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Let $I$ be an ideal of a commutative ring $R$. $M$ be an $R$-module. In Local cohomology: an algebraic introduction with geometric applications of Brodmann M. P., Sharp R. Y we have $$D_I(M)=\mathop {\lim }\limits_{\begin{subarray}{c} \longrightarrow \\ \end{subarray}} Hom_R(I^{n},M) $$ called ideal transform of $M$ respect to $I$ or $I-transform$ of $M$. I heard someone talked about Deligne's formula but i can not find it. Can anyone help me to find it? I think that, it is $$D_I(M)=\mathop {\lim }\limits_{\begin{subarray}{c} \longrightarrow \\ \end{subarray}} M_a $$ where $M_a$ is the localization of $M$ with respect to multiplicative systems of powers of a single $M$-regular element $a$ in $I$.

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up vote 3 down vote accepted

I've seen the following called Deligne's formula (it is in Hartshorne, Algebraic Geometry, Chapter III, Exercise 3.7), and I think essentially answers your question.

It says that if $Z = V(I)$ (the closed subset of $X = \text{Spec} R$) and $U = X \setminus Z$, then $$ \Gamma(U, \widetilde{M}) = \lim_{\to} \text{ Hom}_R(I^n, M) $$

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