Not signed in (Sign In)

A discussion forum about contributions to the nLab wiki and related areas of mathematics, physics, and philosophy.

Want to take part in these discussions? Sign in if you have an account, or apply for one below

2-category 2-category-theory abelian-categories adjoint algebra algebraic algebraic-geometry algebraic-topology analysis analytic-geometry arithmetic arithmetic-geometry book bundles calculus categorical categories category category-theory chern-weil-theory cohesion cohesive-homotopy-type-theory cohomology colimits combinatorics complex complex-geometry computable-mathematics computer-science constructive cosmology deformation-theory descent diagrams differential differential-cohomology differential-equations differential-geometry digraphs duality elliptic-cohomology enriched fibration foundation foundations functional-analysis functor gauge-theory gebra geometric-quantization geometry graph graphs gravity grothendieck group group-theory harmonic-analysis higher higher-algebra higher-category-theory higher-differential-geometry higher-geometry higher-lie-theory higher-topos-theory homological homological-algebra homotopy homotopy-theory homotopy-type-theory index-theory integration integration-theory internal-categories k-theory lie-theory limits linear linear-algebra locale localization logic mathematics measure measure-theory modal modal-logic model model-category-theory monad monads monoidal monoidal-category-theory morphism motives motivic-cohomology nlab noncommutative noncommutative-geometry number-theory of operads operator operator-algebra order-theory pages pasting philosophy physics pro-object probability probability-theory quantization quantum quantum-field quantum-field-theory quantum-mechanics quantum-physics quantum-theory question representation representation-theory riemannian-geometry scheme schemes set set-theory sheaf simplicial space spin-geometry stable-homotopy-theory stack string string-theory superalgebra supergeometry svg symplectic-geometry synthetic-differential-geometry terminology theory topology topos topos-theory tqft type type-theory universal variational-calculus

Vanilla 1.1.10 is a product of Lussumo. More Information: Documentation, Community Support.

If you want to take part in these discussions either sign in now (if you have an account), apply for one now (if you don't).

- Discussion Type
- discussion topicConcrete and abstract structures.
- Category Latest Changes
- Started by TobyBartels
- Comments 4
- Last comment by Mirco Richter
- Last Active Jul 28th 2012

Concrete, abstract: group actions, groups; concrete categories, categories; Cartesian spaces, vector spaces; von Neumann algebras, $W^*$-alebras; material sets, structural sets; etc. At concrete structure.

- Discussion Type
- discussion topiccategory with star-morphisms
- Category Latest Changes
- Started by porton
- Comments 16
- Last comment by porton
- Last Active Jul 26th 2012

- I've added the following two new (related) pages to nLab wiki:

category with star-morphisms

abrupt category

These concept aroused in my research of what I call cross-composition product (a stub in the wiki).

See my research, especially this manuscript for details and examples.

- Discussion Type
- discussion topicoverview over group schemes
- Category Latest Changes
- Started by Stephan A Spahn
- Comments 1
- Last comment by Stephan A Spahn
- Last Active Jul 21st 2012

I wrote an overview over some constructions on- and examples of group schemes.

- Discussion Type
- discussion topicnotes from String-Math 2012
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 21st 2012

as some of you will have seen, I had spent part of the last week with attending talks at

*String-Math 2012*and posting some notes about these, to the $n$Café (here). For many of these notes I added material to existing $n$Lab entries (mostly just references) or created $n$Lab entries (mostly just stubs).But since at the same time I was also finalizing the writup of an article as well as doing yet some other things, the whole undertaking was a bit time-pressured. As a result, I decided it would be too much to announce every single $n$Lab edit that I did here on the $n$Forum.

So I ask you for understaning that hereby I just collectively announce these edits here: those who care should please scan through the list of blue links here and see if they spot pointers to $n$Lab entries where they would like to check out the recent edits.

