[1] Univ Ljubljana, Fac Math & Phys, Ljubljana, Slovenia
[2] Univ Maribor, Fac Nat Sci & Math, Maribor, Slovenia
[3] Inst Math Phys & Mech, Ljubljana, Slovenia
[4] Cent China Normal Univ, Fac Math & Stat, Wuhan 430079, Hubei, Peoples R China
[5] Luoyang Normal Univ, Dept Math, Luoyang 471002, Peoples R China