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Maria Emilia Maietti | 18 May 2013 10:35
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Call for Papers- special issue APAL


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Call for Papers: Fourth Workshop on Formal Topology (4WFTop)
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Special Issue of Annals of Pure and Applied Logic
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The Fourth Workshop on Formal Topology was held in Ljubljana in June 2012:

http://4wft.fmf.uni-lj.si/

The proceedings of this workshop will be published as a special issue of
the Annals of Pure and Applied Logic, with the following guest editors:

Thierry Coquand, Maria Emilia Maietti, Giovanni Sambin, Peter Schuster.

These proceedings are open for high-level research papers on topics from
or closely related to formal topology, that is, constructive and/or
point-free topology including its applications and its foundations.

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Submissions by email to: 4WFTop.apal <at> math.unipd.it
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Please let us know if you plan to submit a paper as soon as possible
Deadline for submissions: Thursday, 31 October 2013
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Richard Garner | 16 May 2013 11:46
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CT2013 - early bird registration

Dear all,

This is a reminder that early bird registration for Category Theory 2013
closes TOMORROW, Friday 17th May. After that, the cost of registration goes
up by 10% across the board.

Any registrations submitted by fax, post or other means at a time
verifiably before the 18th in your local timezone will be deemed eligible
for the early bird rates; any submitted thereafter will not.

The registration page may be found at:

http://web.science.mq.edu.au/groups/coact/seminar/ct2013/registration.html

If you believe you have submitted a registration form already, but have
received no confirmation of its receipt, please email me to let me know.

Richard Garner (for the CT2013 organising committee)

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hilde | 14 May 2013 11:27
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GlynnFest Workshop, May 31st and June 1st, Cambridge University Computer Laboratory

We are happy to announce a workshop to honour Glynn Winskel on the occasion  of his 60th birthday.

The workshop will take place at Cambridge University Computer Laboratory on  May 31st and June 1st.

The speakers will be:

- Samson Abramsky, University of Oxford
- Henrik R. Andersen, Configit
- Steve Brookes, Carnegie-Mellon University
- Pierre-Louis Curien, University of Paris 7
- Olivier Danvy, University of Aarhus
- Marcelo Fiore, University of Cambridge
- Thomas T. Hildebrandt, IT University of Copenhagen
- Martin Hyland, University of Cambridge
- Kim G. Larsen, University of Aalborg
- Ugo Montanari, University of Pisa
- Mogens Nielsen, University of Aarhus
- Prakash Panangaden, McGill University
- Andy Pitts, University of Cambridge
- Gordon Plotkin, University of Edinburgh
- Vladimiro Sassone, University of Southampton

Participation to the workshop is open, but attendees are kindly requested to register in advance.
More details about the workshop venue and program and about the registration procedure can be found at the
following link
http://www.itu.dk/research/models/wiki/index.php/GlynnFestWorkshop

Best wishes on behalf of the organizing committee,
Thomas Hildebrandt
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majordomo | 13 May 2013 14:50
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Zig Zags?

Dear Toby,

Thank you for your explanations. I really admire your elegant idea to use zig zags instead of 2-pullbacks. 
But I am an old fashioned mathematician and I like precise definitions with which I can (try to) prove
precise results.
You suggest to use zig zags. Could you please tell me what are the maps in the zig zags: anafunctors?
functors? (in both cases what properties do you assume about them?), equivalences of categories? (in
that case in what sense?)
How do you compose your zig zags? You say "directly", do you mean by mere concatenation?
Obviously there would be a huge amount of such zig zags, thus you would probably want to work up to some
identification. Could you please tell me, with precision, when two such zig zags between two categories A
and B should be identified?
In the case of spans, using 2-pullbacks, what are the maps in your spans, when should two such spans between A
and B be identified?
You say, I quote you:

Indeed, so one must also define natural isomorphism of equivalences
If you have any difficulty, the answer is in Makkai's anafunctor paper

Thank you for the tactfulness with which you point out my alleged ignorance, but I happen to know what
anafunctors are. I have even given a very simple definition of them. It is still in the nLab, under my name (I
have checked that recently)
But due to my limited imagination, I don't know how to use anafunctors here. Could you please tell me how they
can be used to decide when two spans or two zig zags are equivalent.
Finally, I cannot help thinking about the poor people who are not familiar with anafunctors. Are they
doomed forever to never understand what it means for two categories to be equivalent?
I look forward to your answers.

