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- Discussion Type
- discussion topicGrothendieck duality
- Category Latest Changes
- Started by zskoda
- Comments 5
- Last comment by hilbertthm90
- Last Active Sep 1st 2011

I addede a paragraph at Poincaré duality about the generalizations, and created the entry (so far only descent bibliography) Grothendieck duality; the list of examples expanded at duality. All prompted by seeing the today’s arXiv article of Drinfel’d and Boyarchenko.

- Discussion Type
- discussion topicHadwiger's theorem
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Sep 1st 2011

stub for Hadwiger’s theorem

- Discussion Type
- discussion topicsymplectic geometry - contents
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 31st 2011

started a contents-page symplectic geometry - contents and added it as a floating toc to relevant entries (there should be more, have not included everything yet)

- Discussion Type
- discussion topichigher Kleinian- and Cartanian geometry
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 31st 2011

started entries

collecting some evident ideas and observations, most of which we have discussed at one point or other over at the $n$Café.

- Discussion Type
- discussion topicrigid body dynamics
- Category Latest Changes
- Started by Urs
- Comments 6
- Last comment by zskoda
- Last Active Aug 31st 2011

prompted by the discussion in another thread I have started creating stubs

But not much real content there so far.

- Discussion Type
- discussion topicinstanton Floer homology
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by zskoda
- Last Active Aug 30th 2011

I am starting an entry instanton Floer homology branched off from Floer homology

- Discussion Type
- discussion topicquantum group Fourier transform
- Category Latest Changes
- Started by zskoda
- Comments 4
- Last comment by zskoda
- Last Active Aug 30th 2011

New entry quantum group Fourier transform (and related stubs coregular action, dual bialgebra, S. L. Woronowicz).

- Discussion Type
- discussion topicsymbol map
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Aug 30th 2011

created symbol map and added a sentence to this effect to the Properties-section at Clifford algebra

- Discussion Type
- discussion topicSupergravity and superstrings
- Category Latest Changes
- Started by Tim_Porter
- Comments 5
- Last comment by zskoda
- Last Active Aug 30th 2011

Can someone have a look at Supergravity and Superstrings - A Geometric Per? I don’t suspect this is really Spam but it is a strange entry!

- Discussion Type
- discussion topiccocommutative coalgebra
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by zskoda
- Last Active Aug 29th 2011

created cocommutative coalgebra

- Discussion Type
- discussion topicHamiltonian dynamics on Lie groups
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 29th 2011

- Discussion Type
- discussion topicstrict Lie 2-algebra
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by zskoda
- Last Active Aug 29th 2011

created stub for strict Lie 2-algebra

- Discussion Type
- discussion topictrees as terminal coalgebra
- Category Latest Changes
- Started by Todd_Trimble
- Comments 28
- Last comment by TobyBartels
- Last Active Aug 28th 2011

I added some discussion at terminal coalgebra that the category of trees (equivalently, the category of forests $Set^{\omega^{op}}$) is a terminal coalgebra for the small-coproduct cocompletion (as endofunctor on $Cat$); this is a special case of Adamek’s theorem. I linked to this discussion by adding a section at tree. (There is of course closely related discussion at pure set as well.)

It seems to me that the nLab is a bit thin on general matters of recursion. I’ve been looking a bit at the discussion in Paul Taylor’s book, and I am becoming partial to the general idea that in some sense coalgebras and corecursion often come first; after that one may base recursive schemata on the notion of

*well-founded coalgebras*. For example, (ill-founded) trees are really simple conceptually, or at least have a very simple uniform description: as presheaves $\omega^{op} \to Set$. This is just a simple-minded application of Adamek’s theorem. Later, one can peek inside and gets the initial algebra for the small-coproduct completion as the category of well-founded trees, but this is by no means as simple (one can’t just apply Adamek’s theorem for constructing initial algebras – the hypotheses don’t hold here!).

- Discussion Type
- discussion topicvacuum
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 27th 2011

wrote something at vacuum.

I mainly wanted the link to point somewhere. I don’t claim that what I have there presently is a good discussion. So I have labeled it “under construction” for the moment.

- Discussion Type
- discussion topicmeasure coalgebra
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 27th 2011

since it was mentioned on the category theory mailing list I went to the entry measure coalgebra and edited a bit: I have added some hyperlinks and Definition- and Proposition-environments.

Somebody who created the entry should look into this issue: currently the entry mentions a ground field right at the beginning, which however never reappears again. It’s clear that everything can be done over an arbitrary ground field, I guess, but currently this is not discussed well.

