Affiliation: | 1. School of Mathematics & Information Science, Shangqiu Normal University, Shangqiu, 476000, P. R. China 2. School of Mathematical Sciences, Kaili University, Kaili, 556011, P. R. China 3. Southeast Guizhou Vocational College of Technology for Nationalities, Kaili, 556011, P. R. China 4. College of Mathematics and Statistics, Guizhou University of Finance and Economics, Guiyang, 550004, P. R. China
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