1. School of Mathematical and Statistical Sciences; Arizona State University; Tempe AZ 85287 USA
2. Extraordinary Professor; Department of Mathematics and Applied Mathematics, University of Pretoria; Pretoria 0002 South Africa
3. Department of Mathematics and Applied Mathematics; University of Pretoria; Pretoria 0002 South Africa
4. Department of Applied Mathematics and Sciences; Khalifa University of Science and Technology; Abu Dhabi 127788 UAE