María A. Hernández Cifre
Departamento de Matemáticas
Universidad de Murcia
Campus de Espinardo, 30100-Murcia, Spain
mhcifre@um.es

Eugenia Saorín Gómez
ALTA - Institute for Algebra, Geometry, Topology and their Applications
Department of Mathematics and Computer Science
University of Bremen
28359-Bremen, Germany
esaoring@uni-bremen.de