A simple criterion for a functor to be finitary is presented: we call F finitely bounded if for all objects X every finitely generated subobject of FX factorizes through the F-image of a finitely generated subobject of X. This is equivalent to F being finitary for all functors between `reasonable' locally finitely presentable categories, provided that F preserves monomorphisms. We also discuss the question when that last assumption can be dropped. The answer is affirmative for functors between categories such as Set, K-Vec (vector spaces), boolean algebras, and actions of any finite group either on Set or on K-Vec for fields K of characteristic 0.
All this generalizes to locally $\lambda$-presentable categories, $\lambda$-accessible functors and $\lambda$-presentable algebras. As an application we obtain an easy proof that the Hausdorff functor on the category of complete metric spaces is $\aleph_1$-accessible.
Keywords: Finitely presentable object, finitely generatd object, (strictly) locally finitely presentable category, finitary functor, finitely bounded functor
@article{TAC_2019_34_a34,
author = {Jiri Adamek and Stefan Milius and Lurdes Sousa and Thorsten Wissmann},
title = {On {Finitary} {Functors}},
journal = {Theory and applications of categories},
pages = {1134--1164},
year = {2019},
volume = {34},
language = {en},
url = {http://geodesic.mathdoc.fr/item/TAC_2019_34_a34/}
}
Jiri Adamek; Stefan Milius; Lurdes Sousa; Thorsten Wissmann. On Finitary Functors. Theory and applications of categories, Tome 34 (2019), pp. 1134-1164. http://geodesic.mathdoc.fr/item/TAC_2019_34_a34/