Homomorphism reconfiguration via homotopy

TL;DR

This paper studies the reconfiguration problem for graph homomorphisms, providing a polynomial-time algorithm for graphs H without 4-cycles, linking topological properties to computational complexity.

Contribution

It introduces a polynomial-time algorithm for reconfiguration in graphs H with no C4, extending the understanding of homomorphism reconfiguration complexity.

Findings

Polynomial-time algorithm for H without C4

Reconfiguration paths characterized by topological conditions

Links between topological properties and computational complexity

Abstract

We consider the following problem for a fixed graph H: given a graph G and two H-colorings of G, i.e. homomorphisms from G to H, can one be transformed (reconfigured) into the other by changing one color at a time, maintaining an H-coloring throughout. This is the same as finding a path in the Hom(G,H) complex. For H=K_k this is the problem of finding paths between k-colorings, which was shown to be in P for k<=3 and PSPACE-complete otherwise by Cereceda et al. 2011. We generalize the positive side of this dichotomy by providing an algorithm that solves the problem in polynomial time for any H with no C_4 subgraph. This gives a large class of constraints for which finding solutions to the Constraint Satisfaction Problem is NP-complete, but finding paths in the solution space is P. The algorithm uses a characterization of possible reconfiguration sequences (paths in Hom(G,H)), whose main part is a purely topological condition described in algebraic terms of the fundamental groupoid of H seen as a topological space.