  New formulae for the resonant scattering and the production amplitudes near
an inelastic threshold are derived. It is shown that the Flatte formula,
frequently used in experimental analyses, is not sufficiently accurate. Its
application to data analysis can lead to a substantial distortion of the
effective mass spectra and of the resonance pole positions.
  A unitary parameterization, satisfying a generalized Watson theorem for the
production amplitudes, is proposed. It can be easily applied to study
production processes, multichannel meson-meson interactions and the resonance
properties, including among others the scalar resonances a0(980) and f_0(980).
