On pure monomorphisms and pure epimorphisms in accessible categories

Abstract

In all $\kappa$-accessible additive categories, $\kappa$-pure monomorphisms and $\kappa$-pure epimorphisms are well-behaved, as shown in our previous paper arXiv:2311.02418. This is known to be not always true in $\kappa$-accessible nonadditive categories. Nevertheless, mild assumptions on a $\kappa$-accessible category are sufficient to prove good properties of $\kappa$-pure monomorphisms and $\kappa$-pure epimorphisms. In particular, in a $\kappa$-accessible category with finite products, all $\kappa$-pure monomorphisms are $\kappa$-directed colimits of split monomorphisms, while in a $\kappa$-accessible category with finite coproducts, all $\kappa$-pure epimorphisms are $\kappa$-directed colimits of split epimorphisms. We also discuss what we call Quillen exact classes of monomorphisms and epimorphisms, generalizing the additive concept of one-sided exact category.