Harmonicity of derivatives of the Newtonian potential
= Harmonicity of derivatives of the Newtonian potential
Since $\nabla^2(1/r)=0$ away from the origin, every constant-coefficient derivative of $1/r$ is harmonic there as well.
= Harmonicity of derivatives of the Newtonian potential
Since $\nabla^2(1/r)=0$ away from the origin, every constant-coefficient derivative of $1/r$ is harmonic there as well.