An infinite class of chiral-invariant pion-nucleon Lagrange functions is discussed. Each member of this class is shown to be equivalent, under canonical transformation, to the one in which the commutator of the axial current with the meson field is an isotopic spin scalar. If the chiral symmetry is broken in this special canonical frame in a manner that ensures the partial conservation of the axial current, then the theory is unique.
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Lowell S. Brown (1967) studied this question.
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