Shapiro's lemma gives a natural isomorphismIt follows by applying the Hom functor adjunction for a coinduced module to a projective resolution and observing that restriction from to preserves projective modules.
For a -module and a -module ,Evaluation at gives the forward map. If is -linear, its inverse sends to .
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Shapiro's Lemma is a result in the field of mathematics, specifically in the area of algebraic geometry and the theory of sheaves. It relates to the properties of sections of sheaves over open subsets of a topological space, particularly in relation to the notion of extension of sections.