Affiliation: | (1) Department of Applied Mathematics, Kazan University, Kazan 420008, Russia, RU;(2) Department of Mathematics Education, Chungbuk National University, Cheongju 361-763, Korea (e-mail: shkum@cbucc.chungbuk.ac.kr), KR;(3) Department of Applied Mathematics, Pukyong National University, Pusan 608-737, Korea, KR |