[1] Zhengzhou Univ Light Ind, Coll Math & Informat Sci, Zhengzhou 450002, Henan, Peoples R China
[2] King Abdulaziz Univ, Fac Sci, Dept Math, POB 80203, Jeddah 21589, Saudi Arabia
[3] Univ Craiova, Dept Math, Craiova 200585, Romania
[4] Univ Ljubljana, Fac Educ, Ljubljana 1000, Slovenia
[5] Univ Ljubljana, Fac Math & Phys, Ljubljana 1000, Slovenia