1. Faculty of Mathematics and Computer Science, TU Bergakademie Freiberg, Freiberg, Germany
2. Department of Mathematics, International University, VNU-HCMC, Ho Chi Minh City, Vietnam
3. Department of Mathematics, Vietnam National University-Ho Chi Minh City, Ho Chi Minh City, Vietnam
4. Department of Economic Sciences, Indian Institute of Technology, Kanpur, India
5. Department of Mathematics and Statistics, Indian Institute of Technology, Kanpur, India