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- Discussion Type
- discussion topiccircle of entries surrounding infinitesimal neighour
- Category Latest Changes
- Started by Urs
- Comments 13
- Last comment by Eric
- Last Active Oct 9th 2009

started infinitesimal neighbour and began creating a circle of entries surrounding this:

infinitesimal path infinity-groupoid in a smooth topos; path infinity-groupoid in a smooth topos; simplex in a lined topos

This is heading in the direction of giving a full discussion of for X a microlinear space, mentioned presently already at differential forms in synthetic differential geometry. I though i could just point to the literature for that, but not quite, apparently. Anders Kock discusses this for X a "formal manifold", an object with a cover by Kock-Lawvere vector spaces. But it should work a bit more generally using microlinear spaces, as indicated in the appendix of Models for Smooth Infinitesimal Analysis. There is an obvious general-nonsense definition wich I discuss, but I need yet to insert discussion of that and how this reproduces Kock's definition (but I think it does).

It has been an esteemed insight for me that the best way to think of all these constructions of "combinatorial differential forms" (still have to expand the discussion of those at differential forms in synthetic differential geometry) is by organizing them into their natural simplicial structures and then noticing that the model category structure available in the background allows us to think of the resulting simplicial objects in the topos as interna oo-groupoids. I think this must clearly the nLab way of thinking about this, so I created entries with the respective titles.

You may have noticed that on my personal web I am developing the further step that goes from (infinitesimal) path oo-groupoids of objects in a 1-topos to (infinitesimal) path oo-groupoids of objects in a "smooth (oo,1)-topos". I don't want to impose that fully (oo,1)-material on the main nLab as yet, before this is further developed, but the bits now added that just have oo-groupoids of paths in a 1-topos object is straightforward enough to warrent discussion here. i think.

While working on this, I also did various minor edits on the synthetic differential geometry context cluster, such as

splitting off lined topos from smooth topos

rewriting the introduction at Models for Smooth Infinitesimal Analysis (the previous remarks are by now better explained in the corresponding sub-entries and the new summary is supposed to get the main message of the book across better). Also created section headers there for each of the single models, which I hope I'll eventually describe there in a bit more detail each. Those toposes and they have there are mighty cool, I think, giving not only a well-adapted model for SDG but on top of that an implementation of nonstandard analysis, and of distribution theory. I am thinking that the toposophers among my co-laborants might enjoy looking at their smooth natural number object in a bit more. It's fun and seems to be much more relevant than seems to be widely appreciated.

Notice that at simplex in a lined topos I am asking for a reference. It's this standard construction of simplices as collpsed cylinders on lower dim simplicies. I don't think I should re-invent that wheel. What's the canonical reference for this general construction?

Finally please notice that all entries mentioned above are more or less stubby for the moment and need more work. But I thought it was about time to drop a latest-changes alert here now, before waiting longer.

- Discussion Type
- discussion topicsemiotic information
- Category Latest Changes
- Started by JonAwbrey
- Comments 1
- Last comment by JonAwbrey
- Last Active Oct 9th 2009

Inspired by David Corfield's blog entries on information geometry, I added a 'blink on semiotic information that I hope to develop over time.

- Discussion Type
- discussion topicpolsihing: infinitesimal object
- Category Latest Changes
- Started by Urs
- Comments 5
- Last comment by Urs
- Last Active Oct 9th 2009

I tried to prettify the entry infinitesimal object:

I expanded and restructured the "Idea" section. I tried to emphasize the point that Lawvere's axioms are the right general point of view and that the wealth of constructions in algebraic geoemtry is, from this abstract nonsense point of view, to be regarded as taking place in a

*model*for these axioms. I cite Anders Kocks's latest book for the most simple minded version of how algebraic geometry is a model for sdg, but I think this goes through for more sophisticated versions, too. It would be nice to discuss this eventually elsewhere in some entry on "algebraic geometry as models for smooth toposes".I have also tried to subsume the approach of nonsstandard analysis as yet another special case of Lawvere's general axioms, by referring to Moerdijk-Reyes' topos and in which "objects of invertible infinitesimals/infinities" exist and model nonstandard analysis. This, too, would deserve being expanded on further, but I am thinking for the nLab this abstract-nonsense-first perspective is the right one.

