A randomized polynomial kernel for Subset Feedback Vertex Set

TL;DR

This paper presents a randomized polynomial kernelization for the Subset Feedback Vertex Set problem, significantly improving the understanding of its fixed-parameter tractability and kernelization bounds.

Contribution

It provides the first randomized polynomial kernel for the problem, using matroid-based tools and a new reduction to bound the size of the set S.

Findings

Polynomial kernel with O(k^9) vertices for Subset Feedback Vertex Set

Randomized polynomial kernel for Edge Subset Feedback Vertex Set with O(|S|^2k) vertices

Reduction techniques bounding the size of S to O(k^4)

Abstract

The Subset Feedback Vertex Set problem generalizes the classical Feedback Vertex Set problem and asks, for a given undirected graph $G=(V,E)$, a set $S \subseteq V$, and an integer $k$, whether there exists a set $X$ of at most $k$ vertices such that no cycle in $G-X$ contains a vertex of $S$. It was independently shown by Cygan et al. (ICALP '11, SIDMA '13) and Kawarabayashi and Kobayashi (JCTB '12) that Subset Feedback Vertex Set is fixed-parameter tractable for parameter $k$. Cygan et al. asked whether the problem also admits a polynomial kernelization. We answer the question of Cygan et al. positively by giving a randomized polynomial kernelization for the equivalent version where $S$ is a set of edges. In a first step we show that Edge Subset Feedback Vertex Set has a randomized polynomial kernel parameterized by $|S|+k$ with $O(|S|^2k)$ vertices. For this we use the matroid-based tools of Kratsch and Wahlstr\"om (FOCS '12) that for example were used to obtain a polynomial kernel for $s$-Multiway Cut. Next we present a preprocessing that reduces the given instance $(G,S,k)$ to an equivalent instance $(G',S',k')$ where the size of $S'$ is bounded by $O(k^4)$. These two results lead to a polynomial kernel for Subset Feedback Vertex Set with $O(k^9)$ vertices.