1. Costabel, M., Stephan E., Boundary integral equations for mixed boundary value problems in polygonal domains and Galerkin approximation, THD-preprint 593, Darmstadt 1981. To appear at Banach Center Publications.
2. Costabel, M. and Stephan E., Curvature terms in the asymptotic expansions for solutions of boundary integral equations on curved polygons, THD-preprint 673 Darmstadt (1982).
3. Costabel, M., Stephan E., Wendland W.L., On boundary integral equations of the first kind for the bi-Laplacian in a ploygonal plane domain, THD-preprint 670, Darmstadt 1982.
4. Folland, G.B., Introduction to Partial Differential Equations, Mathematical Notes 17, Princeton University Press, Princeton, N.J. 1976.
5. Grisvard, P., Boundary value problems in non-smooth domains, Univ. of Maryland, MD20742 Lecture Notes 19(1980).