  We propose a new, simple model-independent method to extract information of
near-threshold resonances, such as complex energies and residues. The method is
based on the observation that the Green's function and the T-matrix can be
represented as the sum of all poles, both bound and resonant poles, in the
complex plane of a variable in which the Green's function and the T-matrix are
single-valued functions. The symmetries of poles, which arise from the
unitarity of the S-matrix, naturally impose the sum to obey the proper
threshold behaviors. The imaginary part of Green's function and the T-matrix
are directly related to observables such as scattering cross sections or
invariant or missing mass distributions of hadron resonances. Thus we can
determine their pole positions and residues by fitting their imaginary part to
observables. We also test the new method by regarding the imaginary part of the
$T$-matrix calculated exactly in a model theory as virtual experimental data.
As a model theory, we take double-channel meson-baryon scatterings in the
chiral unitary model with channels, $\overline{K}N (I=0)$, and $\pi\Sigma
(I=0)$. By fitting the imaginary part of the $T$-matrix calculated in the model
theory by that of the uniformized pole-sum, we obtain the pole positions and
residues. Comparing the obtained results with those of the exact calculation in
the model theory, we conclude that our new method works very well.
