1. School of Mathematics and Statistics & Chongqing Key Laboratory of Economic and Social Application Statistics , Chongqing Technology and Business University , Chongqing , 400067 , China
2. School of Mathematics and Statistics , Central South University , Changsha , 410083 , China
3. Institute of Mathematics and Informatics , Bulgarian Academy of Sciences , Acad. G. Bontchev Block 8 , Sofia , 1113 , Bulgaria