1. Associate Professor, Dept. of Mathematics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India. (corresponding author).
2. Senior Research Scholar, Dept. of Mathematics, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India.
3. Distinguished Professor, Regents Professor and Caroline and William N. Lehrer Distinguished Chair, Water Engineering, Dept. of Biological and Agriculture Engineering and Zachry Dept. of Civil Engineering, Texas A&M Univ., College Station, TX 77843-2117.