Signature Catalan Combinatorics

TL;DR

This paper introduces a broad generalization of Catalan combinatorics using signatures derived from compositions and rational parameters, unifying and extending classical, Fuss-Catalan, and rational Catalan objects with explicit bijections and recurrences.

Contribution

It proposes a new framework for generalized Catalan objects indexed by compositions, connecting classical, Fuss-Catalan, and rational Catalan combinatorics with explicit bijections and recurrences.

Findings

Defined $s$-Catalan objects for compositions $s$

Established bijections between generalized Catalan objects

Derived a recurrence generalizing classical Catalan recurrence

Abstract

The Catalan numbers constitute one of the most important sequences in combinatorics. Catalan objects have been generalized in various directions, including the classical Fuss-Catalan objects and the rational Catalan generalization of Armstrong-Rhoades-Williams. We propose a wider generalization of these families indexed by a composition $s$ which is motivated by the combinatorics of planar rooted trees; when $s=(2,...,2)$ and $s=(k+1,...,k+1)$ we recover the classical Catalan and Fuss-Catalan combinatorics, respectively. Furthermore, to each pair $(a,b)$ of relatively prime numbers we can associate a signature that recovers the combinatorics of rational Catalan objects. We present explicit bijections between the resulting $s$-Catalan objects, and a fundamental recurrence that generalizes the fundamental recurrence of the classical Catalan numbers. Our framework allows us to define signature generalizations of parking functions which coincide with the generalized parking functions studied by Pitman-Stanley and Yan, as well as generalizations of permutations which coincide with the notion of Stirling multipermutations introduced by Gessel-Stanley. Some of our constructions differ from the ones of Armstrong-Rhoades-Williams, however as a byproduct of our extension, we obtain the additional notions of rational permutations and rational trees.