S-wave pi-pi phase shifts and scattering lengths. The model-independent   analysis
  The model-independent analysis of the S- and P-wave pi-pi phase shifts was carried out. This analysis was based on the using of the Roy equations only and all available experimental data from the threshold up to dipion mass 1GeV. The S-wave lengths obtained obviously confirm the standard ChPT version. Moreover, additional arguments were found in favor of the ratio of the S-wave phase shifts being independent of energy from the threshold up to 900MeV. 
V.N. Maiorov and O.O. Patarakin