1. School of Mathematics, Hangzhou Normal University, Hangzhou 311121, China
2. Key Laboratory of Cryptography of Zhejiang Province, Hangzhou Normal University, Hangzhou 311121, China
3. Department of Mathematics, Faculty of Sciences, Bartın University, Kutlubey Campus, 74100 Bartın, Turkey
4. Department of Mathematics and Statistics, Texas Tech University, Lubbock TX 79409-1042, USA