses of Baxter slicings: the $m$-skinny slicings and the $m$-row-restricted slicings, for $m \in \mathbb{N}$. Using functional equations and the kernel method, their generating functions are computed in some special cases, and we conjecture that they are algebraic for any $m$.