- Javier Gutiérrez García
Web: http://www.ehu.es/javiergutierrezgarcia/,
Departament of Mathematics, University of the Basque Country UPV/EHU, Apdo. 644, 48080 Bilbao, Spain.
- Iraide Mardones-Pérez
Departament of Mathematics, University of the Basque Country UPV/EHU,
Apdo. 644, 48080 Bilbao, Spain.
- María Angeles de Prada Vicente
Departament of Mathematics, University of the Basque Country UPV/EHU,
Apdo. 644, 48080 Bilbao, Spain.