1. Department of Mathematics & Statistics; King Fahd University of Petroleum and Minerals; Dhahran 31261 Saudi Arabia
2. Department of Mathematics, Statistics and Physics; Qatar University; Doha 2713 Qatar
3. DST-NRF Centre of Excellence in Mathematical and Statistical Sciences and Centre for Differential Equations, Continuum Mechanics and Applications; University of the Witwatersrand; Johannesburg Wits 2050 South Africa