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- Discussion Type
- discussion topicoo-groupoid
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Mar 11th 2010

expanded the entry infinity-groupoid

- Discussion Type
- discussion topicomega-nerve
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Mar 11th 2010

expanded omega-nerve

added to Kan complex a section on how to think of them as oo-groupoids.

- Discussion Type
- discussion topicMore on quantum channels
- Category Latest Changes
- Started by Ian_Durham
- Comments 1
- Last comment by Ian_Durham
- Last Active Mar 11th 2010

- I filled in a bit of stuff on open systems and reversibility under quantum channels and operations. There's some category-theoretic stuff I have to add to it including figuring out a category-theoretic proof for one of the lemmas. Don't have time to do it right now.

- Discussion Type
- discussion topicquantum channel
- Category Latest Changes
- Started by Urs
- Comments 37
- Last comment by Urs
- Last Active Mar 9th 2010

I see that Ian Durham created quantum channel

- Discussion Type
- discussion topicWhitehead tower in an (oo,1)-topos
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Tim_Porter
- Last Active Mar 9th 2010

brief remark on my personal web on Whitehead systems in a locally contractible (oo,1)-topos.

So the homotopy fibers of the morphism that induces the Chern character in an (oo,1)-topos are something like the "rationalized universal oo-covering space": all non-torsion homotopy groups are co-killed, or something like that.

Is there any literature on such a concept?

- Discussion Type
- discussion topicKähler differential
- Category Latest Changes
- Started by Urs
- Comments 23
- Last comment by Tim_Porter
- Last Active Mar 9th 2010

I spent quite a bit of time now working on the entry Kähler differential

The motivation was that I had I pointed out the general idea of Kähler differentials at MathOverflow to somebody here. But when I then checked the entry, i found it didn't actually convey the right general picture.

Have a look at what I did, I did invest quite a bit of time in order to bring both the fully general nonsense nPOV, but lead up to it gently and understandably.

So there is a big "Idea and definition" section now that is meant to explain what is really going on, in the large and in the small.

Then the previous content of the entry, on Kähler differentials over ordinary rings and over smooth rings regarded as ordinary rings, I made subsections in a section titled "Specific deifnitions". I added more subsections to this. A stubby one with a pointer to the C^oo-ring case that is discussed in detail at Fermat theory, and then a bit on modules over monoids in general abelian categories, which is what the MO question had been about.

(They are funny at MO. I think this is quite a deep question. Also Tim Porter pointed out rather non-trivial literature on it. But the poster is being verbally abused for asking a question

that asks more of the respondent than you have contributed yourself.

:-o And now they closed down the thread!

Boy, I am just glad that among us we allow us to ask questions even before we are experts on something. Everything else is unhealthy. )

Anyway, the section on C^oo rings is a bit short. When you read the entry, you'll notice that there is an obvious question now: "So is it also true for C^oo-rings that the overcat. etc. pp-" The answer is YES, we checked this and it works all very nicely and reproduces the stuff reviewed at Fermat theory precisely. But this has been found / thought about by two graduate students, and I don't want to publicize this too much right now. But later.

- Discussion Type
- discussion topictrace + partial trace
- Category Latest Changes
- Started by Ian_Durham
- Comments 1
- Last comment by Ian_Durham
- Last Active Mar 9th 2010

- Based on Urs' comments, I have tentatively merged "partial trace" with the article on "trace" and included a redirect. What do people think about that? If we agree we like the change, can we delete the old partial trace page and, if so, how?

Also, the partial trace needs a diagram. I'm a little sketchy at this point on how to draw them in itex so if someone else is interested in taking a crack at it, it would be appreciated.

- Discussion Type
- discussion topicrational homotopy equivalence
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Mar 8th 2010

created rational homotopy equivalence

- Discussion Type
- discussion topicpartial trace
- Category Latest Changes
- Started by Ian_Durham
- Comments 4
- Last comment by Ian_Durham
- Last Active Mar 8th 2010

- Just added a page on partial trace that is presently linked from quantum operations and channels which I also added to. However, note that the partial trace is not specific to physics so it needs embellishing by the mathematicians among us.

- Discussion Type
- discussion topicmodel structures on cosimplicial rings and dg-rings
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Urs
- Last Active Mar 8th 2010

started model structure on cosimplicial rings and model structure on dg-algebras

want to eventually expand on this: does anyone happen have an electronic copy of the article by Jardine referenced at model structure on dg-algebras?

