Affiliations: Ji-Cai Liu (corresponding author), Department of Mathematics, Wenzhou University, No. 586 Meiquan Road, Wenzhou 325035, P. R. China, e-mail: jcliu2016@gmail.com; He-Xia Ni, Department of Applied Mathematics, Nanjing Audit University, No. 86 Yushan West Road, Nanjing 211815, P. R. China, e-mail: nihexia@yeah.net