1. College of Big Data, Qingdao Huanghai University, Linghai Road 1145, Qingdao 266427, China
2. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qianwangang Road 579, Qingdao 266590, China
3. School of Mathematics, Shandong University, Jinan 250100, China
4. Faculty of Economics and Business Administration, Sofia University “St. Kl. Ohridski”, 1000 Sofia, Bulgaria