1. Department of Applied Mathematics, Xi'an University of Technology , Xi'an 710054 , PR China
2. School of Computer Science and Engineering, Xi'an University of Technology , Xi'an 710048 , PR China
3. University of North Carolina at Pembroke , Pembroke, NC 28372 , USA
4. Department of Mathematics, University of Sargodha , Sargodha 40100 , Pakistan