Affiliation: | (1) Department of Mathematics, Shantou University, 515 063 Shantou, Guangdong, China;(2) Department of Mathematics, Jia Ying University, 514 015 Meizhou, Guangdong, China;(3) Mathematical Institute of the Serbian Academy of Science, Knez Mihailova 35/I, 11000 Beograd, Serbia |