In these excursuses, additional literature on individual points of our topic is considered, special considerations are also presented – historical-geographical, chronological, genealogical. In some places, reasons for my disagreement with the views expressed by other authors are presented.
Some of the relevant issues have been discussed in the literature for so long and so diligently that established points of view have already been outlined. "Discussions" on such issues do not advance our science at all and are very similar to the famous debates between the party of sharp-pointers and the party of blunt-pointers: no one can convince anyone of anything, and the positions on the discussed issues acquire a purely religious meaning, the meaning of symbols of faith.
In such cases, I will simply point out that the question is religious and there is one (blunt-end) belief and another (sharp-end) belief.
Due to the fact that there are very few sources on our topic, and there are many researchers, each episode has been discussed in the literature (starting with N. M. Karamzin) many times. An exhaustive review – who, what and why wrote about each given episode – is not part of my task. I prefer to focus on new literature (mainly from the last 30 years) and then mainly on monographs, turning to individual articles and older monographs only in special cases.