1. School of Information Science and Engineering, Dalian Polytechnic University, Dalian, Liaoning 116034, China
2. Department of Mathematics, Liaoning Normal University, Dalian, Liaoning 116029, China
3. Huludao No. 6 Senior High School, Huludao, Liaoning 125000, China
4. Center for Studies of Marine Economy and Sustainable Development, Liaoning Normal University, Dalian, Liaoning 116029, China