Local error bounds for post-processed finite element calculations

In this paper we produce tight guaranteed bounds for the error in the point wise values of the derivatives of a post-processed finite element solution to a potential flow problem, in which the boundary condition is purely norm al velocity. Our approach has to be modified for the problems with Dirichle t boundary conditions. The aim is to produce a tight envelope of certainty within which the exact value is guaranteed to lie. Our numerical experiment s produce narrow envelopes at interior points and at points close to or on the boundary. Copyright (C) 1999 John Wiley & Sons, Ltd.