[1] Univ Leicester, Dept Math, Univ Rd, Leicester LE1 7RH, Leics, England
[2] Natl Tech Univ Athens, Sch Appl Math & Phys Sci, Dept Math, Zografos 15780, Greece
[3] Fdn Res & Technol Hellas, Inst Appl & Computat Math, Iraklion 70013, Greece
[4] Univ York, Dept Chem, York YO10 5DD, N Yorkshire, England
[5] Durham Univ Lower Mountjoy, Dept Engn, South Rd, Durham DH1 3LE, England
[6] Sch Math Sci, Univ Pk, Nottingham NG7 2RD, England