[1] Univ New England, Sch Sci & Technol, Armidale, NSW 2351, Australia
[2] Natl Tsing Hua Univ, Dept Math, Hsinchu 300044, Taiwan
[3] Chang Gung Univ, Dept Nat Sci, Ctr Gen Educ, Taoyuan 333, Taiwan
[4] Mem Univ Newfoundland, Dept Math & Stat, St John, NF A1C 5S7, Canada