Finding a Sparse Connected Spanning Subgraph in a non-Uniform Failure Model

TL;DR

This paper investigates a generalized spanning tree problem under a non-uniform failure model, providing fixed-parameter tractability results for various parameters and establishing complexity boundaries.

Contribution

It offers a comprehensive FPT analysis of the Unweighted Flexible Graph Connectivity problem, including algorithms and hardness results for multiple parameters.

Findings

FPT algorithms for vertex deletion distance to cluster graphs and treewidth.

Hardness results for small parameters based on Hamiltonian Cycle relationship.

FPT algorithms when parameterized by the number of unsafe edges and solution size below upper bound.

Abstract

We study a generalization of the classic Spanning Tree problem that allows for a non-uniform failure model. More precisely, edges are either \emph{safe} or \emph{unsafe} and we assume that failures only affect unsafe edges. In Unweighted Flexible Graph Connectivity we are given an undirected graph $G = (V,E)$ in which the edge set $E$ is partitioned into a set $S$ of safe edges and a set $U$ of unsafe edges and the task is to find a set $T$ of at most $k$ edges such that $T - \{u\}$ is connected and spans $V$ for any unsafe edge $u \in T$. Unweighted Flexible Graph Connectivity generalizes both Spanning Tree and Hamiltonian Cycle. We study Unweighted Flexible Graph Connectivity in terms of fixed-parameter tractability (FPT). We show an almost complete dichotomy on which parameters lead to fixed-parameter tractability and which lead to hardness. To this end, we obtain FPT-time algorithms with respect to the vertex deletion distance to cluster graphs and with respect to the treewidth. By exploiting the close relationship to Hamiltonian Cycle, we show that FPT-time algorithms for many smaller parameters are unlikely under standard parameterized complexity assumptions. Regarding problem-specific parameters, we observe that Unweighted Flexible Graph Connectivity} admits an FPT-time algorithm when parameterized by the number of unsafe edges. Furthermore, we investigate a below-upper-bound parameter for the number of edges of a solution. We show that this parameter also leads to an FPT-time algorithm.