1. Department of Mathematics, Anna University-Regional Campus, Coimbatore 641046, India e-mail:
2. Department of Mathematics, Bharathiar University, Coimbatore 641046, India;
3. Department of Mathematics, Sungkyunkwan University, Suwon 440-746, South Korea e-mail:
4. School of Electrical Engineering, Chungbuk National University, 1 Chungdao-ro, Cheongju 362-763, South Korea e-mail:
5. School of Basic Sciences, Indian Institute of Technology Mandi, Mandi 175001, HP, India e-mail: