[1] Sungkyunkwan Univ, Dept Math, Suwon 440746, South Korea
[2] Nanjing Univ Aeronaut & Astronaut, Coll Sci, Nanjing, Jiangsu, Peoples R China
[3] Uludag Univ, Fac Arts & Sci, Dept Math, TR-16059 Bursa, Turkey
[4] Selcuk Univ, Dept Math, Fac Sci, TR-42075 Campus, Konya, Turkey
[5] Univ Split, Fac Sci, HR-21000 Split, Croatia