1. Aharonov, D., Schiffer, M.M., Zalcman, L.: Potato kugel. Isr. J. Math.40, 331?339 (1981)
2. Armitage, D.H., Goldstein, M.: The volume mean value property of harmonic functions. Complex Variables, Theory Appl.13, 185?193 (1990)
3. Armitage, D.H., Goldstein, M.: Characterizations of balls and strips via harmonic functions. In: Fuglede, B. et al. (eds.) Approximation by solutions of partial differential equations, pp. 1?9. London New York: Kluwer 1992
4. Bliedtner, J., Hansen, W.: Potential theory?An analytic and probabilistic approach to balayage (Universitext). Berlin Heidelberg New York: Springer 1986
5. Br�del, W.: Funktionen mit Gaussischen Mittelwerteigenschaften f�r konvexe Kurven und Bereiche. Dtsch. Math.4, 3?15 (1939)