A Partially Synchronizing Coloring

TL;DR

This paper generalizes the road coloring problem to k-synchronizing colorings of directed graphs, providing theoretical results, a subquadratic algorithm, and visualization tools for such colorings.

Contribution

It extends the road coloring problem to k-synchronizing colorings, offering a characterization, an efficient algorithm, and visualization methods.

Findings

A characterization of k-synchronizing colorings based on cycle lengths.

Development of a subquadratic algorithm for k-synchronizing coloring.

Implementation of the algorithm in the TESTAS package with visualization tools.

Abstract

Given a finite directed graph, a coloring of its edges turns the graph into a finite-state automaton. A k-synchronizing word of a deterministic automaton is a word in the alphabet of colors at its edges that maps the state set of the automaton at least on k-element subset. A coloring of edges of a directed strongly connected finite graph of a uniform outdegree (constant outdegree of any vertex) is k-synchronizing if the coloring turns the graph into a deterministic finite automaton possessing a k-synchronizing word. For k=1 one has the well known road coloring problem. The recent positive solution of the road coloring problem implies an elegant generalization considered first by Beal and Perrin: a directed finite strongly connected graph of uniform outdegree is k-synchronizing iff the greatest common divisor of lengths of all its cycles is k. Some consequences for coloring of an arbitrary finite digraph are presented. We describe a subquadratic algorithm of the road coloring for the k-synchronization implemented in the package TESTAS. A new linear visualization program demonstrates the obtained coloring. Some consequences for coloring of an arbitrary finite digraph and of such a graph of uniform outdegree are presented.