ABSTRACT
The averaging technique was later generalized by Kancev and Lazarov [ 1 1 ] to tetrahedral elements , by Goodsell and Whiteman [7] to quadratic elements , by Krizek and Neittaanmaki [ 13] to global estimates and L''''-norm error estimates , by Hlavacek and Krizek [8] , Goodsell and Whiteman [6] to elliptic systems , by Kancev [ 10] to nonlinear elliptic problems, by Thomee, Xu and Zhang [18] to parabolic problems, by Lin and Zhu [ 1 5] to piecewise uniform triangulations and by Levine [14] to quasiuni form triangulations . However, an "optimal" averaging scheme for the gradient of linear elements on fully irregular meshes was up to now an open problem.
