Repulsive electromagnetic stresses in the Casimir piston
  This manuscript has been withdrawn, since the authors have detected numerical inaccuracies that invalidate their main results concerning the existence of repulsive Casimir forces within a rectangular piston. Formulas presented in the manuscript keep still valid. The authors deeply apologize for this mistake. 
L. Garc\'ia, L. E. Gonz\'alez, M. Lomnitz, C. Villarreal