On the mean-value property of harmonic functions
Myron Goldstein, Wellington H. Ow · Proceedings of the American Mathematical Society · 1971
In this note we show that if the areal mean-value theorem holds for a plane domain (subject to a mild regularity condition) for all integrable harmonic functions, then the domain must be a disk. It is also shown that if a plane domain with finite area has at least two boundary components which are continua then the mean-value property cannot hold for the class of all integrable harmonic functions with single-valued harmonic conjugates.