1. Departamento de Matemáticas, Universidad del Valle, Calle 13 No 100-00, Cali, Valle del Cauca, Colombia
2. School of Mathematics, University of the Witwatersrand, Private Bag X3, Wits 2050, South Africa
3. Department of Mathematics, Faculty of Sciences, University of Ostrava 30 Dubna 22, 701 03 Ostrava 1, Czechia