1. Department of Primary Education, Hunan Vocational College for Nationalities, Yueyang, Hunan, China
2. School of Mathematics and Statistics, Anhui Normal University, Wuhu, Anhui, China
3. Departamento de Estatística, Análise Matemática e Optimización, Facultade de Matemáticas, Universidade de Santiago de Compostela, Santiago de Compostela, Spain + Instituto de Matemáticas, Universidade de Santiago de Compostela, Santiago de Compostela, Spain