1. Department of Mathematics, Imperial College London, South Kensington, London, SW7 2AZ, United Kingdom, www2.imperial.ac.uk/~dbuck/
2. Department of Mathematics and Information Sciences, Yamaguchi University, 1677-1 Yoshida, Yamaguchi‐shi, Yamaguchi 753-8513, Japan,
3. Department of Mathematics, California State University, Fullerton, Fullerton, CA 92831, USA, http://mathfaculty.fullerton.edu/mrathbun/
4. Department of Mathematics, Saitama University, 255 Shimo‐Okubo, Sakura‐ku, Saitama 338-8570, Japan, http://www.rimath.saitama-u.ac.jp/lab.en/kshimoka/index-e.html