(in the process I also created a quick stub for differential graded ring)

- Discussion Type
- discussion topicQuantum operations
- Category Latest Changes
- Started by Ian_Durham
- Comments 4
- Last comment by Urs
- Last Active Mar 6th 2010

- Based on where the discussion was headed, I renamed the quantum channels page quantum operations and channels (but included suitable redirects) and added a few To Do items (including describing quantum operations) since, in order to fully understand the reversibility stuff, open quantum systems should be discussed. I don't have time right now to fill in all the details, but will hopefully get a chance to sometime in the next few days (spring break is rapidly approaching its end which means my time will get eaten up again...).

Incidentally, from the open systems stuff I will eventually link to a new page on closed time-like curves (CTCs) since they are (or can be) related and I think category theory might serve to help shed some light on how they function. This brings up the question: why isn't there a relativity section on nLab? I thought John Baez had done some work applying categories to quantum gravity? Maybe no one ever got to it?

- Discussion Type
- discussion topichomotopy coherent nerve
- Category Latest Changes
- Started by Urs
- Comments 23
- Last comment by Urs
- Last Active Mar 5th 2010

edited homotopy coherent nerve a bit

I tried to bring out the structure more by adding more subsections. Have a look at the new table of contents. Then I did a bunch of trivial edits like indenting some equations etc. Have a look at "See changes" if you want to see it precisely.

- Discussion Type
- discussion topicChapman complement theorem
- Category Latest Changes
- Started by Tim_Porter
- Comments 1
- Last comment by Tim_Porter
- Last Active Mar 5th 2010

I put a summary of the Chapman complement theorem at shape theory. I remember a discussion about duality on the blog some time ago and this may be relevant.

- Discussion Type
- discussion topicUnitary operators
- Category Latest Changes
- Started by Ian_Durham
- Comments 28
- Last comment by DavidRoberts
- Last Active Mar 5th 2010

- I just added a page on unitary operators. I also have a query there about whether unitary operators on a given Hilbert space form a category.

- Discussion Type
- discussion topicWick rotation
- Category Latest Changes
- Started by Ian_Durham
- Comments 2
- Last comment by DavidRoberts
- Last Active Mar 5th 2010

- I was hunting around for things a newbie could contribute to and noticed an empty link to Wick rotation so I filled it in.

- Discussion Type
- discussion topicMore quantum channel
- Category Latest Changes
- Started by DavidRoberts
- Comments 2
- Last comment by Ian_Durham
- Last Active Mar 4th 2010

Some more discussion (Ian and myself) at quantum channel about the definition of QChan when taking into account classical information.

- Discussion Type
- discussion topicHamilton operator
- Category Latest Changes
- Started by Urs
- Comments 8
- Last comment by Ian_Durham
- Last Active Mar 4th 2010

started something at Hamilton operator

- Discussion Type
- discussion topicenrichment through lax monoidal functors
- Category Latest Changes
- Started by Todd_Trimble
- Comments 3
- Last comment by Mike Shulman
- Last Active Mar 2nd 2010

I added a small subsection to the definition of an enriched category over which describes them as lax monoidal functors where the codomain is the monoidal category of endospans on in the bicategory of spans.

- Discussion Type
- discussion topicKalb-Ramond field
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Mar 2nd 2010

expanded and polished Kalb-Ramond field. In particular I added more references.

- Discussion Type
- discussion topicSibe Mardesic
- Category Latest Changes
- Started by Tim_Porter
- Comments 3
- Last comment by TobyBartels
- Last Active Mar 2nd 2010

This is really just for Zoran although anyone else is welcome to help. I felt there needed to be a little more here, but you are also closely involved with this so please, check that what I have added is alright. Thanks. Tim

- Discussion Type
- discussion topicrational homotopy theory
- Category Latest Changes
- Started by Urs
- Comments 7
- Last comment by Tim_Porter
- Last Active Mar 1st 2010

I wanted to add to rational homotopy theory a section that gives a summary overview of the two Lie theoretic approaches, Sullivan's and Quillen's, indicating the main ingredients and listing the relevant references, by collecting some of the information accumulated in the blog discussion.

But, due to my connection problem discused in another thread, even after trying repeatedly for about 45 minutes, the nLab software still regards me as a spammer and won't let me edit the entry.

