  The scattering lengths of the pi Sigma systems are key quantities in
understanding the structure of the Lambda(1405) resonance and the subthreshold
extrapolation of the barK N interaction. We demonstrate that the pi Sigma
scattering lengths can be extracted from the threshold cusp phenomena in the
weak Lambda_c -> pi pi Sigma decays, analogously with Cabibbo's method for
determination of the pi pi scattering length. We show that the substantial cusp
effect should be observed in the spectrum, when the pi Sigma interaction in I=0
is strongly attractive to generate a near-threshold singularity, such as a
bound state or a virtual state.
