Scattering and production amplitudes involving scalar resonances are known, according to Watsons theorem, to share the same phase $delta(s)$. We show that, at low energies, the production amplitude is fully determined by the combination of $delta(s)$ with another phase $omega(s)$, which describes intermediate two-meson propagation and is theoretically unambiguous. Our main result is a simple and almost model independent expression, which generalizes the usual $K$-matrix unitarization procedure and is suited to be used in analyses of production data involving scalar resonances.
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