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Applicability constraints of the Equivalence Theorem

In this work we study the applicability of the Equivalence Theorem, either for unitary models or within an effective lagrangian approach. There are two types of limitations: the existence of a validity energy window and the use of the lowest order in the electroweak constants. For the first kind, we consider some methods, based on dispersion theory or the large $N$ limit, that allow us to extend the applicability. For the second, we have obtained numerical estimates of the effect of neglecting higher orders in the perturbative expansion.