Set Cover, Set Packing and Hitting Set for Tree Convex and Tree-Like Set Systems

A set system is a collection of subsets of a given finite universe. A tree convex set system has a tree defined on the universe, such that each subset in the system induces a subtree. A circular convex set system has a circular ordering defined on the universe, such that each subset in the system induces a circular arc. A tree-like set system has a tree defined on the system, such that for each element in the universe, all subsets in the system containing this element induce a subtree. A circular-like set system has a circular ordering defined on the system, such that for each element in the universe, all subsets in the system containing this element induce a circular arc. In this paper, we restrict the trees to be stars, combs, triads, respectively, and restrict the set system to be unweighted. We show tractability of Triad Convex Set Cover, Circular-like Set Packing, and Triad-like Hitting Set, intractability of Comb Convex Set Cover and Comb-like Hitting Set. Our results not only complement the known results in literatures, but also rise interesting questions such as which other kind of trees will lead to tractability or intractability results of Set Cover, Set Packing and Hitting Set for tree convex and tree-like set systems.