“…Later the same quantity b(Λ) was studied by other authors, for instance [8], [9], and used in several papers; we cite [7], [10] and [15]; for an extensive study and generalizations see [13], [12], [1] (see [14] for a complete list of references); in particular the author of [15] discovers an alternative equivalent definition, which will be stated and used later (see Definition 2.1).…”