1. Department of Natural Science in the Center for General Education, Chang Gung University, Guishan, Taoyuan 333, Taiwan, and Community Medicine Research Center, Chang Gung Memorial Hospital, Keelung Branch, Keelung 204, Taiwan, and National Center for Theoretical Sciences, National Taiwan University, Taipei 106, Taiwan.
2. Corresponding author. School of Mathematics and Statistics, Shaanxi Normal University, Xi’an, Shaanxi 710119, China.
3. Department of Mathematics and Statistics, Memorial University of Newfoundland, St. John’s, NL A1C 5S7, Canada.