⁹As we shall see, within the realms of finite elasticity, Ericksen [9] has showed that in the absence of body forces, deformations of the form: r = r(R), θ = Θ, z = Z, where (R,Θ,Z) and (r,θ,z) are the coordinates of a material particle in the reference and current configuration respectively, is not possible in some compressible materials.