If a function 
is harmonic in a sphere, then the value of 
at the center of the sphere is
the arithmetic mean of its value on the surface.
Gauss's Harmonic Function Theorem
Explore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Gauss's Harmonic Function Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/GausssHarmonicFunctionTheorem.html