1. Laboratory of Foundational Aspects of Computer Science, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, 7701, South Africa, margaret.archibald@uct.ac.za
2. The John Knopfmacher Centre for Applicable Analysis and Number Theory, Department of Mathematics, University of the Witwatersrand, P. O. Wits, 2050, Johannesburg, South Africa, arnold.knopfmacher@wits.ac.za
3. Department of Mathematics, University of Haifa, 31905 Haifa, Israel, tmansour@univ.haifa.ac.il