Very respectfully yours,
Jean
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Emily Riehl | 10 May 2013 15:05
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on a subcategory of algebras for a monad

Hi,

I received the following question from a grad student that I was unable to
answer, but maybe you can (shared with permission). The subcategory Comp_M
he introduces below can equally be defined to be the inverter of the
counit of the monadic adjunction. But I don't see how this universal
property helps understand limits in the subcategory. We suspect a left
adjoint to the inclusion is unlikely.

Can you help? Or have you seen something like this before?

Best,
Emily

***
??
Hi folks,
??
I'm interested in closure properties of a particular subcategory of the
category of algebras of a monad. To be more precise, let C be a locally
presentable category and M be a monad on C. The category of algebras Alg_M
has all limits, and they are computed in C. Denote by Comp_M the full
subcategory of Alg_M of "M-complete objects" (does anyone have a better
name?), with objects those X in C such that the unit X -> MX is an
isomorphism, viewed in the natural way as M-algebras (using the inverse MX
-> X).
??
My question: Is Comp_M closed under (actually: sequential) limits, computed
as limits in Alg_M?
??
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David Roberts | 9 May 2013 05:07
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(In)accessible comonads and (non)Grothendieck toposes

Hi all,

I am just wondering where it was first stated (for both directions) that the category of coalgebras for a
comonad on a Grothendieck topos E is again Grothendieck if and only if the underlying endofunctor of E is
accessible. 

A modern argument might go as: the topos of coalgebras is Grothendieck if and only if it is locally
presentable if and only if the endofunctor is accessible, the original probably just mentioned
preservation of filtered colimits.

Many thanks,

David Roberts

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Tom Leinster | 8 May 2013 16:55
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Reminder: CT2013 abstracts

Dear all,

This is a reminder that if you wish to take advantage of the extended 1
June deadline for submitting abstracts to CT2013, you are requested to
email me **by 10 May** saying that you intend to submit an abstract.
This is to help with planning.

Abstracts received after 10 May will not be accepted unless you have
emailed before then.  I am acknowledging all such emails, so if you have
written to me and not received a reply, there has probably been a
technological problem - in which case, please try again.

Best wishes,
Tom

--

-- 
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

---------- Forwarded message ----------
Date: Wed, 1 May 2013 21:29:23
From: Tom Leinster <Tom.Leinster <at> ed.ac.uk>
To: categories <at> mta.ca
Subject: categories: CT2013 abstracts

Dear all,

The original deadline for submitting abstracts to Category Theory 2013
(Macquarie University, Sydney) was 1 May.
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Marek Zawadowski | 8 May 2013 11:34
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Samuel Eilenberg Centenary Conference - Second Announcement

-------- SECOND ANNOUNCEMENT-------------------------------------------------

Dear Colleagues,
below is the Second Announcement of the Samuel Eilenberg Centenary Conference
which will be held in Warsaw, July 22-26, 2013. The organizers cordially
invite you to participate in the event commemorating one of the founders of
our field.

-------- SCIENTIFIC PROGRAM--------------------------------------------------

Ten plenary speakers confirmed their attendance and some titles and abstracts
of their lectures are already posted at:

http://eilenberg100.ptm.org.pl/programme

All participants are welcome to propose contributed talks by filling an
appropriate entry in the registration form and uploading an abstract. Note
that the deadline for submissions of titles and abstracts of the contributed
talks is May 31, 2013.

-------- REGISTRATION--------------------------------------------------------

The organizers of the conference cordially invite you to register for the
meeting at:

http://eilenberg100.ptm.org.pl/registration

The conference fee is 300 PLN (approx 70 Euro or 100 USD), and it will raise
to 400 PLN on June 1, 2013.
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Marek Zawadowski | 7 May 2013 23:05
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PhD program in Warsaw

Dear Categorists,

Warsaw Center of Mathematics and Computer Science (WCMCS) has opened
registration for PhD programs in mathematics and computer science at the
University of Warsaw and Mathematical Institute of Polish Academy of
Science. A number of PhD scholarships financed by WCMCS will be awarded to
the most promising graduate students. The details of the program are
available at wcmcs.edu.pl/projects. You can apply on-line at
http: //wcmcs.edu.pl/submit-application/admissions-fellowships
The deadline for application is June 7th, 2013.