In order to satisfy links I then created

- Discussion Type
- discussion topicdilaton gravity
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 26th 2011

added to dilaton the action functional of

*dilaton gravity*(Perelman’s functional)Also references and maybe something else, I forget.

- Discussion Type
- discussion topictemporal gauge
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 26th 2011

unmotivated stub for temporal gauge (which I happened to need at Yang-Mills instanton and instanton Floer homology)

- Discussion Type
- discussion topicCharacteristic forms and geometric invariants
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 26th 2011

The reference

*Characteristic forms and geometric invariants*was missing from a few entries (for instance Chern-Simons theory and secondary characteristic class.) So I added it in. Since it is an important reference, I thought I should give it a “category: reference”-page of its own.

- Discussion Type
- discussion topicstring theory -- contents
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 24th 2011

I shouldn’t be doing this. But in a clear case of procrastination of more urgent tasks, I created a floatic TOC string theory - contents and added it to some relevant entries.

- Discussion Type
- discussion topicfivebrane
- Category Latest Changes
- Started by Urs
- Comments 5
- Last comment by Urs
- Last Active Aug 24th 2011

I notice that in recent preprints (see equation (2.1) in today’s 1108.4060) people are getting awefully close to rediscovering nonabelian 2-connections in the worldvolume theory of NS-fivebranes (but they are forgetting the associator! :-).

This follows a famous old conjecture by Witten, which says that the worldvolume theory of a bunch of fivebranes on top of each other (what physicsist call a “stack” of fivebranes) should be a nonabelian principal 2-bundle/gerbe-gauge theory. If you have followed Witten’s developments since then (with his latest on Khovanov homology) you’ll know that he is suggesting that this theory is at the very heart of a huge cluster of concepts (geometric Langlands duality and S-duality being part of it).

So I should eventually expand the entry

*fivebrane*. I’ll start with some rudiments now, but will have to interrupt soon. Hopefully more later.

- Discussion Type
- discussion topicVolume pseudoforms
- Category Latest Changes
- Started by TobyBartels
- Comments 2
- Last comment by TobyBartels
- Last Active Aug 24th 2011

I wrote about these on Hodge star and volume form.

- Discussion Type
- discussion topicopinions on development of mathematics
- Category Latest Changes
- Started by zskoda
- Comments 6
- Last comment by TobyBartels
- Last Active Aug 24th 2011

I could not find a better title, for the new entry, unfortunately: opinions on development of mathematics (should be mainly bibiliography entry). I need some place to start collecting the titles which talk about generalities of mathematical development, what is important, what is not. This is relevant for but it is

*not*philosophy. Not only because of traditional focus of philosophy on “bigger” things like true nature of beings, meaning, ethics, cognition and so on, but more because the latter is very opinionated in the usual sense, even politics. Though we should of course, choose those which have important content, it is useful to collect those. We can have netries like math and society, even math funding for other external things of relevance, eventually. This was quick fix as I have no time now.

- Discussion Type
- discussion topicconnection on a 2-bundle
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 23rd 2011

At connection on a 2-bundle I have started an Examples-section and added References. Both long overdue.

- Discussion Type
- discussion topicreal closed field
- Category Latest Changes
- Started by Todd_Trimble
- Comments 19
- Last comment by Mike Shulman
- Last Active Aug 23rd 2011

I added a little bit of material to ordered field, namely that a field is orderable iff it is a real field (i.e., $-1$ is not a sum of squares). More importantly, at real closed field, I have addressed an old query of Colin Tan:

Colin: Is it true that real closure is an adjoint construction to the forgetful functor from real closed fields to orderable fields?

by writing out a proof (under Properties) that indeed the forgetful functor from category of real closed fields and field homomorphisms to the category of real fields and field homomorphisms has a left adjoint (the real closure). Therefore I am removing this query from that page over to here.

