Localization of Module Homomorphism is Exact
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Theorem
Let $A$ be a commutative ring with unity.
Let $S \subseteq A$ be a multiplicatively closed subset.
Let $S^{-1}A$ be the localization of $A$ at $S$.
Let:
- $M_1 \stackrel {f_1} \longrightarrow M_2 \stackrel {f_2} \longrightarrow M_3$
be an exact sequence of $A$-modules.
For $i=1,2,3$, let $\struct { S^{-1}M_i, \iota_i}$ be the localization of $M_i$ at $S$.
For $i=1,2$, let:
- $S^{-1}f_i : S^{-1}M_i \to S^{-1}M_{i+1}$
be the unique $S^{-1}A$-homomorphism such that:
- $\iota_{i+1} \circ f_i = \paren {S^{-1}f_i} \circ \iota_i$
Then:
- $S^{-1}M_1 \stackrel {S^{-1}f_1} \longrightarrow S^{-1}M_2 \stackrel {S^{-1}f_2} \longrightarrow S^{-1}M_3$
is an exact sequence of $S^{-1}A$-modules.
Proof
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Also known as
The operation $S^{-1}$ is called exact.
Sources
- 1969: M.F. Atiyah and I.G. MacDonald: Introduction to Commutative Algebra: Chapter $3$: Rings and Modules of Fractions