Best regards,
Marek Zawadowski

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Eduardo J. Dubuc | 7 May 2013 19:26
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indexed_vs_fibrations

On 07/05/13 13:11, Jean B?nabou wrote:

>  For
> more than forty years now I have struggled to convince the category
> community that fibered categories are much better than indexed ones,
> and that the latter ought to be briefly described for the sake of
> completeness, but immediately discarded.

Pare and Scumacher (SLN 661) among other considerations to justify their
choice of indexed categories (= cloven fibrations) over fibrations say:

   "We have tried to make our theory conform as closely as possible to
actual mathematical practice"

Grothendiek (SGA 1, SLN 224, http://arxiv.org/abs/math/0206203) among
other considerations to justify his choice of fibrations over cloven
fibrations (= indexed categories) say:

"Il est d?ailleurs probable que, contrairement a l?usage encore
preponderant maintenant, lie a d?anciennes habitudes de pensee, il
finira par s?averer plus commode dans les problemes universels, de ne
pas mettre l?accent sur une  solution supposee choisie une fois pour
toutes, mais de mettre toutes les solutions sur un pied d?egalite"

"actual mathematical practice" = "l?usage encore preponderant maintenant"

Grothendieck ads  "lie a d'anciennes habitudes de pensee"

Interesting enough, categorical thinking is hard to swallow.
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Jean Bénabou | 7 May 2013 10:23
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"Terminolgy" re-visited

Dear all,

I cannot type any form of LaTeX, and do not know the "standard" ways to introduce indices,exponentials and
so on, using only the typing which is admitted on this list. Thus I shall use "non standard" notations, very
simple, which I shall explain precisely.
If A is a category an object a of A can be identified with a functor "name of a" which I denote by "a": 1 --> A .
If  F: A --> C and G: B --> C are functors I denote by  F/G the comma category they define, and by F//G their
  2-pull-back sometimes called their pseudo pull-back.
I shall call "weak equivalence" a functor F: A --> B  full and faithful and essentially surjective (ff-es)
and say that A is weakly equivalent (we) to B if there is such an F. This defines  a preorder relation which I
denote by 
W(A,B). It is symmetric iff the Axiom of choice (AC) holds.
A strong equivalence between  A and B is a pair of adjoint functors  F: A --> B  and  F': B --> A  such that the
adjunction morphisms are isos. I shall say that  A and B are strongly equivalent if such a pair exists and
denote by  SE(A,B) this equivalence relation.
If S is a category I denote by Fib/S the 2-category of fibrations over S
I denote by S° the dual of S . An S indexed category C is a pseudo functor   C: S° --> Cat .  For each map  
f: s --> t  of  S , I denote by f*: C(t) --> C(s) the value of  C on f. If g: t --> u is another map of S, I denote by  c(f,g)
the isomorphism f*g* --> (gf)*. I shall assume C normalized i.e. for each object  s of S, id(s)* = id(C(s))
For an indexed functor  F: C --> C'  i use the following notations: If s is an object of S, F(s): C(s) --> C'(s) , 
if  f: s --> t is a map of S  is the iso of functors  F(s)f* --> f*F(t)
(I'm aware of the ambiguity of denoting by the same  f* the "re-indexing functors" defined by C and C' but it is
the best I can do with the limited typographic means I use, without having too cumbersome notations) 
If S is has pull-backs, I denote by Cat(S) the category of categories internal to S

 
Answer to Toby Bartels:
(i) You propose to compose the spans by pullbacks. Here is an example of two spans  A <-- X --> B and 
B <-- Y --> C such that the functors  A <--X  and  Y --> C are isos, the functors X --> B and  B <-- Y have unique quasi
inverses, and if  Z is the pullback none of the functors  A <-- Z  and  Z --> B is a weak equivalence:
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Gmane