  The time evolution of an oscillator coupled to an infinite string with a
discontinuous mass density is investigated. It is shown that the equation of
motion of the oscillator leads to a nonlinear characteristic equation due to
the frequency-dependent nature of the point impedance of the string. Then, the
method of perturbation theory is applied to solve the characteristic equation
to first order in perturbation parameter. Finally, response of the oscillator
to an impulse is obtained by means of the Green's function method.
