Strong Hub Sets and Strong Hub Number of Hypergraphs

This paper introduces and investigates the concepts of strong hub sets and the strong hub number in hypergraphs, extending the notion of hub sets defined for graphs. For a hypergraph H = (V, E), a subset S ⊆ V (H) is said to be a strong hub set if, for every pair of distinct vertices u, v ∈ V (H) − S, either u and v are adjacent or there exists a strong S-hyperpath joining them. The minimum cardinality of such a set is called the strong hub number of H, denoted by h∗(H). Fundamental properties of strong hub sets are established, and relationships between the strong hub number and various hypergraph parameters such as the domination number and connectivity are explored. The effects of vertex deletion and weak deletion on h∗(H) are studied, leading to several sharp bounds and recursive characterizations. In particular, it is shown that under weak deletion of a non cut vertex, the strong hub number remains invariant, while deletion of a cut vertex reduces it according to the structure of the resulting components. Special attention is devoted to hypertrees, where structural simplicity allows a precise characterization. This characterization provides an exact formula for h∗(H) in acyclic hypergraphs and establishes the foundational link between connectivity and hub-based path structure in hypertrees.