I think I can guarantee, though, that in all cases I did edits that should be entirely uncontroversial, their main defect being that in many cases they leave one wish for more exhaustive discussion.

- Discussion Type
- discussion topiccategorification in representation theory
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 17th 2012

added some references by Catahrina Stroppel at the end of

*categorification in representation theory*(also added the words “representation theory” to the entry itself :-)

- Discussion Type
- discussion topicNew page: [[structured set]]
- Category Latest Changes
- Started by TobyBartels
- Comments 4
- Last comment by TobyBartels
- Last Active Jul 16th 2012

I've been meaning to write this for a while. Now I need to look at Bourbaki this weekend to explain their approach.

- Discussion Type
- discussion topicTowards QFT
- Category Latest Changes
- Started by fpaugam
- Comments 2
- Last comment by TobyBartels
- Last Active Jul 15th 2012

- Hi guys,

The situation with my habilitation has been resolved.

I decided to postone it to more favourable times.

You can refer to my book and link it.

Best,

Frédéric

- Discussion Type
- discussion topicRadon–Nikodym derivatives.
- Category Latest Changes
- Started by TobyBartels
- Comments 2
- Last comment by hilbertthm90
- Last Active Jul 14th 2012

New page: Radon–Nikodym derivative.

- Discussion Type
- discussion topicPure and mixed states.
- Category Latest Changes
- Started by TobyBartels
- Comments 3
- Last comment by TobyBartels
- Last Active Jul 14th 2012

I moved some material from state to create pure state (redirect from mixed state).

- Discussion Type
- discussion topicquantum affine algebra
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 13th 2012

I have created a stub

*quantum affine algebra*as a means to collect some references, alluded to here.If there is any expert on the matter around, he or she should please feel invited to add an illuminating Idea-section to the entry.

- Discussion Type
- discussion topiccalculus, types and trees
- Category Latest Changes
- Started by Stephan A Spahn
- Comments 10
- Last comment by Mike Shulman
- Last Active Jul 12th 2012

I created types and calculus and seven trees in one. Both entries as yet contain just references.

It would be nice to have more articles expanding on the reltion of calculus and (higher) category theory /type theory.

- Discussion Type
- discussion topicconjugacy class
- Category Latest Changes
- Started by Urs
- Comments 9
- Last comment by zskoda
- Last Active Jul 11th 2012

created

*conjugacy class*

- Discussion Type
- discussion topicmetaplectic representation
- Category Latest Changes
- Started by zskoda
- Comments 2
- Last comment by Urs
- Last Active Jul 11th 2012

New stub metaplectic representation, for now containing only some references.

- Discussion Type
- discussion topicdiagram of LCTVS properties
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 10th 2012

Maybe I am not searching correctly, but it seems to me that until 2 minutes ago the rather remarkable

*diagram of LCTVS properties*was linked to from exactly none non-meta $n$Lab page. It was effectively invisble unless one explicitly searched for “SVG”.Let me know if there is a reason for it remaining invisible. Assuming that there isn’t, I have now added it to

*locally convex space*and to*functional analysis - contents*(which I restructured slightly, moving the*two*such overview diagrams prominently to the top, where they can be recognized as what they are).

- Discussion Type
- discussion topicmetaplectic structure
- Category Latest Changes
- Started by Urs
- Comments 8
- Last comment by Urs
- Last Active Jul 10th 2012

stub for

*metaplectic structure*

- Discussion Type
- discussion topicreduction of structure groups
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Jul 10th 2012

added a few more

*Examples*to*reduction of structure groups*.

- Discussion Type
- discussion topicmetalinear structure
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 10th 2012

created

*metalinear structure*. Added it to*square roots of line bundles - table*. Linked to it from Theta characteristic and so forth.

- Discussion Type
- discussion topicmetalinear group
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 10th 2012

started

*metalinear group*

- Discussion Type
- discussion topic(infinity,1)-pullback
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 10th 2012

Danny Stevenson was so kind and completed spelling out the proof of the pasting law for $\infty$-pullbacks here at

*(infinity,1)-pullback*.

