1. Department of Applied Mathematics, Hong Kong Polytechnic University, Hung Hom Hong Kong, P. R. China
2. Department of Mathematics, Kano University of Science and Technology, Wudil, Nigeria
3. School of Nursing, Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China
4. Division of Biostatistics, JC School of Public Health and Primary Care, Chinese University of Hong Kong, Hong Kong, P. R. China
5. Department of Mathematical Sciences, Bayero University, Kano, Nigeria
6. Collage of Health Sciences, Bayero University, Kano, Nigeria