1. School of Mathematical Sciences , Harbin Engineering University , Harbin , 150001 , P. R. China
2. Faculty of Applied Mathematics , AGH University of Science and Technology , al. Mickiewicza 30, 30-059 Kraków , Poland ; and Institute of Mathematics “Simion Stoilow” of the Romanian Academy, P.O. Box 1-764, 014700 Bucharest, Romania