- Discussion Type
- discussion topicKirchhoff's laws
- Category Latest Changes
- Started by Urs
- Comments 7
- Last comment by Urs
- Last Active Jul 10th 2012

I created a stub for

*Kirchhoff’s laws*to go with the $n$Café-discussion here. Maybe somebody feels like expanding it, I don’t really have the time for this right now.

- Discussion Type
- discussion topicsymplectic reduction - table
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 5th 2012

I have created an entry

*symplectic reduction - table*and included it into relevant entries.

- Discussion Type
- discussion topicHamiltonian action
- Category Latest Changes
- Started by Urs
- Comments 12
- Last comment by Urs
- Last Active Jul 4th 2012

I wrote

*Hamiltonian action*.I tried to say precisely what the action is by. In the literature (but also in a previous version of our

*moment map*entry) there is often (for instance on Wikipedia, but also in many other sources) an imprecise (not to say: wrong) statement, where an action by Hamiltonian vector fields is not distinguished from one*by Hamiltonians*.

- Discussion Type
- discussion topicsymplectic reduction
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by jim_stasheff
- Last Active Jul 4th 2012

I have decided to splitt off a stand-alone entry

*symplectic reduction*from*BRST-BV formalism*(which used to be the redirect). Still just a stub. Lots of material and references still needs to be copied or moved from the latter to the former.

- Discussion Type
- discussion topicgeometric quantization - contents
- Category Latest Changes
- Started by Urs
- Comments 14
- Last comment by Urs
- Last Active Jul 3rd 2012

I have started a table

*geometric quantization - contents*and added it as a floating TOC to the relevant entries.Parts of this remain a bit unfinished. The $n$Lab is pretty much unusable in the last hours. I’ll give up now, have wasted too much time with this already. Maybe later it has recovered.

- Discussion Type
- discussion topicgeometric quantization extensions - table
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 2nd 2012

I have created a table

*geometric quantization extensions - table*.Mostly I have been editing aspects of the entries listed in this table here and there. Also included the table in the Properties-section of various of these entries.

- Discussion Type
- discussion topicrepresentation theory - contents
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jul 2nd 2012

I have started a table of contents

*representation theory - contents*(based on the link list at*representation theory*) and started adding it as a floating table of contents to relevant entries.But I ran out of steam before being entirely satisfied with the result.

- Discussion Type
- discussion topicIHL manifold
- Category Latest Changes
- Started by Urs
- Comments 10
- Last comment by DavidRoberts
- Last Active Jul 2nd 2012

stub for

*IHL manifold*.

- Discussion Type
- discussion topicinfinite dimensional manifold
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Tim_Porter
- Last Active Jul 1st 2012

I realized that

*infinite-dimensional manifold*and all entries related to it are still very stubby.I am not attempting to change this now, but I thought a first step to make progress is to list what stubs we actually have. I came up with this list

and added it to

*manifolds and cobordisms - contents*and made sure that all these entries point to each other.Now somebody go and add more content to these entries! :-)

- Discussion Type
- discussion topichigher geometric quantization
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jun 30th 2012

In case you see the activity in the logs and are wondering, I should say that I have been working on a new entry

*higher geometric quantization*(that used to redirect to n-plectic infinity-groupoid).I have started adding some survey-tables. But not done yet with the entry as a whole.

- Discussion Type
- discussion topichomotopy-homology-cohomology
- Category Latest Changes
- Started by Urs
- Comments 7
- Last comment by Urs
- Last Active Jun 30th 2012

I felt it was time for another table:

*homotopy-homology-cohomology*The structure is just a first attempt, begun in a brief moment of leisure. I’ll try to think about how to improve on it. Let me know what you think.

I have started to include this into relevant entries.