- Discussion Type
- discussion topicconstructible
- Category Latest Changes
- Started by zskoda
- Comments 6
- Last comment by Mike Shulman
- Last Active Aug 23rd 2011

I have created a stub for constructible universe. I did not go through the version of the definition via definability. Now constructible sets are sets in the constructible universe. The notion of course, intentionally reminds the constructible sets in topology and algebaric geometry as exposed e.g. in the books on stratified spaces, on perverse sheaves (MacPherson e.g.) and in Lurie’s Higher Topos Theory. Now I wanted to create constructible set but I was hoping that there is a common definition for all these cases or at least logically defendable unique point of view, rather than partial similarity of definitions. I mean one always have some business of unions, complements etc. starting with some primitive family, say with open sets, or algebraic sets, or open sets relative strata etc. and inductively constructs more. Now, all the operations mentioned seem to have sense in some class of lattices. Maybe in Heyting lattices or at least in Boolean lattices. On the other hand, google spits out several references on

*constructible lattices**one of the authors is certain Janowitz), but the definition there is disappointing. I mean I would like that one has some sort of constructible completion of certain kind of a lattice and talk about the constructible elements as the elements of constructible completion. I am sure that the nLab community could nail the wanted common generalization down or to give a reference if the literature has it already.

- Discussion Type
- discussion topic2-pullback
- Category Latest Changes
- Started by Tim_Porter
- Comments 2
- Last comment by TobyBartels
- Last Active Aug 22nd 2011

Query from Stephen on 2-pullback.

- Discussion Type
- discussion topictorsion of a metric connection
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 22nd 2011

some quick content at torsion of a metric connection

- Discussion Type
- discussion topicisometry group
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 22nd 2011

stub for isometry group

- Discussion Type
- discussion topicProper homotopy theory
- Category Latest Changes
- Started by Tim_Porter
- Comments 13
- Last comment by Andrew Stacey
- Last Active Aug 22nd 2011

I have started an entry on proper homotopy theory. This is partially since it will be needed in discussing some parts of strong shape theory, but it may also be useful for discussing duality and various other topics, including studying non-compact spaces in physical contexts. This is especially true for non-compact manifolds. (I do not know what fibre bundles etc. look like in the proper homotopy setting!)

- Discussion Type
- discussion topicOrdinary mathematics
- Category Latest Changes
- Started by TobyBartels
- Comments 15
- Last comment by TobyBartels
- Last Active Aug 20th 2011

A term that I see sometimes in discussions of foundations: ordinary mathematics.

- Discussion Type
- discussion topicPlace
- Category Latest Changes
- Started by fpaugam
- Comments 3
- Last comment by fpaugam
- Last Active Aug 18th 2011

- Following Zoran's suggestion, i have written a short entry place to describe the different meanings of this term in arithmetic and analytic geometry.

- Discussion Type
- discussion topicLebesgue space
- Category Latest Changes
- Started by TobyBartels
- Comments 11
- Last comment by TobyBartels
- Last Active Aug 18th 2011

I added some more to Lebesgue space about the cases where $1 \lt p \lt \infty$ fails.

- Discussion Type
- discussion topicMicrolocalization
- Category Latest Changes
- Started by fpaugam
- Comments 8
- Last comment by fpaugam
- Last Active Aug 18th 2011

- I have added a page on microlocalization a la Sato and Kashiwara-Schapira. It is complementary and different of the page on microlocal analysis (the approach is more algebraic). Perhaps both should be merged. Mathematically, the theory of ind-sheaves by Kashiwara-Schapira completes the bridge between classical analysis (a la Hormander) and Sato's approach, even if these two domains have quite different aims.

I saw that there is also a page called algebraic microlocalization, but i don't like this name much (perhaps because of my ignorance): localization is allways algebraic (or sheaf theoretic), and microlocal analysis describes the corresponding thing in the analysis community.

What do you think (in particular, Zoran)?

- Discussion Type
- discussion topicaffine connection, frame bundle
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active Aug 18th 2011

New entry frame bundle. Correction at affine connection: affine connection is the principal $GL_n(k)$-connection on the frame bundle of the manifold, not the connection on the tangent bundle, though the latter is a special case of the corresponding associated bundle connection. Urs, do you agree with the correction ? (I think that wikipedia, linked there, terminologically agrees).

- Discussion Type
- discussion topic[[Dijkgraaf-Witten theory]]
- Category Latest Changes
- Started by domenico_fiorenza
- Comments 2
- Last comment by zskoda
- Last Active Aug 17th 2011

Added another proof (this one not using the universal coefficients theorem) of the isomorphism $H^n(M,U(1))\cong U(1)$ for $M$ a closed oriented $n$-manifold to Dijkgraaf-Witten theory.

- Discussion Type
- discussion topicsimplicial localization
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 17th 2011

At simplicial localization I have added in some basic definitions and some basic properties.

