[1] Comenius Univ, Fac Math Phys & Informat, Dept Math Anal & Numer Math, Bratislava 84248, Slovakia
[2] Guizhou Univ, Dept Math, Guiyang 550025, Guizhou, Peoples R China
[3] Slovak Acad Sci, Math Inst, Bratislava 81473, Slovakia
[4] Xiangtan Univ, Dept Math, Xiangtan 411105, Hunan, Peoples R China