1. Departamento de Matemáticas; Universidad de los Andes; Carrera 1 No. 18 A-10 Bogotá Colombia
2. Universidad Antonio Nariño; Calle 58A No. 37-94 Bogotá Colombia
3. School of Science and the Environment; Grenfell Campus; Memorial University of Newfoundland; 20 University Dr Corner Brook NL A2H 5G4 Canada