  In semiclassical theories for chaotic systems such as Gutzwiller's periodic
orbit theory the energy eigenvalues and resonances are obtained as poles of a
non-convergent series g(w)=sum_n A_n exp(i s_n w). We present a general method
for the analytic continuation of such a non-convergent series by harmonic
inversion of the "time" signal, which is the Fourier transform of g(w). We
demonstrate the general applicability and accuracy of the method on two
different systems with completely different properties: the Riemann zeta
function and the three disk scattering system. The Riemann zeta function serves
as a mathematical model for a bound system. We demonstrate that the method of
harmonic inversion by filter-diagonalization yields several thousand zeros of
the zeta function to about 12 digit precision as eigenvalues of small matrices.
However, the method is not restricted to bound and ergodic systems, and does
not require the knowledge of the mean staircase function, i.e., the Weyl term
in dynamical systems, which is a prerequisite in many semiclassical
quantization conditions. It can therefore be applied to open systems as well.
We demonstrate this on the three disk scattering system, as a physical example.
The general applicability of the method is emphasized by the fact that one does
not have to resort a symbolic dynamics, which is, in turn, the basic
requirement for the application of cycle expansion techniques.
