## 19.10 Grothendieck's AB conditions

This and the next few sections are mostly interesting for “big” abelian categories, i.e., those categories listed in Categories, Remark 4.2.2. A good case to keep in mind is the category of sheaves of modules on a ringed site.

Grothendieck proved the existence of injectives in great generality in the paper [Tohoku]. He used the following conditions to single out abelian categories with special properties.

Definition 19.10.1. Let $\mathcal{A}$ be an abelian category. We name some conditions

1. $\mathcal{A}$ has direct sums,

2. $\mathcal{A}$ has AB3 and direct sums are exact,

3. $\mathcal{A}$ has AB3 and filtered colimits are exact.

Here are the dual notions

1. $\mathcal{A}$ has products,

2. $\mathcal{A}$ has AB3* and products are exact,

3. $\mathcal{A}$ has AB3* and filtered limits are exact.

We say an object $U$ of $\mathcal{A}$ is a generator if for every $N \subset M$, $N \not= M$ in $\mathcal{A}$ there exists a morphism $U \to M$ which does not factor through $N$. We say $\mathcal{A}$ is a Grothendieck abelian category if it has AB5 and a generator.

Discussion: A direct sum in an abelian category is a coproduct. If an abelian category has direct sums (i.e., AB3), then it has colimits, see Categories, Lemma 4.14.12. Similarly if $\mathcal{A}$ has AB3* then it has limits, see Categories, Lemma 4.14.11. Exactness of direct sums means the following: given an index set $I$ and short exact sequences

$0 \to A_ i \to B_ i \to C_ i \to 0,\quad i \in I$

in $\mathcal{A}$ then the sequence

$0 \to \bigoplus \nolimits _{i \in I} A_ i \to \bigoplus \nolimits _{i \in I} B_ i \to \bigoplus \nolimits _{i \in I} C_ i \to 0$

is exact as well. Without assuming AB4 it is only true in general that the sequence is exact on the right (i.e., taking direct sums is a right exact functor if direct sums exist). Similarly, exactness of filtered colimits means the following: given a directed set $I$ and a system of short exact sequences

$0 \to A_ i \to B_ i \to C_ i \to 0$

over $I$ in $\mathcal{A}$ then the sequence

$0 \to \mathop{\mathrm{colim}}\nolimits _{i \in I} A_ i \to \mathop{\mathrm{colim}}\nolimits _{i \in I} B_ i \to \mathop{\mathrm{colim}}\nolimits _{i \in I} C_ i \to 0$

is exact as well. Without assuming AB5 it is only true in general that the sequence is exact on the right (i.e., taking colimits is a right exact functor if colimits exist). A similar explanation holds for AB4* and AB5*.

Comment #2315 by Linyuan Liu on

I'm currently reading the Tohoku paper. In the original paper the AB5 condition states that filtered union of sub objects distributes over intersection with another sub object. I've been thinking for several days of why this condition implies exactness of filtered colimits. Is there any references concerning this? Perhaps it's a stupid question...

Comment #2391 by on

This is not a stupid question. First of all, he says AB3 should be satisfied and the thing you say. In particular Grothendieck's AB5 implies the existence of colimits. Now let's for example try to show that colimits over $\mathbf{N}$ are exact if Grothendieck's AB5 holds. So assume given a short exact sequence of systems $0 \to (A_n) \to (B_n) \to (C_n) \to 0$ over $\mathbf{N}$. It is clear that $\colim A_n \to \colim B_n \to \colim C_n \to 0$ is exact by looking at Homs into another object and using the mapping property of $\colim$. OK, now let $K \subset \colim A_n$ be the kernel of the first map. Let $A'_n \subset \colim A_n$ be the image of $A_n$. Observe that $\colim A_n = \sum A'_n$ as the map $\bigoplus A_n \to \colim A_n$ is surjective. Then Grothendieck's axiom AB5 says it suffices to show that $K \cap A'_n = 0$. I claim that $\Ker(A_n \to A'_n) = \bigcup \Ker(A_n \to A_{n + m})$. To see this I suggest you think about the exact sequence and use Grothendieck's axiom AB5 about unions inside the object in the middle. A similar argument shows that the inverse image of $K$ in $A_n$ is $\bigcup \Ker(A_n \to B_{n + m})$. Since the maps $A_{n + m} \to B_{n + m}$ are injective we conclude. Presumably there are shorter proofs as well.

Anyway, it doesn't matter as we will use AB5 as stated throughout the document.

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