[1] TU Dortmund, Inst Angew Math, D-44227 Dortmund, Germany
[2] Univ Cent Aisa, Bishkek 720001, Kyrgyzstan
[3] UET Taxila, Dept Basic Sci, Taxila, Pakistan
[4] Quaid I Azam Univ, Dept Math, Islamabad, Pakistan
[5] COMSATS Inst Informat Technol, Dept Math, Abbottabad 22060, Pakistan