1. Dept. of Mathematics, Lucknow Univ., Lucknow-226002, India.
2. Dept. of Mathematics, Banaras Hindu Univ., Varanasi-221005, India.
3. Dept. of Mathematics, Banaras Hindu Univ., Varanasi-221005, India (corresponding author).
4. Dept. of Applied Mathematics, I.S.M., Dhanabad-826004, India.