  We consider the minimum vertex cover problem in hypergraphs in which every
hyperedge has size k (also known as minimum hitting set problem, or minimum set
cover with element frequency k). Simple algorithms exist that provide
k-approximations, and this is believed to be the best possible approximation
achievable in polynomial time. We show how to exploit density and regularity
properties of the input hypergraph to break this barrier. In particular, we
provide a randomized polynomial-time algorithm with approximation factor k/(1
+(k-1)d/(k Delta)), where d and Delta are the average and maximum degree,
respectively, and Delta must be Omega(n^{k-1}/log n). The proposed algorithm
generalizes the recursive sampling technique of Imamura and Iwama (SODA'05) for
vertex cover in dense graphs. As a corollary, we obtain an approximation factor
k/(2-1/k) for subdense regular hypergraphs, which is shown to be the best
possible under the unique games conjecture.
