1. Department of Mathematics, Bharathiar University, Coimbatore 641 046, India
2. Department of Mathematics, Sungkyunkwan University, Suwon 440-746, Republic of Korea
3. Department of Applied Mathematics, Kongju National University, Chungcheongnam-do 32588, Republic of Korea
4. Department of Mathematics, Yunnan University, Kunming, Yunnan 650091, China