Motivated by the need for succinct representations of all “small” transversals (or hitting sets) of a hypergraph of fixed rank, we study the complexity of computing such a representation. Next, the existence of a minimal hitting set of at least a given size arises as a subproblem. We give one algorithm for hypergraphs of any fixed rank, and we largely improve an earlier algorithm (by H. Fernau,...

A classical problem in query optimization is to find the optimal ordering of a set of possibly correlated selections. We provide an abstraction of this problem as a generalization of set cover called pipelined set cover, where the sets are applied sequentially to the elements to be covered and the elements covered at each stage are discarded. We show that several natural heuristics for this NP-...

This article studies crime rates’ response to economic conditions. Using a longitudinal data set covering about 20 countries over the period 1970-2010, we investigate whether crime rates respond asymmetrically to increases in unemployment and recoveries in economic conditions. We find a positive response of crime rates on variation in unemployment rates, but we do not find compelling evidence o...

The number of studies seeking to empirically characterize the reduced-form relationship between a country economic growth and the quantity of various pollutants produced has recently increased significantly. In several cases researchers have found evidence in favor of an inverted-U “environmental Kuznets” curve. In the case of a major greenhouse gas, CO2, however, the evidence is at best mixed....

Let X = {x1, ..., xn} be a set, and let P = {P1, ..., Pm} be a set of subsets of X. The goal is to find the smallest sub-collection of sets R ⊆ P whose union covers all of X. More concretely, we can say that each xi, 1 ≤ i ≤ n represents a “skill”, and each Pj, 1 ≤ j ≤ m represents a “person”. In this case, xi ∈ Pj if person Pj has the skill xi. Then, as an employer, the goal is to find the min...