1 School of Science and Technology, University of New England Armidale, NSW 2351, Australia
2 Department of Mathematics, National Tsing Hua University Hsinchu 300044, Taiwan
3 Department of Natural Science, Center for General Education Chang Gung University Guishan, Taoyuan 333, Taiwan
4 Department of Mathematics and Statistics Memorial University of Newfoundland St. John’s, NL A1C 5S7, Canada