Subject: categories: Three questions about fibrations

From: Michael Abbott <[email-masked]>
Date: Thu, 15 Jun 2000 18:02:23 +0100

I am wondering if anyone can give references for three remarks Wesley Phoa
makes in the chapter on fibrations of his paper "An introduction to
fibrations, topos theory, the effective topos and modest sets".  

1. Essentially Algebraic Theories

In the footnote on page 7 Phoa comments: 
	"[fibrations] are the models for a first-order, 'essentially
algebraic' theory".

I'm not quite sure what he means here, and this sounds like it must be a
standard and well known connection.  I'd be glad of a reference.

2. Splitting Fibrations

At the bottom of page 14 Phoa observes: 
	"Every fibration .. is equivalent to a split fibration (there is an
elegant proof due to John Power).  However, it's not clear how to extend
this result to more complicated structures.  This is the coherence problem
..."

Now I know that any fibration is equivalent to a split fibration via the
"fibred Yoneda lemma" (Borceux, "Handbook of Categorical Algebra 2", 8.2.7
and Jacobs, "Categorical Logic and Type Theory"), but I don't think that's
the only splitting available, and I'm not sure that this correspondence
helps very with coherence questions.
	I am aware of another, different, equivalent splitting, and I wonder
if there are any references.  In particular, can anyone guess what reference
by John Power Phoa was referring to?
	In particular, I'd be very interested in any other observations on
the "coherence problem".

3. Generalising the Definition

In the footnote on page 12, in reference to the definition of a fibration,
Phoa says: 
	"If one really wants to take .. 2-categorical issues seriously, one
needs .. a more sophisticated definition of 'fibration'."

I can make some promising looking guesses.  Any references?

Many thanks.
	Michael Abbott

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From: "Robert A.G. Seely" <[email-masked]>
Date: Fri, 16 Jun 2000 00:43:33 -0400 (EDT)

> fibrations, topos theory, the effective topos and modest sets".  

At this point, I think your best bet is to get a copy of Bart Jacobs
excellent book "Categorical Logic and Type Theory" - which (in spite
of its title?) is all about fibrations (and logic and type theory -
actually I think it is well-titled, though you might not have guessed
it was what you needed from the title).  (There is a review of this
book on my web page, link below.)

-= rags =-

==================
R.A.G. Seely
<[email-masked]>
<http://www.math.mcgill.ca/rags>

---

From: Claudio Hermida <[email-masked]>
Date: Fri, 16 Jun 2000 19:12:29 +0000

> fibrations, topos theory, the effective topos and modest sets".

I think I can provide some meaningful answers to these questions, but of
course, one should always take such (throwaway) remarks in an informal
exposition with a pinch of black pepper.

> standard and well known connection.  I'd be glad of a reference.

Fibrations as finite-limit theories: This follows from the analysis in
[S], that exhibits fibrations as (adjoint) pseudo-algebras in any 2-category
with comma-objects. In particular, for any category B with pullbacks, the
2-category Cat(B) of internal categories in B admits them.

>         In particular, I'd be very interested in any other observations on

Split fibrations: Since fibrations are pseudo-algebras, it follows from Power's

coherence theorem [P] (which applies in this situation) that every fibration is

equivalent (over its base category) to a split one. The construction there does

not appeal to Yoneda. Yet, given that fibrations are actually properties, that
is adjoint pseudo-algebras, one can give a neat explicit description of the
associated split one as follows:

Given p:E->B, consider the free fibration over it, E/p -> B. The unit
\eta: E -> E/p takes X |-> X,id_(pX). The right adjoint r: E/p -> E amounts to
a choice of cartesian lifting: (Y,u:I->pY) |-> u*(Y). Thus we get a comonad
\eta o r: E/p -> E/p. The resulting Kleisli category (E/p)_(\eta r) is fibred
over B and gives the corresponding split fibration. The equivalence is given by
the composite E -\eta-> E/p -J-> (E/p)_(\eta r).

The above construction is a special case of the theory in [H], specially
section 11. The paper and the references there in provide further material on
coherence.

> I can make some promising looking guesses.  Any references?

Generalised fibrations: The only meaningful generalisation (in the categorical
context) proposed so far in the literature is that of [S1], fibrations "up to
equivalence". However I can see no impediment whatsoever at taking 2-category
theory seriously with the standard (Grothendieck) notion of fibration, so I
cannot guess what the author means.

References:

[S] R. Street, Fibrations and Yoneda Lemma in a 2-category, Category Seminar,
LNM 420, 1973.

[S1] R. Street, Fibrations in bicategories, Cahiers Top.Geom.Diff.Cat, 21,
1980.

[P] A.J.Power, A general coherence result, JPAA, 57(2):165-173, 1989.

[H] C. Hermida, From coherence structure to universal properties, (to appear
JPAA) (available at http://www.cs.math.ist.utl.pt/s84.www/cs/claudio.html)

Claudio Hermida