Institution: | a Department of Applied Mathematics, Beijing University of Technology, 100 Ping Le Yuan, Chao Yang District, Beijing 100022, PR China b Department of Mathematics, Saga University, 1 Honjo-Machi, Saga 840-8502, Japan c Department of Mathematics, University of Oslo, N-0316 Oslo, Norway d Department of Mathematics, University of Indiana, Bloomington, IN 47405, USA |