Identifying codes in hereditary classes of graphs and VC-dimension

TL;DR

This paper explores the size and approximability of identifying codes in hereditary graph classes, revealing a VC-dimension-based dichotomy and hardness results for approximation algorithms.

Contribution

It establishes a VC-dimension-based dichotomy for identifying code sizes and analyzes the approximability limits in hereditary graph classes.

Findings

Hereditary classes with infinite VC-dimension have infinitely many graphs with small identifying codes.

Finite VC-dimension classes have polynomial lower bounds on identifying code size.

Finding smallest identifying codes is log-APX-hard in classes with infinite VC-dimension.

Abstract

An identifying code of a graph is a subset of its vertices such that every vertex of the graph is uniquely identified by the set of its neighbours within the code. We show a dichotomy for the size of the smallest identifying code in classes of graphs closed under induced subgraphs. Our dichotomy is derived from the VC-dimension of the considered class C, that is the maximum VC-dimension over the hypergraphs formed by the closed neighbourhoods of elements of C. We show that hereditary classes with infinite VC-dimension have infinitely many graphs with an identifying code of size logarithmic in the number of vertices while classes with finite VC-dimension have a polynomial lower bound. We then turn to approximation algorithms. We show that the problem of finding a smallest identifying code in a given graph from some class is log-APX-hard for any hereditary class of infinite VC-dimension. For hereditary classes of finite VC-dimension, the only known previous results show that we can approximate the identifying code problem within a constant factor in some particular classes, e.g. line graphs, planar graphs and unit interval graphs. We prove that it can be approximate within a factor 6 for interval graphs. In contrast, we show that on C_4-free bipartite graphs (a class of finite VC-dimension) it cannot be approximated to within a factor of c.log(|V|) for some c>0.