On the semi-sub-hypergroups of a hypergroup (Q1175436)

The paper studies properties of the semi-sub-hypergroups (as the author calls the sets closed under the hyperoperation) and the closed sub- hypergroups of hypergroups. The study of subsets generated by a set of elements leads to the definition of correlated elements and non- correlated elements. Properties of these elements are the main object of the paper. An application in geometry appears in Helly's theorem [cf. \textit{W. Prenowitz}, \textit{J. Jantosciak}: Join Geometries (1979; Zbl 0421.52001)].