Green's function in the interior of penetrable bodies with inhomogeneous compressibility by
sources placed inside them is evaluated through a Schwinger–Lippmann volume integral
equation. In the case of a radial inhomogeneous sphere, the radial part of the unknown
Green's function can be expanded in a double Dini's series, which allows analytical
evaluation of the involved cumbersome integrals. The simple case treated here can be
extended to more difficult situations involving inhomogeneous density as well as to the …