nLab anabicategory

An anabicategory is a particular notion of weak 2-category appropriate in the absence of the axiom of choice (including in many internal contexts). It is derived from the notion of bicategory by replacing the composition functor?s $\circ: B(y,z) \times B(x,y) \to B(x,z)$ with anafunctors (and therefore the associators and unitors with ananatural transformations).

Zoran Škoda: I understand that there is a version of $Cat$ using anafunctors, but it is not clear to me what are the axioms for a variant of 2-category which this example belongs to. I do not understand what you mean by replacing hom-functor with anafunctor in that definition: I mean which definition of bicategory is phrased in terms of hom-functors; standard definition talks associators and so on…Please be more explicit.

Mike: Is this version clearer?

Toby: Or look at the complete definition in the reference that I just added.

If Cat is defined as consisting of (small) categories, anafunctors, and ananatural transformations (as is most appropriate in the absence of choice), then $Cat$ is more naturally an anabicategory rather than any stricter notion.

Mike: Is that only because of the non-canonicity of pullbacks in $Set$? If our version of $Set$ has canonical chosen pullbacks, do we get an ordinary bicategory?

Toby: H'm … as I recall, you get an ordinary bicategory using canonical pullbacks and general anafunctors, but if you move to saturated anafunctors, then you still only get an anabicategory. I should check this and then rewrite (here and at Cat) to say it correctly. (Of course, if you really use anafunctors, then you should only want an anabicategory.)

Reference

• Michael Makkai; Avoiding the axiom of choice in general category theory; section 3 (which is part 4 here).
Revised on August 27, 2009 07:14:26 by Toby Bartels (71.104.230.172)