 Solution

Put $r_0=|{\mathbf{x}}_0|$ and ${\mathbf{x}}_0^{\ast }=a^2{\mathbf{x}}_0/r_0^2$ for $0<r_0<a$. The Dirichlet Green function inside a sphere isOn $|\mathbf{x}|=a$, the distance identity $|\mathbf{x}-{\mathbf{x}}_0^{\ast }|=(a/r_0)|\mathbf{x}-{\mathbf{x}}_0|$ cancels the two terms. The inverse point lies outside the sphere, so it introduces no interior source. This weighted inversion is the Kelvin transform. For a central source, take the limit to get $G_B(\mathbf{x};0)=-(4\pi {)}^{-1}(1/|\mathbf{x}|-1/a)$.