Sayed Mahmoud
Department of Mathematical Sciences, Faculty of Science and Technology, Hirosaki University, Hirosaki 036-1861, Japan
Xiaojun Chen
Department of Applied Mathematics, Hong Kong Polytechnic Uniersity, Hung Hom, Kowloon, Hong Kong, China
Robert Jankowski
Faculty of Civil and Environmental Engineering, Gdansk University of Technology, ul. Narutowicza 11/12, 80-952 Gdansk, Poland