1. Babuška, J., Kellog, R. B., Pitkäranta, J.: Direct and inverse error estimates for finite elements with mesh refinements. Num. Math.33, 447–471 (1979).
2. Bank, R. E., Dupont, T.: An optimal order process for solving elliptic finite element equations. Math. Comp.36, 35–51 (1981).
3. Braess, D., Hackbusch, W.: A new convergence proof for the multigrid method including theV-cycle. Preprint, Mathematisches Institut der Ruhr-Universität Bochum (1982).
4. Hackbusch, W.: On the convergence of multi-grid iterations. Beiträge Num. Math.9, 213–329 (1981).
5. Kondrat'ev, V. A.: Boundary problems for elliptic equations with conical or angular points. Trans. Moscow Math. Soc.16, 227–313 (1967).