On the parameterized complexity of Compact Set Packing

The Set Packing problem is, given a collection of sets $\mathcal{S}$ over a ground set $\mathcal{U}$, to find a maximum collection of sets that are pairwise disjoint. The problem is among the most fundamental NP-hard optimization problems that have been studied extensively in various computational regimes. The focus of this work is on parameterized complexity, Parameterized Set Packing (PSP): Given $r \in {\mathbb N}$, is there a collection $\mathcal{S}' \subseteq \mathcal{S}: |\mathcal{S}'| = r$ such that the sets in $\mathcal{S}'$ are pairwise disjoint? Unfortunately, the problem is not fixed parameter tractable unless $\mathsf{W[1] = FPT}$, and, in fact, an "enumeration" running time of $|\mathcal{S}|^{\Omega(r)}$ is required unless the exponential time hypothesis (ETH) fails. This paper is a quest for tractable instances of Set Packing from parameterized complexity perspectives. We say that the input $(\mathcal{U},\mathcal{S})$ is "compact" if $|\mathcal{U}| = f(r)\cdot\Theta(\textsf{poly}( \log |\mathcal{S}|))$, for some $f(r) \ge r$. In the Compact Set Packing problem, we are given a compact instance of PSP. In this direction, we present a "dichotomy" result of PSP: When $|\mathcal{U}| = f(r)\cdot o(\log |\mathcal{S}|)$, PSP is in $\textsf{FPT}$, while for $|\mathcal{U}| = r\cdot\Theta(\log (|\mathcal{S}|))$, the problem is $W[1]$-hard; moreover, assuming ETH, Compact PSP does not even admit $|\mathcal{S}|^{o(r/\log r)}$ time algorithm. Although certain results in the literature imply hardness of compact versions of related problems such as Set $r$-Covering and Exact $r$-Covering, these constructions fail to extend to Compact PSP. A novel contribution of our work is the identification and construction of a gadget, which we call Compatible Intersecting Set System pair, that is crucial in obtaining the hardness result for Compact PSP.