1. Department of Mathematics, Zografou Campus, National Technical University, Athens 15780, Greece
2. Faculty of Applied Mathematics, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
3. Department of Mathematics, University of Craiova, Craiova 200585, Romania, China-Romania Research Center in Applied Mathematics
4. College of Science, Hunan University of Technology and Business, Key Laboratory of Hunan Province for Statistical Learning and Intelligent Computation, Changsha, Hunan 410205, China