**Author details**

Vladimir A. Testov Mathematics Department, Vologda State University, Vologda, Russia

\*Address all correspondence to: vladafan@inbox.ru

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