- Discussion Type
- discussion topicassociated oo-bundle
- Category Latest Changes
- Started by Urs
- Comments 10
- Last comment by Urs
- Last Active Jun 29th 2012

started an entry associated infinity-bundle

in order to summarize the thesis by Matthias Wendt on associated $\infty$-bundles in arbitrary $(\infty,1)$-toposes, generalizing the classical old results by Stasheff and May from $\infty Grpd$.

Also added some remarks on the relation to the discussion at principal infinity-bundle. Hopefully to be continued tomorrow.

- Discussion Type
- discussion topicstructures in a cohesive oo-topos
- Category Latest Changes
- Started by Urs
- Comments 12
- Last comment by Urs
- Last Active Jun 28th 2012

I am now going through the section Structures in a cohesive oo-topos and polish and expand the discussions there.

First thing I went through is the subsection Geometric homotopy and Galois theory. It gives the definition of the fundamental $\infty$-groupoid functor, a proposition on its consistency (which we had mentioned elsewhere), the definition of locally constant $\infty$-stacks in the sense of $Disc Aut(F)$-principal $\infty$-bundles, and then the central theorem of Galois theory, proven by applying the $\infty$-Yoneda lemma iteratively.

(This is material appearing in one form or other in other entries and at this point does not invoke the $\infty$-locality, but I want to have here all in one place a nice comprehensive discussion of the whole situation in a cohesive $\infty$-topos.)

- Discussion Type
- discussion topicNekrasov functions and AGT conjecture
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active Jun 25th 2012

Stubs (just to provide the links to the basic references so far) for Nekrasov functions, AGT conjecture, and, less related, spectral networks.

- Discussion Type
- discussion topicsuppressed reference
- Category Latest Changes
- Started by fpaugam
- Comments 1
- Last comment by fpaugam
- Last Active Jun 22nd 2012

- Hi guys,

I suppressed the reference to my course on global analytic geometry. These notes were not well written enough and i put them into the basket. Please, don't pull back the reference.

Cheers,

Fred

- Discussion Type
- discussion topicminimal fibration
- Category Latest Changes
- Started by Stephan A Spahn
- Comments 8
- Last comment by Urs
- Last Active Jun 21st 2012

I created minimal fibration which could be merged with minimal Kan fibration. The idea-section says that this notion is needed to give a well defined notion of n-category. However there are other applications which I didn’t mention.

- Discussion Type
- discussion topicring extension
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Tim_Porter
- Last Active Jun 20th 2012

started a stub for

*ring extension*to go with the Café-discussion here.

- Discussion Type
- discussion topiccarrying
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Mike Shulman
- Last Active Jun 16th 2012

created

*carrying*

- Discussion Type
- discussion topicinfinite-dimensional Chern-Simons theory
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by DavidRoberts
- Last Active Jun 15th 2012

just heard an interesting talk by Steven Rosenberg on CS invariants on infinite-dimensional manifolds. So I created an entry

*infinite-dimensional Chern-Simons theory*in order to record some references

- Discussion Type
- discussion topicover quasi-category
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Jun 14th 2012

and handful of further details at over quasi-category

- Discussion Type
- discussion topicOnline Resources, math institutions, math archives
- Category Latest Changes
- Started by zskoda
- Comments 6
- Last comment by zskoda
- Last Active Jun 9th 2012

I made redirects to Online Resources, namely the math blogs, online resources. Before we were complaining to Online Resources for many reasons including that it is not of all resources but only of blogs and wikis in relevant areas. No list of main institutes and archives like arXiv, numdam, jstor etc. there. As the list is long, and hard to scroll, I suggest not to add those to the current page. I think we should rename the current page to math blogs eventually and keep Online Resources (especially because of John's reference in his AMS Notices paper) as a redirect and create new pages for other stuff as well as organize the whole system around a top page math resources which will link to math blogs, math archives, math institutions (and maybe more) as well as very comprehensive central AMS-kept list of math resources.

