Affiliation: | 1. Department of Mathematics, University of West Bohemia, P.O. Box 314, 306 14 Pilsen, Czech Republic;2. Institute of Theoretical Computer Science (ITI), Charles University, Prague, Czech Republic;3. Department of Mathematics, Beijing Institute of Technology, Beijing, 100081, PR China;4. Department of Mathematics, Jiangxi Normal University, Nanchang 330022, PR China;5. Department of Mathematics, College of Science and Technology, Nihon University, Tokyo 101-8308, Japan |