Hitting and harvesting pumpkins
Résumé
The $c$-pumpkin is the graph with two vertices linked by $c \geq 1$ parallel edges. A $c$-pumpkin-model in a graph $G$ is a pair $\{A, B\}$ of disjoint subsets of vertices of $G$, each inducing a connected subgraph of $G$, such that there are at least $c$ edges in $G$ between $A$ and $B$. We focus on hitting and packing $c$-pumpkin-models in a given graph in the realm of approximation algorithms and parameterized algorithms. We give a fixed-parameter tractable (FPT) algorithm running in time $2^{\mathcal{O}(k)} n^{\mathcal{O}(1)}$ deciding, for any fixed $c \geq 1$, whether all $c$-pumpkin-models can be hit by at most $k$ vertices. This generalizes known single-exponential FPT algorithms for Vertex Cover and Feedback Vertex Set, which correspond to the cases $c=1,2$ respectively. Finally, we present an $\mathcal{O}(\log n)$-approximation algorithm for both the problems of hitting all $c$-pumpkin-models with a smallest number of vertices and packing a maximum number of vertex-disjoint $c$-pumpkin-models.