# Spherical mean

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The spherical mean of a function $u$ (shown in red) is the average of the values $u(y)$ (top, in blue) with $y$ on a "sphere" of given radius around a given point (bottom, in blue).

In mathematics, the spherical mean of a function around a point is the average of all values of that function on a sphere of given radius centered at that point.

## Definition

Consider an open set U in the Euclidean space Rn and a continuous function u defined on U with real or complex values. Let x be a point in U and r > 0 be such that the closed ball B(xr) of center x and radius r is contained in U. The spherical mean over the sphere of radius r centered at x is defined as

$\frac{1}{\omega_{n-1}(r)}\int\limits_{\partial B(x, r)} \! u(y) \, \mathrm{d} S(y)$

where ∂B(xr) is the (n−1)-sphere forming the boundary of B(xr), dS denotes integration with respect to spherical measure and ωn−1(r) is the "surface area" of this (n−1)-sphere.

Equivalently, the spherical mean is given by

$\frac{1}{\omega_{n-1}}\int\limits_{\|y\|=1} \! u(x+ry) \, \mathrm{d}S(y)$

where ωn−1 is the area of the (n−1)-sphere of radius 1.

The spherical mean is often denoted as

$\int\limits_{\partial B(x, r)}\!\!\!\!\!\!\!\!\!\!\!-\, u(y) \, \mathrm{d} S(y).$

The spherical mean is also defined for Riemannian manifolds in a natural manner.

## Properties and uses

• From the continuity of $u$ it follows that the function
$r\to \int\limits_{\partial B(x, r)}\!\!\!\!\!\!\!\!\!\!\!-\, u(y) \,\mathrm{d}S(y)$
is continuous, and its limit as $r\to 0$ is $u(x).$
• Spherical means are used in finding the solution of the wave equation $u_{tt}=c^2\Delta u$ for $t>0$ with prescribed boundary conditions at $t=0.$
• If $U$ is an open set in $\mathbb R^n$ and $u$ is a C2 function defined on $U$, then $u$ is harmonic if and only if for all $x$ in $U$ and all $r>0$ such that the closed ball $B(x, r)$ is contained in $U$ one has
$u(x)=\int\limits_{\partial B(x, r)}\!\!\!\!\!\!\!\!\!\!\!-\, u(y) \,\mathrm{d}S(y).$
This result can be used to prove the maximum principle for harmonic functions.

## References

• Evans, Lawrence C. (1998). Partial differential equations. American Mathematical Society. ISBN 0-8218-0772-2.
• Sabelfeld, K. K.; Shalimova, I. A. (1997). Spherical means for PDEs. VSP. ISBN 90-6764-211-8.
• Sunada, Toshikazu (1981). "Spherical means and geodesic chains in a Riemannian manifold". Trans. A.M.S. 267: 483–501.