On Detecting $H$-Induced Minors for Small $H$

TL;DR

This paper advances the understanding of the $H$-Induced Minor problem by providing polynomial algorithms for specific small graphs and completing the classification for all graphs on five vertices.

Contribution

It solves an open problem by showing polynomial-time algorithms for detecting certain small $H$-induced minors, including a long-standing open case.

Findings

Polynomial-time algorithms for detecting specific small $H$-induced minors.

NP-hardness results for detecting some substructures individually.

Complete classification of $H$-Induced Minor for graphs on five vertices.

Abstract

We consider the $H$-Induced Minor problem: for a fixed graph~$H$, decide whether a given graph $G$ contains $H$ as an induced minor. While the problem is known to be NP-complete for some trees~$H$ on more than $2^{300}$ vertices, the complexity for small trees remains unresolved. In particular, the case where $H$ is the $7$-vertex tree consisting of a path on five vertices with a pendant vertex attached to the second and fourth vertex was a long-standing open problem. We show that this case is polynomial-time solvable by developing algorithms that detect a sequence of carefully chosen substructures. Complementing this, we prove that detecting some of these substructures individually is NP-hard. We also give polynomial-time algorithms for three cases where $H$ is a graph on five vertices (that is not a tree). In this way, we completed the classification of $H$-Induced Minor for graphs $H$ on five vertices and answered an open problem of Dallard, Dumas, Hilaire and Perez (2025).