The Laplace equation is unchanged under a rotation of coordinates, and hence we can expect that a fundamental solution may be obtained among solutions that only depend upon the distance r from the source point. If we choose the volume to be a ball of radius a around the source point, then Gauss's divergence theorem implies that − − 1 = ∭ ∭ V ∇ ∇ ⋅ ⋅ ∇ ∇ u d V = ∬ ∬ S d u d r d S = 4 π π a 2 d u d r | r = a .