Author: Nikolay Alexeevich  Klyuzhev. Candidate of sciences in technics, professor, the Pskov branch of the St. Petersburg State Engineering-Economical University, Pskov, Russia
E-mail: klujev-33@yandex.ru
Phone: 577-044

Author: Maria Milanova Stamatova. Student, Plovdiv University “Paisii Hilendarski”, Department of Mathematics and Informatics,  Plovdiv, Bulgaria
E-mail: matias123tz@abv.bg