Defective coloring is perfect for minors

TL;DR

This paper proves that the defective chromatic number of any minor-closed graph family matches the simple lower bound, confirming a conjecture and providing optimal minors for infinite graphs with bounded degree partitions.

Contribution

It confirms a conjecture that the defective chromatic number equals the lower bound for all minor-closed families, resolving a key open problem.

Findings

Defective chromatic number equals the simple lower bound for minor-closed families.

Provides the optimal list of unavoidable minors for infinite graphs with bounded degree partitions.

Establishes a linear relation between clustered chromatic number and forbidden minors' tree-depth.

Abstract

The defective chromatic number of a graph class is the infimum $k$ such that there exists an integer $d$ such that every graph in this class can be partitioned into at most $k$ induced subgraphs with maximum degree at most $d$. Finding the defective chromatic number is a fundamental graph partitioning problem and received attention recently partially due to Hadwiger's conjecture about coloring minor-closed families. In this paper, we prove that the defective chromatic number of any minor-closed family equals the simple lower bound obtained by the standard construction, confirming a conjecture of Ossona de Mendez, Oum, and Wood. This result provides the optimal list of unavoidable finite minors for infinite graphs that cannot be partitioned into a fixed finite number of induced subgraphs with uniformly bounded maximum degree. As corollaries about clustered coloring, we obtain a linear relation between the clustered chromatic number of any minor-closed family and the tree-depth of its forbidden minors, improving an earlier exponential bound proved by Norin, Scott, Seymour, and Wood and confirming the planar case of their conjecture.