Nikolai Nikolaevich Dobrovol’sky
Tula State University, Tula State L. N. Tolstoy Pedagogical University, Tula.
Russian Federation

candidate of physical and mathematical sciences, assistant of the department of applied mathematics and computer science, Tula State University; associate Professor of algebra, mathematical analysis and geometry, Tula State L. N. Tolstoy Pedagogical University, Tula.

Nikolai Mihailovich Dobrovol’skii
Tula State L. N. Tolstoy Pedagogical University, Tula.
Russian Federation

doctor of physical and mathematical sciences, professor, head of the department of algebra, mathematical analysis and geometry

Irina Yuryevna Rebrova
Tula State L. N. Tolstoy Pedagogical University, Tula.
Russian Federation

candidate of physical and mathematical Sciences, associate professor, dean of the faculty of mathematics, physics and computer science, 

Alexandr Valer’evich Rodionov
Tula State L. N. Tolstoy Pedagogical University, Tula.
Russian Federation

senior lecturer of the Department of algebra, mathematical analysis and geometry