The Stacks project

Lemma 19.11.4. Let $\mathcal{A}$ be a Grothendieck abelian category with generator $U$.

  1. If $|M| \leq \kappa $, then $M$ is the quotient of a direct sum of at most $\kappa $ copies of $U$.

  2. For every cardinal $\kappa $ the isomorphism classes of objects $M$ with $|M| \leq \kappa $ form a set.

Proof. For (1) choose for every proper subobject $M' \subset M$ a morphism $\varphi _{M'} : U \to M$ whose image is not contained in $M'$. Then $\bigoplus _{M' \subset M} \varphi _{M'} : \bigoplus _{M' \subset M} U \to M$ is surjective. It is clear that (1) implies (2). $\square$

Comments (2)

Comment #9496 by on

I was a little confused by the way (2) is phrased. At the beginning I understood that what was being asserted is that for each cardinal there is an object in with . Maybe it's me alone, but I would phrase it as "the collection of isomorphisms classes of objects with is a set" or something like that.

There are also:

  • 10 comment(s) on Section 19.11: Injectives in Grothendieck categories

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