Identifying the Minimal Transversals of a Hypergraph and Related Problems

The paper considers two decision problems on hypergraphs, hypergraph saturation and recognition of the transversal hypergraph, and discusses their significance for several search problems in applied computer science. Hypergraph saturation (i.e., given a hypergraph $\mathcal{H}$, decide if every subset of vertices is contained in or contains some edge of $\mathcal{H}$) is shown to be co-${\bf NP}$-complete. A certain subproblem of hypergraph saturation, the saturation of simple hypergraphs (i.e., Sperner families), is shown to be under polynomial transformation equivalent to transversal hypergraph recognition; i.e., given two hypergraphs $\mathcal{H}_{1}$, $\mathcal{H}_{2}$, decide if the sets in $\mathcal{H}_{2}$ are all the minimal transversals of $\mathcal{H}_{1}$. The complexity of the search problem related to the recognition of the transversal hypergraph, the computation of the transversal hypergraph, is an open problem. This task needs time exponential in the input size; it is unknown whether an output-polynomial algorithm exists. For several important subcases (for instance, if an upper or lower bound is imposed on the edge size or for acyclic hypergraphs) output-polynomial algorithms are presented. Computing or recognizing the minimal transversals of a hypergraph is a frequent problem in practice, which is pointed out by identifying important applications in database theory, Boolean switching theory, logic, and artificial intelligence (AI), particularly in model-based diagnosis.