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1. • CommentRowNumber1.
   • CommentAuthorUrs
   • CommentTimeDec 21st 2011
   • PermaLink
   Author: Urs
   Format: MarkdownItexdiscovered the following remnant discussion at [[full functor]], which hereby I move from there to here *** _Mathieu says:_ I agree that, for functors, there is no reason to say “fully faithful” rather than “full and faithful”. But for arrows in a 2-category (like in the new version of the entry on [[subcategory|subcategories]]), there are reasons. I quote myself (from my thesis): «Remark: we say _fully faithful_ and not _full and faithful_, because the condition that, for all $X:\C$, $C(X,f)$ be full is not equivalent in $\Grpd$ to $f$ being full. Moreover, in $\Grpd$, this condition implies faithfulness. We will define (Definition 197) a notion of full arrow in a $\Grpd$-category which, in $\Grpd$ and $\Symm2\Grp$ (symmetric 2-groups), gives back the ordinary full functors.» Note that this works only for some good groupoid enriched categories, not for $\Cat$, for example. _Mike_ says: Do you have a reason to care about full functors which are not also faithful? I've never seen a very compelling one. (Maybe I should just read your thesis...) I agree that "full morphism" (in the representable sense) is not really a useful/correct concept in a general 2-category, and that therefore "full and faithful" is not entirely appropriate, so I usually use "ff" in that context. I've changed the entry above a bit to reflect your comment; is it satisfactory now? Maybe all this should actually go at [[full and faithful functor]] (and/or [[fully faithful functor]])?

   discovered the following remnant discussion at full functor, which hereby I move from there to here

   ---

   Mathieu says: I agree that, for functors, there is no reason to say “fully faithful” rather than “full and faithful”. But for arrows in a 2-category (like in the new version of the entry on subcategories), there are reasons. I quote myself (from my thesis): «Remark: we say fully faithful and not full and faithful, because the condition that, for all $X:\C$, $C(X,f)$ be full is not equivalent in $\Grpd$ to $f$ being full. Moreover, in $\Grpd$, this condition implies faithfulness. We will define (Definition 197) a notion of full arrow in a $\Grpd$-category which, in $\Grpd$ and $\Symm2\Grp$ (symmetric 2-groups), gives back the ordinary full functors.» Note that this works only for some good groupoid enriched categories, not for $\Cat$, for example.

   Mike says: Do you have a reason to care about full functors which are not also faithful? I’ve never seen a very compelling one. (Maybe I should just read your thesis…) I agree that “full morphism” (in the representable sense) is not really a useful/correct concept in a general 2-category, and that therefore “full and faithful” is not entirely appropriate, so I usually use “ff” in that context. I’ve changed the entry above a bit to reflect your comment; is it satisfactory now? Maybe all this should actually go at full and faithful functor (and/or fully faithful functor)?