1. Corresponding author. The Institute of Electrical Engineering, Yanshan University, Qinhuangdao, 066004, China, and The Institute of Mathematics, Würzburg University, Würzburg, 97074, Germany.
2. The Institute of Mathematics, Würzburg University, Würzburg, 97074, Germany.
3. The Institute of Electrical Engineering, Yanshan University, Qinhuangdao, 066004, China.