This work was completed while the first author was visiting the Institute of Mathematics and Statistics at the University of Goias, whose warm hospitality is gratefully acknowledged. The authors thank Dr. Benar Fux Svaiter for bringing this problem to our attention, for sending us a copy of [11] and [12] as well as for the criticism concerning the first version of this manuscript. The authors also thank the two anonymous referees and the editor for their valuable comments.
M. Marques Alves is partially supported by CNPq grants 305414/2011-9 and 479729/2011-5 and PRONEX-Optimization. J.G. Melo is Partially supported by Procad/NF and by PRONEX-Optimization.