“…This error estimator essentially consists of the elementwise error of the finite element functions with respect to the strong form of the differential equation and of jumps across inter-element boundaries of that boundary operator which naturally links the strong and weak forms of the differential equation. Some of the results of § §6-8 are completely new, others are generalizations of, and improvements upon, results previously obtained in [4,5,7,8,9,19,25,27,28]. We then trivially have a-l(\\F(u)\\Y-) < ||" -"oik < Q-x(\\F(u)\\Y.)…”