Exact multi-covering problems with geometric sets

Abstract

The b-EXACT MULTICOVER problem takes a universe U of n elements, a family F of m subsets of U, a function dem:U→{1,…,b} and a positive integer k, and decides whether there exists a subfamily(set cover) F′ of size at most k such that each element u ∈ U is covered by exactly dem(u) sets of F′. The b-EXACT COVERAGE problem also takes the same input and decides whether there is a subfamily F′⊆F such that there are at least k elements that satisfy the following property: u ∈ U is covered by exactly dem(u) sets of F′. Both these problems are known to be NP-complete. In the parameterized setting, when parameterized by k, b-EXACT MULTICOVER is W[1]-hard even when b = 1. While b-EXACT COVERAGE is FPT under the same parameter, it is known to not admit a polynomial kernel under standard complexity-theoretic assumptions, even when b = 1. In this paper, we investigate these two problems under the assumption that every set satisfies a given geometric property π. Specifically, we consider the universe to be a set of n points in a real space Rd, d being a positive integer. When d = 2 we consider the problem when π requires all sets to be unit squares or lines. When d > 2, we consider the problem where π requires all sets to be hyperplanes in Rd. These special versions of the problems are also known to be NP-complete. When parameterized by k, the b-EXACT COVERAGE problem has a polynomial size kernel for all the above geometric versions. The b-EXACT MULTICOVER problem turns out to be W[1]-hard for squares even when b = 1, but FPT for lines and hyperplanes. Further, we also consider the b-EXACT MAX. MULTICOVER problem, which takes the same input and decides whether there is a set cover F′ such that every element u ∈ U is covered by at least dem(u) sets and at least k elements satisfy the following property: u ∈ U is covered by exactly dem(u) sets of F′. To the best of our knowledge, this problem has not been studied before, and we show that it is NP-complete (even for the case of lines). In fact, the problem turns out to be W[1]-hard in the general setting, when parameterized by k. However, when we restrict the sets to lines and hyperplanes, we obtain FPT algorithms.