proportional to the displacement (i.e., if Hooke’s law is satisfied), the equilibrium
displacement x is proportional to g; 9 is then proportional to g:, and the medium is
linear. However, if the restraining force is a nonlinear function of the displacement, the
equilibrium displacement x and the polarization density 9 are nonlinear functions of
8 and, consequently, the medium is nonlinear. The time dynamics of an anharmonic
oscillator model describing a dielectric medium with these features is discussed in Sec.