1. A. Boukricha, Das Picard-Prinzip und verwandte Fragen bei Störung von harmonischen Räumen, Math. Ann. 239 (1979), 247–270.
2. C. Constantinescu A. Cornea, “Potential theory on harmonic spaces,” Springer Verlag, Berlin — Heidelberg — New York, 1972.
3. A. Cornea, R. Wittmann, An approximation theorem for cones of potentials. Analysis 3 (1983), 317–335.
4. B. Franchi, E. Laconelli, Hölder regularity theorem for a class of linear non-uniformly elliptic operators with measurable coefficients, Ann. Sc. Norm. Super. Pisa Serie IV 10 (1983), 523–541.
5. W. Hansen, Kernels on harmonic spaces, (unpublished) (1979).