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The current commutation relations^1 and partially conserved axial-vector current (PCAC) assumption^2, 3 allow the calculation of the matrix elements for emission and absorption of any number of soft pions^4 and, therefore, in particular, determine the scattering length of a pion on any target particle. In this note we give a simple formula for pion scattering on any particle but a pion^5, and then extend this result to the more difficult case of pion-pion scattering.
Steven Weinberg (Mon,) studied this question.