A category $C$ is well-powered if every object has a small poset of subobjects.
Assuming that by ‘subobject’ we mean (an equivalence class of) monomorphisms, this means that for every object $X$, the (generally large) preordered set of monomorphisms with codomain $X$ is equivalent to a small poset, or equivalently that this preordered set is essentially small. Variations exist that use notions of subobject other than monomorphisms.
If $C^{op}$ is well-powered, we say that $C$ is well-copowered (although “cowell-powered” is also common).
A well-powered category with binary products is always locally small, since morphisms $f: A \to B$ can be identified with particular subobjects of $A \times B$ (their graphs).
Conversely, any locally small category with a subobject classifier must obviously be well-powered. In particular, a topos is locally small if and only if it is well-powered.
There are interesting conditions and applications of the preorder on the sets of subobjects in well-powered categories, cf. e.g. property sup.
Every Grothendieck topos (indeed, every locally small elementary topos) is well-powered (by the existence of a subobject classifier and the smallness of hom sets).
More generally, every locally presentable category is well-powered, since it is a full reflective subcategory of a presheaf topos, so its subobject lattices are subsets of those of the latter.
Indeed, every category $C$ with a small dense subcategory $A$ is well-powered, since the restricted Yoneda embedding $C \to [A^{op},Set]$ is then fully faithful and preserves monomorphisms, so it embeds the subobject posets of $C$ as sub-posets of those of $[A^{op},Set]$. This includes in particular all accessible categories.
On the other hand, a locally small category with a strong generator can fail to be well-powered; a counterexample is here.
Every locally presentable category, indeed every accessible category with pushouts, is well-copowered. This is shown in Adamek-Rosicky, Proposition 1.57 and Theorem 2.49. Whether this is true for all accessible categories depends on what large cardinal properties hold: by Corollary 6.8 of Adamek-Rosicky, if Vopenka's principle holds then all accessible categories are well-copowered, while by Example A.19 of Adamek-Rosicky, if all accessible categories are well-copowered then there exist arbitrarily large measurable cardinals.
Last revised on February 22, 2019 at 11:51:10. See the history of this page for a list of all contributions to it.