Department of Mathematics, College of Science, Yanbian University, Yanji, China
Department of Mathematics, College of Science, Yanbian University, Yanji, China
Copyright © 2015 Xing Jin, Yongjie Piao et al. This is
an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.