1. Group of Analysis and Applied Mathematics, Department of Mathematics, Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam.
2. Department of Mathematics, Wayne State University, Detroit, MI 48202 USA.
3. Group of Analysis and Applied Mathematics, Department of Mathematics, Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam, and Department of Mathematics, Wayne State University, Detroit, MI 48202 USA.