Enumeration of non-crossing pairings on bit strings

TL;DR

This paper generalizes classical Catalan structure enumeration to non-crossing pairings on complex bitstrings, providing explicit formulas and bounds relevant to random matrix theory and free probability.

Contribution

It derives explicit formulas and upper bounds for counting non-crossing pairings on arbitrary bitstrings, extending classical Catalan enumeration results.

Findings

Explicit formulas for pairing enumeration functions

Upper bounds in terms of Fuss-Catalan numbers

Connections to random matrices and free probability

Abstract

A non-crossing pairing on a bitstring matches 1s and 0s in a manner such that the pairing diagram is nonintersecting. By considering such pairings on arbitrary bitstrings $1^{n_1} 0^{m_1} ... 1^{n_r} 0^{m_r}$, we generalize classical problems from the theory of Catalan structures. In particular, it is very difficult to find useful explicit formulas for the enumeration function $\phi(n_1, m_1, ..., n_r, m_r)$, which counts the number of pairings as a function of the underlying bitstring. We determine explicit formulas for $\phi$, and also prove general upper bounds in terms of Fuss-Catalan numbers by relating non-crossing pairings to other generalized Catalan structures (that are in some sense more natural). This enumeration problem arises in the theory of random matrices and free probability.