1. The John Knopfmacher Centre for Applicable Analysis and Number Theory, School of Mathematics, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa, Charlotte.Brennan@wits.ac.za
2. Department of Mathematics, University of Venda, Private bag x5050, Thohoyandou, 0950, South Africa, Simon.Mavhungu@univen.ac.za