1. S. Agmon, Multiple Layer Potential and the Dirichlet Problem for Higher Order Elliptic Equations in the Plane, Comm. Pure Appl. Math. 10 (1957) 179?239.
2. A. Cialdea, Teoremi di completezza connessi con equazioni ellittiche di ordine superiore in due variabili in un campo con contorno angoloso, Rend. Circ. Mat. di Palermo (2), 34 (1985) 32?49.
3. G. Fichera, Linear elliptic equations of higher order in two independent variables and singular integral equations, with applications to anisotropic inhomogeneous elasticity, in: Partial Differential Equations and Continuum Mechanics, R. E. Langer (editor), Madison (1961) 55?80.
4. G. Fichera, Generalized biharmonic problem and related eigenvalue problems, in Blanch Anniversary Volume, Aerospace Research Laboratories, Office of Aerospace Research, United States Air Force (1967) 37?44.
5. G. Fichera & L. De Vito, Funzioni analitiche di una variabile complessa, III Ediz. Veschi, Roma (1971).