1. School of Mathematics, Statistics and Actuarial Science,University of Kent, Canterbury, UK
2. The Ohio State University, Columbus, OH, USA
3. Department of Mathematics, China University of Mining and Technology—Beijing Campus, Beijing, People’s Republic of China
4. Department of Mathematical Sciences, Loughborough University School of Science, Loughborough, UK
5. Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK