## 19.3 G-modules

We will see later (Differential Graded Algebra, Section 22.17) that the category of modules over an algebra has functorial injective embeddings. The construction is exactly the same as the construction in More on Algebra, Section 15.55.

Lemma 19.3.1. Let $G$ be a topological group. Let $R$ be a ring. The category $\text{Mod}_{R, G}$ of $R\text{-}G$-modules, see Étale Cohomology, Definition 59.57.1, has functorial injective hulls. In particular this holds for the category of discrete $G$-modules.

Proof. By the remark above the lemma the category $\text{Mod}_{R[G]}$ has functorial injective embeddings. Consider the forgetful functor $v : \text{Mod}_{R, G} \to \text{Mod}_{R[G]}$. This functor is fully faithful, transforms injective maps into injective maps and has a right adjoint, namely

$u : M \mapsto u(M) = \{ x \in M \mid \text{stabilizer of }x\text{ is open}\}$

Since $v(M) = 0 \Rightarrow M = 0$ we conclude by Homology, Lemma 12.29.5. $\square$

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