作者单位:Univ Banja Luka, Fac Nat Sci & Math, Banja Luka 78000, Republika Srpsk, Bosnia & Herceg[1]Univ Banja Luka, Fac Nat Sci & Math, Dept Math & Informat, Banja Luka 78000, Republika Srpsk, Bosnia & Herceg[2]Univ Belgrade, Fac Math, Belgrade 11000, Serbia[3]