The Stacks project

Proof. Let us denote $\text{gr}^ n : \textit{Mod}(\mathcal{A}) \to \textit{Mod}(\mathcal{O})$ the functor in the statement of the lemma. Consider a homomorphism $f : \mathcal{M} \to \mathcal{N}$ of graded $\mathcal{A}$-modules. The kernel and cokernel of $f$ as maps of graded $\mathcal{O}$-modules are additionally endowed with multiplication maps as in Definition 24.4.1. Hence these are also the kernel and cokernel in $\textit{Mod}(\mathcal{A})$. Thus $\textit{Mod}(\mathcal{A})$ is an abelian category and taking kernels and cokernels commutes with $\text{gr}^ n$.

To prove the existence of limits and colimits it is sufficient to prove the existence of products and direct sums, see Categories, Lemmas 4.14.11 and 4.14.12. The same lemmas show that proving the commutation of limits and colimits with $\text{gr}^ n$ follows if $\text{gr}^ n$ commutes with direct sums and products.

Let $\mathcal{M}_ t$, $t \in T$ be a set of graded $\mathcal{A}$-modules. Then we can consider the graded $\mathcal{A}$-module whose degree $n$ term is $\bigoplus _{t \in T} \mathcal{M}_ t^ n$ (with obvious multiplication maps). The reader easily verifies that this is a direct sum in $\textit{Mod}(\mathcal{A})$. Similarly for products.

Observe that $\text{gr}^ n$ is an exact functor for all $n$ and that a complex $\mathcal{M}_1 \to \mathcal{M}_2 \to \mathcal{M}_3$ of $\textit{Mod}(\mathcal{A})$ is exact if and only if $\text{gr}^ n\mathcal{M}_1 \to \text{gr}^ n\mathcal{M}_2 \to \text{gr}^ n\mathcal{M}_3$ is exact in $\textit{Mod}(\mathcal{O})$ for all $n$. Hence we conclude that (3) holds as filtered colimits are exact in $\textit{Mod}(\mathcal{O})$; it is a Grothendieck abelian category, see Cohomology on Sites, Section 21.19. $\square$