[1] Govt Coll Univ, Abdus Salam Sch Math Sci, Lahore, Pakistan
[2] Fed Polytech, Sch Sci & Technol, PMB 0231,Off Dass Rd, Bauchi, Nigeria
[3] Tech Univ Iasi, Dept Theoret Mech, R-6600 Iasi, Romania
[4] Univ Teknol, Fac Sci, Dept Math Sci, Skudai 81310, Malaysia