The Stacks Project

Tag 08XT

Example 45.3.6. Let $R$ be a reduced ring. Let $\mathfrak p \subset R$ be a minimal prime so that $K = R_\mathfrak p$ is a field (Algebra, Lemma 10.24.1). Then $K$ is an injective $R$-module. Namely, we have $\mathop{\rm Hom}\nolimits_R(M, K) = \mathop{\rm Hom}\nolimits_K(M_\mathfrak p, K)$ for any $R$-module $M$. Since localization is an exact functor and taking duals is an exact functor on $K$-vector spaces we conclude $\mathop{\rm Hom}\nolimits_R(-, K)$ is an exact functor, i.e., $K$ is an injective $R$-module.

The code snippet corresponding to this tag is a part of the file dualizing.tex and is located in lines 236–246 (see updates for more information).

\begin{example}
\label{example-reduced-ring-injective}
Let $R$ be a reduced ring. Let $\mathfrak p \subset R$ be a minimal prime
so that $K = R_\mathfrak p$ is a field
(Algebra, Lemma \ref{algebra-lemma-minimal-prime-reduced-ring}).
Then $K$ is an injective $R$-module. Namely, we have
$\Hom_R(M, K) = \Hom_K(M_\mathfrak p, K)$ for any $R$-module
$M$. Since localization is an exact functor and taking duals is
an exact functor on $K$-vector spaces we conclude $\Hom_R(-, K)$
is an exact functor, i.e., $K$ is an injective $R$-module.
\end{example}

There are no comments yet for this tag.

Add a comment on tag 08XT

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the lower-right corner).