Deletion to Scattered Graph Classes II -- Improved FPT Algorithms for Deletion to Pairs of Graph Classes

TL;DR

This paper develops improved fixed-parameter tractable algorithms for the problem of deleting vertices to ensure each connected component belongs to one of two specified hereditary graph classes, extending previous work with more efficient solutions.

Contribution

It introduces a general FPT and approximation algorithm for deletion problems targeting pairs of hereditary graph classes, including cases with infinite forbidden sets, using branching techniques.

Findings

Designed a simpler, more efficient FPT algorithm for pairs of graph classes.

Extended applicability to classes with infinite forbidden sets.

Covered several popular graph class pairs with the new algorithms.

Abstract

Let $\Pi$ be a hereditary graph class. The problem of deletion to $\Pi$, takes as input a graph $G$ and asks for a minimum number (or a fixed integer $k$) of vertices to be deleted from $G$ so that the resulting graph belongs to $\Pi$. This is a well-studied problem in paradigms including approximation and parameterized complexity. Recently, the study of a natural extension of the problem was initiated where we are given a finite set of hereditary graph classes, and the goal is to determine whether $k$ vertices can be deleted from a given graph so that the connected components of the resulting graph belong to one of the given hereditary graph classes. The problem is shown to be FPT as long as the deletion problem to each of the given hereditary graph classes is fixed-parameter tractable, and the property of being in any of the graph classes is expressible in the counting monodic second order (CMSO) logic. While this was shown using some black box theorems, faster algorithms were shown when each of the hereditary graph classes has a finite forbidden set. In this paper, we do a deep dive on pairs of specific graph classes ($\Pi_1, \Pi_2$) in which we would like the connected components of the resulting graph to belong to, and design simpler and more efficient FPT algorithms. We design a general FPT algorithm and approximation algorithm for pairs of graph classes (possibly having infinite forbidden sets) satisfying certain conditions. These algorithms cover several pairs of popular graph classes. Our algorithm makes non-trivial use of the branching technique and as a black box, FPT algorithms for deletion to individual graph classes.