Three spheres inequality

In mathematics, the three spheres inequality bounds the $L^2$ norm of a harmonic function on a given sphere in terms of the $L^2$ norm of this function on two spheres, one with bigger radius and one with smaller radius.

Statement of the three spheres inequality

Let $u$ be an harmonic function on ${\mathbb{ℝ}}^n$. Then for all $0<r_1<r<r_2$ one has

$\Vert u{\Vert }_{L^2(S_r)}\le \Vert u{\Vert }_{L^2(S_{r_1})}^{\alpha }\Vert u{\Vert }_{L^2(S_{r_2})}^{1-\alpha }$

where $S_{\rho }:=\{x\in {\mathbb{ℝ}}^n:|x|=\rho \}$ for $\rho >0$ is the sphere of radius $\rho$ centred at the origin and where

$\alpha :=\frac{\log (r_2/r)}{\log (r_2/r_1)}.$

Here we use the following normalisation for the $L^2$ norm:

$\Vert u{\Vert }_{L^2(S_{\rho })}^2:={\rho }^{1-n}\underset{{\mathbb{𝕊}}^{n-1}}{\int }|u(\rho \hat{x}){|}^{2}d\sigma (\hat{x}).$

References

• Korevaar, J.; Meyers, J. L. H. (1994), "Logarithmic convexity for supremum norms of harmonic functions", Bull. London Math. Soc. 26 (4): 353–362, doi:10.1112/blms/26.4.353