Homomorphism Indistinguishability and Game Comonads for Restricted Conjunction and Requantification

TL;DR

This paper extends homomorphism indistinguishability to logics with restricted requantification, introducing new combinatorial and categorical tools to analyze graph classes with bounded width and reusability constraints.

Contribution

It generalizes Lovász-type theorems for restricted logics, introduces novel path and tree decompositions with reusability, and develops comonadic frameworks for these logics.

Findings

Bounded pathwidth and treewidth classes are homomorphism distinguishing closed.

New comonads encapsulate restricted-reusability pebble games.

Categorical characterizations unify logical, combinatorial, and categorical perspectives.

Abstract

The notion of homomorphism indistinguishability offers a combinatorial framework for characterizing equivalence relations of graphs, in particular equivalences in counting logics within finite model theory. That is, for certain graph classes, two structures agree on all homomorphism counts from the class if and only if they satisfy the same sentences in a corresponding logic. This perspective often reveals connections between the combinatorial properties of graph classes and the syntactic structure of logical fragments. In this work, we extend this perspective to logics with restricted requantification, refining the stratification of logical resources in finite-variable counting logics. Specifically, we generalize Lov\'asz-type theorems for these logics with either restricted conjunction or bounded quantifier-rank and present new combinatorial proofs of existing results. To this end, we introduce novel path and tree decompositions that incorporate the concept of reusability and develop characterizations based on pursuit-evasion games. Leveraging this framework, we establish that classes of bounded pathwidth and treewidth with reusability constraints are homomorphism distinguishing closed. Finally, we develop a comonadic perspective on requantification by constructing new comonads that encapsulate restricted-reusability pebble games. We show a tight correspondence between their coalgebras and path/tree decompositions, yielding categorical characterizations of reusability in graph decompositions. This unifies logical, combinatorial, and categorical perspectives on the notion of reusability.