1. Department of Applied Mathematics, National Hsinchu University of Education, Hsinchu, Taiwan
2. Department of Mathematical Sciences, Korea Advanced Institute of Science and Technology, Daejeon, Korea
3. Division of Mathematical Models, National Institute for Mathematical Sciences, Daejeon, Korea
4. Department of Mathematics, Center for Advanced Study in Theoretical Sciences (CASTS), National Center for Theoretical Sciences (NCTS), National Taiwan University, Taipei, Taiwan
5. Department of Mathematics, Pennsylvania State University, University Park, Penn., U.S.A.