3.7.155. Minimum hitting set cardinality
Denotes that, by reduction to the problem of finding the cardinality of a minimum hitting set, deciding whether a constraint has a solution or not, or getting a sharp lower bound for one of its arguments, was shown to be NP-hard. The cardinality of a minimum hitting set problem can be described as follows: given a collection of subsets of a set , find the minimum cardinality of such that contains at least one element from each subset in .