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13 votes

Non-small objects in categories

In the category $\mathsf{Top}$ of topological spaces and continuous maps the only $\lambda$-presentable objects are discrete spaces. This appears 1.14(6) in Locally presentable and Accessible categori …
Ivan Di Liberti's user avatar
10 votes
Accepted

Can the dual of a finitely-accessible category be accessible?

In Accessible Categories: The Foundations of Categorical Model Theory by Makkai and Paré, there is the example of a finitely accessible self-dual category. Apparently the example is due to Isbell. Thi …
Ivan Di Liberti's user avatar
7 votes

Example: Accessible category without colimits

The category Hil of Hilbert spaces, considered as a full subcategory of Ban is $\aleph_1$-accessible but not locally presentable, in fact it is self dual. The category Lin of linear orders and strict …
Ivan Di Liberti's user avatar
4 votes

Relation between Ind-completion and "additive"-ind-completion

Let $\mathcal{V}$ be a cocomplete monoidal category which can be presented by a limit theory, so that $\mathcal{V} = \mathsf{Lex}(\mathbb{T},\text{Set})$, of course this is the case of your question. …
Ivan Di Liberti's user avatar
4 votes

Is every accessible category well-powered?

Some considerations, not a full answer (yet). In Accessible categories and models for linear logic, at page 2, Barr claims that every accessible category is well powered. He even claims that is obse …
Ivan Di Liberti's user avatar
3 votes

Which abelian groups are $\aleph_1$-filtered colimits of finitely-generated abelian groups?

Ycor answered the question, so I shall add a couple of remarks providing some insight about why these results should be true at all. Ind-completions are nested $$\text{Ind}(C) \supset \text{Ind}_{\ale …
Ivan Di Liberti's user avatar
2 votes

What are the reflective subcategories of the category of presentable categories?

The following is a very long comment and works in $1$-category theory. I claim that you can characterize very well coreflective subcategories. My strategy works even for reflective. There is a …
Ivan Di Liberti's user avatar