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108 SANDRA RICARDO AND WITOLD RESPONDEK
(S. Ricardo) Department of Mathematics School of Sciences and Technology University of Tra´s-os-Montes e Alto Douro 5001-801 Vila Real
Portugal
and Institute of Systems and Robotics University of Coimbra - Po´lo II Pinhal de Marrocos
3030-290 Coimbra
Portugal.
E-mail address: sricardo@utad.pt
(W. Respondek) INSA–Rouen Laboratoire de Mathe´mathiques 76801 Saint-Etienne-du-Rouvray France.
E-mail address: Witold.Respondek@insa-rouen.fr