I know it is not only about math here, but math is a short abbreviation for page name.

Up to now I have realized a large part of an above program, see math archives, math institutions and the supposed top resource page math resources, except that I was cautious not to rename the page Online Resources as people may disagree even with keeping the old redirect and because it may be tricky with the cache bug, while the page is of central importance. I think it would be useful if the pages like math institutions and the top page math resources stay not much longer than they are now, to have quick links and nice readability/visibility. This is the most effective organization, I think. For smaller institutions societies and alternative small lists of resources, it is better to go via links at AMS, EMS and IMU which are already efefctively linked. We can not do better there than those societies do, apart from listing few extra

**main**resources of our main interest. We can have a separate page just for categories or some other things. But the list of blogs is of different character, unlike going to AMS page or jstor, one does not need to go that quickly through list of less-organized stuff like blogs. So the blog list math blogs should grow indefinitely...I have chosen plural as before in these pages, without singular redirect at the moment.

- Discussion Type
- discussion topicRelation between type theory and category theory
- Category Latest Changes
- Started by DavidRoberts
- Comments 4
- Last comment by Mike Shulman
- Last Active Jun 6th 2012

Added the reference

Maria Emilia Maietti, Modular correspondence between dependent type theories and categories including pretopoi and topoi, Math. Struct. in Comp. Science (2005), vol. 15, pp. 1089–1149

- Discussion Type
- discussion topicfiber integration in ordinary differential cohomology
- Category Latest Changes
- Started by Urs
- Comments 4
- Last comment by Urs
- Last Active Jun 6th 2012

I am starting fiber integration in ordinary differential cohomology, but just two references so far (are there more? the project by Baer seems not to have borne any article, it seems)

- Discussion Type
- discussion topicEnergy
- Category Latest Changes
- Started by TobyBartels
- Comments 16
- Last comment by TobyBartels
- Last Active Jun 5th 2012

I have created energy ex nihilo. Take that, Hermann von Helmholtz!

- Discussion Type
- discussion topicdisplay of a p-divisible group
- Category Latest Changes
- Started by Stephan A Spahn
- Comments 1
- Last comment by Stephan A Spahn
- Last Active Jun 4th 2012

I started display of a p-divisible group.

- Discussion Type
- discussion topicKervaire invariant
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by DavidRoberts
- Last Active Jun 4th 2012

I may have written something at

*Kervaire invariant*, but it is at best a stub for the moment

- Discussion Type
- discussion topicBorn-Oppenheimer approximation
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by zskoda
- Last Active Jun 3rd 2012

I happened to have need for a stub

*Born-Oppenheimer approximation*

- Discussion Type
- discussion topicKac-Moody group
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Urs
- Last Active Jun 3rd 2012

stub for

*Kac-Moody group*, in order to record some references

- Discussion Type
- discussion topicformal loop space
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Jun 2nd 2012

stub for

*formal loop space*, just so as to record Kapranov’s article. Eventually it would be great if the $n$Lab had some paragraphs on the geometric link with the chiral de Rham complex, vertex operator algebras, etc. But I can’t do it now.

- Discussion Type
- discussion topictype theory
- Category Latest Changes
- Started by Mike Shulman
- Comments 48
- Last comment by Mike Shulman
- Last Active Jun 2nd 2012

Some reorganization and added material at type theory. In particular, I added some of the basic syntax of type theories, and also some comments about extensional vs. intensional type theories.

- Discussion Type
- discussion topicpretopos completion
- Category Latest Changes
- Started by zskoda
- Comments 3
- Last comment by zskoda
- Last Active May 31st 2012

- Discussion Type
- discussion topicelementary and abstract elementary classes
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active May 31st 2012

Stubs for elementary class of structures (with redirect elementary class) and a stub for far more general Saharon Shelah’s notion of an abstract elementary class.

