To understand the works included in this collection, in most cases it is required to get acquainted with any of these sources (the most accessible of which is the third). It would be pointless to duplicate the latter, making explanations whenever the terms and facts contained in them are used in the articles of this collection. (True, many articles in the collection provide the necessary wording, but this cannot replace a systematic presentation.) In many cases, familiarity with topology elements is also necessary. Finally, the present collection, as well as the current research literature in any branch of mathematics, suggests the presence of a well-known general mathematical culture.