[1] Univ Catania, Dept Math & Comp Sci, I-95125 Catania, Italy
[2] Gyeongsang Natl Univ, Dept Math, Jinju 52828, South Korea
[3] Gyeongsang Natl Univ, Res Inst Nat Sci, Jinju 52828, South Korea
[4] Univ Parma, Dept Math Phys & Comp Sci, Parco Area Sci 53-A, I-43124 Parma, Italy