A Parameterized Complexity View on Collapsing k-Cores

TL;DR

This paper analyzes the computational complexity of the Collapsed k-Core problem, revealing a complexity dichotomy based on the parameter k, and explores fixed-parameter tractability and kernelization aspects.

Contribution

It provides a detailed parameterized complexity classification of Collapsed k-Core, including hardness results and fixed-parameter algorithms for various parameters.

Findings

Collapsed k-Core is W[1]-hard when parameterized by b for k >= 3.

It is fixed-parameter tractable when parameterized by (b + x) for k <= 2.

The problem admits an FPT algorithm when parameterized by the treewidth of the input graph.

Abstract

We study the NP-hard graph problem Collapsed k-Core where, given an undirected graph G and integers b, x, and k, we are asked to remove b vertices such that the k-core of remaining graph, that is, the (uniquely determined) largest induced subgraph with minimum degree k, has size at most x. Collapsed k-Core was introduced by Zhang et al. [AAAI 2017] and it is motivated by the study of engagement behavior of users in a social network and measuring the resilience of a network against user drop outs. Collapsed k-Core is a generalization of r-Degenerate Vertex Deletion (which is known to be NP-hard for all r >= 0) where, given an undirected graph G and integers b and r, we are asked to remove b vertices such that the remaining graph is r-degenerate, that is, every its subgraph has minimum degree at most r. We investigate the parameterized complexity of Collapsed k-Core with respect to the parameters b, x, and k, and several structural parameters of the input graph. We reveal a dichotomy in the computational complexity of Collapsed k-Core for k <= 2 and k >= 3. For the latter case it is known that for all x >= 0 Collapsed k-Core is W[P]-hard when parameterized by b. We show that Collapsed k-Core is W[1]-hard when parameterized by b and in FPT when parameterized by (b + x) if k <= 2. Furthermore, we show that Collapsed k-Core is in FPT when parameterized by the treewidth of the input graph and presumably does not admit a polynomial kernel when parameterized by the vertex cover number of the input graph.