Bodies for which harmonic functions satisfy the mean value property
Avner Friedman, Walter Littman · Transactions of the American Mathematical Society · 1962
Introduction.Let D be an «-dimensional domain with boundary S, let " be a density function defined on S, and let P be a fixed point in D. As is well known, if S is a sphere and P is its center, then for any harmonic function in 7), continuous in D-\-S, (0.1) u(P)= f u(Q)ß(Q)dSQ, J s