- Discussion Type
- discussion topiccolimit and limit
- Category Latest Changes
- Started by Todd_Trimble
- Comments 2
- Last comment by TobyBartels
- Last Active Aug 14th 2011

- Discussion Type
- discussion topicPostnikov tower and coskeleton
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Urs
- Last Active Aug 11th 2011

added remarks about the relation of Postnikov towers to coskeletons to

- Discussion Type
- discussion topicContents of contents
- Category Latest Changes
- Started by Andrew Stacey
- Comments 10
- Last comment by zskoda
- Last Active Aug 11th 2011

I created contents of contents. I find I’m losing sight of what’s in the nLab since there’s so much of it. Urs (and others) very helpfully go around putting in these “contents” links, but even then you have to be in a section to know that it’s there, so I slurped through the database and extracted all the “contents” pages and stuck them in a single page (via includes). It’s not sorted, but my idea is to update this from the database rather than revising it by hand.

It’s just a first idea at getting some sort of overview; I imagine that this sort of thing can be done much better with some sort of graph showing how the pages link together, but this was quick and easy.

- Discussion Type
- discussion topicabsolute de Rham cohomology
- Category Latest Changes
- Started by zskoda
- Comments 2
- Last comment by zskoda
- Last Active Aug 10th 2011

New stubs absolute de Rham cohomology, L-function, prompted by one answer to my MathOverflow question and having just basic links. By the way, the link to the pdf file of a Kapranov’s article listed at de Rham complex does not seem to work.

- Discussion Type
- discussion topicK3 surfaces
- Category Latest Changes
- Started by hilbertthm90
- Comments 3
- Last comment by zskoda
- Last Active Aug 10th 2011

I made a very, very brief start to K3 surfaces

- Discussion Type
- discussion topicLocally additive spaces and kinematic tangent space
- Category Latest Changes
- Started by Andrew Stacey
- Comments 9
- Last comment by Andrew Stacey
- Last Active Aug 10th 2011

New pages:

locally additive space: Something I’ve been musing on for a bit: inside all these “categories of smooth object” then we have the category of manifolds sitting as a nice subcategory, but that doesn’t give a very nice

*intrinsic*definition of a “manifold”. By that I mean that suppose you knew a category of smooth spaces and took that as your starting point, could you figure out what manifolds were without knowing the answer in advance? “locally additive spaces” are an attempt to characterise manifolds intrinsically.kinematic tangent space: Once out beyond the realm of finite dimensional manifolds, the various notions of tangent space start to diverge and so each acquires a name.

*kinematic*refers to taking equivalence classes of curves. There’s a bit of an overlap here with some of the stuff on Frölicher spaces, but this applies to any (cartesian closed, cocomplete) category of smooth spaces.

Apart from a few little tweaks to do with wikilinks and entities, these were generated by my LaTeX-to-iTeX package. References and all.

- Discussion Type
- discussion topicsimplicial groupoid
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Aug 9th 2011

added looping/delooping statements to model structure on simplicial groupoids and model structure on presheaves of simplicial groupoids.

**Question**: Suppose we take the catgeory of genuine simplicial groupoids $Grpd^\Delta$ (no restriction on the simplicial set of objects). Does $\bar W : Grpd^\Delta \to sSet$ send degreewise fibrations to fibrations?

- Discussion Type
- discussion topicsimplicial Lie algebra
- Category Latest Changes
- Started by Urs
- Comments 4
- Last comment by Urs
- Last Active Aug 8th 2011

am starting an entry simplicial Lie algebra

- Discussion Type
- discussion topicdescent of affine schemes
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active Aug 6th 2011

New entry descent of affine schemes: the fibered category of affine morphisms (SGA I.8.2 th.2.1) satisfies effective descent along any fpqc morphism. This fact is harder than the descent for quasicoherent sheaves of $\mathcal{O}_X$-modules.

- Discussion Type
- discussion topicPoisson sigma-model
- Category Latest Changes
- Started by Urs
- Comments 8
- Last comment by zskoda
- Last Active Aug 6th 2011

stub for Poisson sigma-model. Needs references.

- Discussion Type
- discussion topicmatroid
- Category Latest Changes
- Started by zskoda
- Comments 2
- Last comment by zskoda
- Last Active Aug 5th 2011

- Discussion Type
- discussion topicfinitary vs infinitary extensive
- Category Latest Changes
- Started by Mike Shulman
- Comments 2
- Last comment by Todd_Trimble
- Last Active Aug 5th 2011

The discussion about the finitary vs infinitary case at connected object made me realize that something analogous could be said about finitary vs infinitary extensive categories themselves. I added a remark along those lines to extensive category.

