Parameterized Aspects of Strong Subgraph Closure

TL;DR

This paper introduces and studies the parameterized complexity of the Strong F-closure problem, a generalization of Strong Triadic Closure, analyzing its fixed-parameter tractability and kernelization across various graph classes.

Contribution

It establishes FPT results for Strong F-closure with respect to the number of strong edges and explores kernelization limits, including polynomial kernels on specific graph classes.

Findings

Strong F-closure is FPT for every fixed F when parameterized by the number of strong edges.

No polynomial kernel exists for Strong F-closure even on split graphs.

Polynomial kernels are possible for planar and d-degenerate graphs.

Abstract

Motivated by the role of triadic closures in social networks, and the importance of finding a maximum subgraph avoiding a fixed pattern, we introduce and initiate the parameterized study of the Strong F-closure problem, where F is a fixed graph. This is a generalization of Strong Triadic Closure, whereas it is a relaxation of F-free Edge Deletion. In Strong F-closure, we want to select a maximum number of edges of the input graph G, and mark them as strong edges, in the following way: whenever a subset of the strong edges forms a subgraph isomorphic to F, then the corresponding induced subgraph of G is not isomorphic to F. Hence the subgraph of G defined by the strong edges is not necessarily F-free, but whenever it contains a copy of F, there are additional edges in G to destroy that strong copy of F in G. We study Strong F-closure from a parameterized perspective with various natural parameterizations. Our main focus is on the number k of strong edges as the parameter. We show that the problem is FPT with this parameterization for every fixed graph F, whereas it does not admit a polynomial kernel even when F =P_3. In fact, this latter case is equivalent to the Strong Triadic Closure problem, which motivates us to study this problem on input graphs belonging to well known graph classes. We show that Strong Triadic Closure does not admit a polynomial kernel even when the input graph is a split graph, whereas it admits a polynomial kernel when the input graph is planar, and even d-degenerate. Furthermore, on graphs of maximum degree at most 4, we show that Strong Triadic Closure is FPT with the above guarantee parameterization k - \mu(G), where \mu(G) is the maximum matching size of G. We conclude with some results on the parameterization of Strong F-closure by the number of edges of G that are not selected as strong.