[1] 183 Fleet St, London EC4A 2BB, England
[2] Univ Lethbridge, Dept Math & Comp Sci, Lethbridge, AB T1K 3M4, Canada
[3] Univ Western Australia, Sch Math & Stat, Ctr Math Symmetry & Computat, Crawley, WA 6009, Australia
[4] Univ Milano Bicocca, Dipartimento Matemat & Applicaz, Via Cozzi 53, I-20125 Milan, Italy
[5] King Abdulaziz Univ, Jeddah 21413, Saudi Arabia