1. Departamento de Matemáticas; Universidad de los Andes; Carrera 1 No. 18A-10 111711 Bogotá Colombia
2. Mathematics Grenfell Campus; Memorial University of Newfoundland; Corner Brook NL A2H 6P9 Canada
3. Department of Mathematics; Christopher Newport University; 1 Avenue of the Arts Newport News VA 23606 United States of America