Some of you have remarked small mistakes. Here it is the correct version.
Best regards
Ramon
--
=========================================================================
Ramon Puigjaner
Universitat de les Illes Balears
Departament de Ciencies Matematiques i Informatica
07122 PALMA (Spain)
Phone: +34-971173288 Fax: +34-971173003
e-mail: putxi@uib.es
=========================================================================