- Discussion Type
- discussion topicconnected object
- Category Latest Changes
- Started by Todd_Trimble
- Comments 9
- Last comment by Mike Shulman
- Last Active Aug 4th 2011

In response to a very old query at connected object, I gave a proof that in an infinitary extensive category $C$, that an object $X$ is connected iff $\hom(X, -): C \to Set$ merely preserves binary coproducts.

The proof was written in classical logic. If Toby would like to rework the proof so that it is constructively valid, I would be delighted.

- Discussion Type
- discussion topicDoctrines, theories and semantics
- Category Latest Changes
- Started by fpaugam
- Comments 55
- Last comment by fpaugam
- Last Active Aug 1st 2011

- I have updated the doctrine part by a new n-categorical definition of doctrine, that i use in my book on QFT.

It is more flexible (even if more naive) than the classical notion of doctrine (2-adjunction on Cat, say), because

it contains all kinds of higher categorical operads, properads, etc... in a logical categorical spirit (theories

an semantics).

I guess some similar ideas were already known to specialist (street? etc...), but it makes things confortable

to give a nave to the general notion, in particular for pedagogical purpose and presenting things to non-specialists.

- Discussion Type
- discussion topicDieudonne module
- Category Latest Changes
- Started by hilbertthm90
- Comments 1
- Last comment by hilbertthm90
- Last Active Jul 31st 2011

I created Dieudonne module. What is the policy on accents. Technically this should be written Dieudonné module everywhere. There is a redirect. I also defined this in the affine case, but I’m pretty sure if you replace “affine” by “flat” everything should work still.

- Discussion Type
- discussion topicunitary representation of the super Poincaré group
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 31st 2011

started unitary representation of the super Poincaré group – the super-analog of unitary representation of the Poincaré group – so far mainly in order to record some references.

Also created a stub super Poincaré group

- Discussion Type
- discussion topicSpecker sequences
- Category Latest Changes
- Started by TobyBartels
- Comments 1
- Last comment by TobyBartels
- Last Active Jul 29th 2011

I wrote Specker sequence, a topic in computability theory that also has applications to constructivism.

- Discussion Type
- discussion topicCartier module
- Category Latest Changes
- Started by hilbertthm90
- Comments 6
- Last comment by hilbertthm90
- Last Active Jul 29th 2011

I’ve started the page Cartier module.

- Discussion Type
- discussion topicBohrification
- Category Latest Changes
- Started by Urs
- Comments 20
- Last comment by Urs
- Last Active Jul 29th 2011

created an entry Bohrification

- Discussion Type
- discussion topictwisted spin structure
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 29th 2011

started an entry twisted spin structure. So far the main point is to spell out the general abstract definition and notice that this is what Murray-Singer’s “spin gerbes” are models of.

- Discussion Type
- discussion topicproset and poset
- Category Latest Changes
- Started by Urs
- Comments 7
- Last comment by TobyBartels
- Last Active Jul 29th 2011

I added remarks on Cauchy completion to the Properties-section both at proset and poset.

Also made more explicit at poset the relation to prosets.

I notice that at proset there is a huge discussion section. It would be nice if those involved could absorb into the main text whatever stable insight there is, and move the remaining discussion to the nForum here.

- Discussion Type
- discussion topicWitt cohomology
- Category Latest Changes
- Started by hilbertthm90
- Comments 1
- Last comment by hilbertthm90
- Last Active Jul 29th 2011

I created the page Witt Cohomology.

- Discussion Type
- discussion topicGrothendieck topos
- Category Latest Changes
- Started by Urs
- Comments 5
- Last comment by Urs
- Last Active Jul 28th 2011

Todd has added to Grothendieck topos the statement and proof that any such is total and cototal (and I have added to adjoint functor theorem the statement that this implies that all (co)limit preserving functors between sheaf toposes have (right)left adjoints).

I notice that we should really merge Grothendieck topos with category of sheaves. But I don’t have the energy to do this now.

- Discussion Type
- discussion topicadjoint functor theorem
- Category Latest Changes
- Started by Urs
- Comments 12
- Last comment by Urs
- Last Active Jul 28th 2011

I edited adjoint functor theorem a bit: gave it an Idea-section and a References-section and, believe it or not, a toc.

Then I opened an Examples-section and filled in what I think is an instructive simple example: the right adjoint for a colimit preserving functor on a category of presheaves.