- Discussion Type
- discussion topicamalgamation
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active May 31st 2012

Stub for amalgamation.

- Discussion Type
- discussion topicfirst-order theory
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active May 31st 2012

We already have the entry predicate calculus (or first-order logic); I have created separate stub first-order theory, almost empty now, and which could just have been redirect to first-order logic, though I think eventually it would be good to have them separate, as under first-order theories version one could list lots of standard examples of first-order theories, what does not really fit into predicate logic entry, where one should really deal more with predicate calculus. PLus one should do other views of first-order theories.

- Discussion Type
- discussion topicDixmier-Douady class
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by Urs
- Last Active May 30th 2012

created Dixmier-Douady class

- Discussion Type
- discussion topicconstructible set
- Category Latest Changes
- Started by zskoda
- Comments 4
- Last comment by TobyBartels
- Last Active May 30th 2012

Stub constructible set, not yet precise (e.g. the universe is not a Boolean algebra as it is a proper class), but gives idea what we could work on in the entry.

- Discussion Type
- discussion topicalgebraic dynamics
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active May 29th 2012

Changes to dynamical system. Stub for algebraic dynamics.

- Discussion Type
- discussion topicstructure
- Category Latest Changes
- Started by zskoda
- Comments 15
- Last comment by zskoda
- Last Active May 28th 2012

New entry structure but the $n$Lab is down so I save here the final version of editing, which is probably lost in $n$Lab:

The concept of a

*structure*is formulated as the basic object of mathematics in the work of Bourbaki.In model theory, a structure of a language $L$ is the same as model of $L$ with empty set of extra axioms. Given a first-order language $L$, which consists of symbols (variable symbols, constant symbols, function symbols and relation symbols including $\epsilon$) and quantifiers; a

**structure**for $L$, or $L$-structure is a set $M$ with an**interpretation**for symbols:if $R\in L$ is an $n$-ary relation symbol, then its interpretation $R^M\subset M^n$

if $f\in L$ is an $n$-ary function symbol, then $f^M:M^n\to M$ is a function

if $c\in L$ is a constant symbol, then $c^M\in M$

Interpretation for an $L$-structure inductively defines an interpretation for well-formed formulas in $L$. We say that a sentence $\phi\in L$ is true in $M$ if $\phi^M$ is true. Given a theory $(L,T)$, which is a language $L$ together with a given set $T$ of sentences in $L$, the interpretation in a structure $M$ makes those sentences true or false; if all the sentences in $T$ are true in $M$ we say that $M$ is a model of $(L,T)$.

Some special cases include

**algebraic structures**, which is usually defined as a structure for a first order language with equality and $\epsilon$-relation both with the standard interpretation, no other relation symbols and whose function symbols are interpreted as operations of various arity. This is a bit more general than an**algebraic theory**as in the latter, one needs to have free algebras so for example fields do not form an algebraic theory but are the algebraic structures for the theory of fields.In category theory we may talk about functor forgetting structure (formalizing an intuitive, related and in a way more general sense), see

- Discussion Type
- discussion topicalgebraic group
- Category Latest Changes
- Started by Stephan A Spahn
- Comments 5
- Last comment by zskoda
- Last Active May 28th 2012

I added in the definition of algebraic group the requirement ”field” into ”algebraically closed field”. Alternatively one could omit ”field” in the definition at all since this is implicit in ”variety”.

- Discussion Type
- discussion topicn-angulated category
- Category Latest Changes
- Started by zskoda
- Comments 1
- Last comment by zskoda
- Last Active May 28th 2012

- Discussion Type
- discussion topicCalculus is Topology
- Category Latest Changes
- Started by isomorphisms
- Comments 8
- Last comment by DavidRoberts
- Last Active May 28th 2012

- I recently came across some interesting ideas at inperc.com/wiki/index.php?title=Calculus_is_topology which might be incorporable into the nLab wiki -- although I'm not sure exactly where.