ELEMENTOS FINITOS 6 FINITE ELEMENTS 6 A posteriori error estimates 4 Anisotropic elements 4 Guaranteed upper bound 4 Robustness 4 Singularly perturbed reaction-diffusion problem 4 Vertex-centered finite volume/finite volume element/box method 4 Weighted norms 4 A POSTERIORI ERROR ESTIMATES 3 ANISOTROPIC ELEMENTS 3 CONVECCION-DIFUSION 3 CONVECTION-DIFFUSION 3 DISCONTINUOUS GALERKIN 3 ELEMENTOS ANISOTROPICOS 3 ESTIMADORES DE ERROR A POSTERIORI 3 GALERKIN DISCONTINUO 3 GRADED MESHES 3 MALLAS GRADUADAS 3 NORMAS CON PESOS 3 PROBLEMAS SINGULARMENTE PERTURBADOS 3 REACCION-DIFUSION 3 REACTION-DIFFUSION 3 SINGULARLY PERTURBED PROBLEMS 3 SUPERCERCANIA 3 SUPERCLOSENESS 3 SUPERCONVERGENCE 3 SUPERCONVERGENCIA 3 WEIGHTED NORMS 3