The following physical model of a vibrating inhomogeneous string (or string w/with beads) by Krein provides mechanical interpretation for the study of continued fractions, see [GK]. The model is one-dimensional, and it arises as a restriction of n-dimensional inverse problems with rotational symmetry.
The string is represented by a non-decreasing positive mass function m(x) on a possibly infinite interval [0, l]. The right end of the string is fixed. The ratio of the forced oscillation to an applied periodic force @ the left end of the string is the function of frequency, called coefficient of dynamic compliance of the string, see [KK] and [I2].
The small vertical vibration of the string is described by the following differential equation:
is the density of the string, possibly including atomic masses. One can express the coefficient in terms of the fundamental solution of the ODE:
A fundamental theorem of Krein and Kac, see [KK] & also [I2], essentially states that an analytic function H() is the coefficient of dynamic compliance of a string if and only if the function
is an analytic automorphism of the right half-plane C+, that is real on the real line. The theorem of Herglotz completely characterizes such functions by the following integral representation:
is a positive measure of bounded variation on the half-line .
Exercise(**). Use the theorem above, Fourier transform and a change of variables to characterize the set of Dirichlet-to-Neumann maps for a unit disc with rotation independent conductivity.