Cross-Symmetric Expansion of $\pi \pi$ Amplitude Near Threshold
  The near-threshold expansion of the $\pi \pi$ amplitude is developed using the crossing-covariant independent variables. The independent threshold parameters entering the real part of the amplitude in an explicitly Lorentz-invariant way are free from restrictions of isotopic and crossing symmetries. Parameters of the expansion of the imaginary part are recovered by the perturbative unitarity relations. 
A.A. Bolokhov (1), T.A. Bolokhov (1), I.S. Manida (1), M.V. Polyakov   (2) and S.G. Sherman (2) ((1) St. Petersburg State Un...