An integral representation is shown for the matrix element between the vacuum state and a one-particle state of the T-product of two field operators (two-body Bethe-Salpeter amplitudes). It is a generalization of the representation introduced by Wick. The derivation makes use of the Lorentz invariance, microcausality, asymptotic conditions, spectral conditions plus the stability condition.
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M. Idà (1960) studied this question.