I'll try again tomorrow. Meanwhile, in case a good soul here can help me out, I post the text that I wanted to add to the entry in the next message. It's supposed to go right after the section ""Rational homotopy type".

- Discussion Type
- discussion topicDefinitions
- Category Latest Changes
- Started by Ian_Durham
- Comments 4
- Last comment by Andrew Stacey
- Last Active Feb 27th 2010

- When Urs cleaned up my quantum channel entry he included some empty links to things that needed defining. I created an entry for one (density matrices and operators) but, before I do anymore, wanted to make sure that what I did was appropriate and conforms to the general format for definitions, particularly since it is an applied context and may be somewhat unfamiliar to some people.

- Discussion Type
- discussion topicmore type theory
- Category Latest Changes
- Started by Mike Shulman
- Comments 2
- Last comment by Urs
- Last Active Feb 27th 2010

Wrote identity type and display map and dependent type.

Also, I have a question/correction at internal logic in a presheaf topos

- Discussion Type
- discussion topicEquilogical space
- Category Latest Changes
- Started by SridharRamesh
- Comments 3
- Last comment by SridharRamesh
- Last Active Feb 26th 2010

I've created a stub article for equilogical spaces. I couldn't quite figure out how to make a link while preserving the subscripting; I guess I could rewrite that to avoid the formatting problem, but presumably someone else knows how to do it anyway

- Discussion Type
- discussion topicmotivic cohomology
- Category Latest Changes
- Started by Urs
- Comments 22
- Last comment by Urs
- Last Active Feb 25th 2010

created a stub for motivic cohomology in reply to a question here

Zoran (or anyone), can you say something about how that related to motives?

- Discussion Type
- discussion topiccategory fibered in groupoids
- Category Latest Changes
- Started by Urs
- Comments 42
- Last comment by SridharRamesh
- Last Active Feb 25th 2010

started category fibered in groupoids as I think this deserves a page of its own separated from Grothendieck fibration

I understand that there was some terminological opposition voiced at Grothendieck fibration concerning the term "category cofibered in groupoids", but am I right that this does not imply opposition against "category fibered in groupoids", only that the right term for the arrow-reversed situation should be "opfibration in groupoids"?

- Discussion Type
- discussion topiccoskeleton
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Feb 25th 2010

started expanding simplicial skeleton. But more needs to eventually go here.

- Discussion Type
- discussion topichomotopy lifting property
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Feb 24th 2010

expanded homotopy lifting property

- Discussion Type
- discussion topichomotopy group of an oo-stack
- Category Latest Changes
- Started by Urs
- Comments 76
- Last comment by Urs
- Last Active Feb 24th 2010

I am expanding the entry homotopy group (of an infinity-stack) by putting in one previously missing aspect:

there are two different notions of homotopy groups of oo-stacks, or of objects in an (oo,1)-topos, in general

the "categorical" homotopy groups

the "geometric" homotopy groups.

See there for details. This can be seen by hand in same cases That this follows from very general nonsense was pointed out to me by Richard Williamson, a PhD student from Oxford (see credits given there). The basic idea for 1-sheaves is Grothendieck's, for oo-stacks on topological spaces it has been clarified by Toen.

While writing what I have so far (which I will probably rewrite now) I noticed that the whole story here is actually nothing but an incarnation of Tannak-Krein reconstruction! I think.

It boils down to this statement, I think:

IF we already know what the fundamental oo-groupoid of an object is, then we know that a "locally constant oo-stack" with finite fibers is nothing but a flat oo-bundle, namely a morphism (think about it for n=1, where it is a very familiar statement). The collectin of all these is nothing but the

*representation category*(on finite o-groupoids)For each point this comes with the evident forgetful funtor

that picks the object that we are representing on.

Now, Tannaka-Krein reconstruction suggests that we can reconstruct as the automorphisms of the functor.

And that's precisely what happens. This way we can

*find*from just knowing "locally constant oo-stacks" on X, i.e. from known flat oo-bundles with finite fibers on X.And this is exactly what is well known for the n=1 case, and what Toen shows for oo-stacks on Top.

- Discussion Type
- discussion topicLie theory for stacky Lie groupoids
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Feb 24th 2010

Chenchang Zhu updated the reference to her work at Lie theory for stacky Lie groupoids