The role of counting quantifiers in laminar set systems

TL;DR

This paper demonstrates how to derive laminar trees from set systems using MSO logic, resolving an open question and enhancing understanding of counting quantifiers' expressive power.

Contribution

It shows that laminar trees can be obtained via MSO transductions, resolving an open problem and improving decomposition techniques in graph theory.

Findings

MSO transduction can derive laminar trees from set systems.

Resolved an open question by Courcelle regarding MSO definability.

Provided insights into the expressive power of counting quantifiers in laminar set systems.

Abstract

Laminar set systems consist of non-crossing subsets of a universe with set inclusion essentially corresponding to the descendant relationship of a tree, the so-called laminar tree. Laminar set systems lie at the core of many graph decompositions such as modular decompositions, split decompositions, and bi-join decompositions. We show that from a laminar set system we can obtain the corresponding laminar tree by means of a monadic second order logic (MSO) transduction. This resolves an open question originally asked by Courcelle and is a satisfying resolution as MSO is the natural logic for set systems and is sufficient to define the property ``laminar''. Using results from Campbell et al. [STACS 2025], we can now obtain transductions for obtaining modular decompositions, co-trees, split decompositions and bi-join decompositions using MSO instead of CMSO. We further gain some insight into the expressive power of counting quantifiers and provide some results towards determining when counting quantifiers can be simulated in MSO in laminar set systems and when they cannot.