“…These and some other properties of the extended Hilbert scale are considered in Section 2 of this paper; they are proved in Section 3. Note that the above interpolation and interpolational properties of Hilbert scales are studied in articles [4,5,16,18,19,35,36,38,47,59,63] (see also monographs [39, Chapter 1], [51,Section 1.1], and [68,Chapters 15 and 30]). Among them, of fundamental importance for our investigation is Ovchinnikov's result [59,Theorem 11.4.1] on an explicit description (with respect to equivalence of norms) of all Hilbert spaces that are interpolation ones between arbitrarily chosen compatible Hilbert spaces.…”