Then I inserted some links to the now separate infinitesiaml interval object that I am still working on. I also changed the ideosyncratic terminology "infinitesimal k-cube" and "infinitsimal k-disk" to "cartesian product of inf. intervals" and "k-dimensional infinitesimal interval". Anders Kock calls the latter a "monad", following Leibniz, but I am hesitating to overload monad this way, given that Kock's use of it doesn't seem to be wide spread.

- Discussion Type
- discussion topicinfinitesimal singular simplicial object
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 9th 2009

Spent all day with being distracted from this single thing that I planned to finish this morning: now at least a rough sketch is done

at infinitesimal interval object in the last section with the long section name I mean to define the "infinitesimal singular simplicial complex" in a new way.

Anders Kock defines this guy for "formal manifols", roughly, for spaces that have an atlas by vector spaces. There the simple definition applies recalled at infinitesimal singular simplicial complex.

But there should be a definition for arbitrary microlinear spaces, And it should be such that it is almost manfestly the inifnitesimal version of the path oo-groupoid construction described at interval object. This is what I am aiming to describe here.

One crucial thing is that we want that morphisms out of the objects in degree k of the infinitesimal singular simplicial complex that vanish on degenerate k-simplices are automatically fiberwise skew-linear. Seeing this in the construction that I am presenting there seems to be different to the way Anders Kock describes it in the other setup. This is the main thing I need to check again when i am more awake.

- Discussion Type
- discussion topichomotopy - contents
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 8th 2009

I created homotopy - contents and added it as a floating table of contents to relevant entries.

This was motivated from the creation of infinitesimal interval object and the desire to provide a kind of map that shows how that concept sits in the greater scheme of things. The homotopy - contents was supposed to be a step in that diretion.

I really meant to expand at infinitesimal interval object on something I already meant to provide yesterday, but then that table of contents kept distracting me, and the fact that I came across mapping cone while editing it and felt compelled to improved that entry first, which I did

- Discussion Type
- discussion topicinterval object: induced path oo-groupoids
- Category Latest Changes
- Started by Urs
- Comments 6
- Last comment by Urs
- Last Active Oct 8th 2009

I removed my recent material at simplex in a lined topos and instead inserted this now, expanded, at

where it belongs. There is now a section there that discusses how interval objects gives rise to cubical and simplicial path oo-categories.

The proposition I state there I have carefully checked. Should be correct. But haven't typed the proof, it doesn't lend itself to being typed (straightforward but tedious, as one says).

But if it is indeed correct, this must be standard well-known stuff. Does anyone have a reference?!

I also restructured and edited the rest of the entry a bit, trying to make it a bit nicer. THis entry deserves more attention, it is a pivotal entry.

Tomorrw when I am more awake I'll remove simplex in a lined topos and redirect links to it suitably to interval oject.

- Discussion Type
- discussion topichowto and faq
- Category Latest Changes
- Started by Mike Shulman
- Comments 1
- Last comment by Mike Shulman
- Last Active Oct 8th 2009

I moved the instructions on making diagrams from FAQ to HowTo, which seemed a better fit, and added a comment about including images as another method. I also made the individual questions at FAQ into ### headers, rather than numbered lists, so that they would show up in the automatic table of contents.

- Discussion Type
- discussion topicsmooth loci
- Category Latest Changes
- Started by Urs
- Comments 2
- Last comment by TobyBartels
- Last Active Oct 8th 2009

created stub for smooth loci

(should it be "smooth locus" instead?)

- Discussion Type
- discussion topicgeneralized smooth algebra
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Oct 7th 2009

sty addition to generalized smooth algebra: remark on terminology added and section on "internal definition" added.

planning to polish thinmgs later

- Discussion Type
- discussion topicIsrael Gelfand
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 7th 2009

created entry on Israel Gelfand with the material that John posted to the blog.

turns out the "Timeline" entry was already requesting it

- Discussion Type
- discussion topicmicrolinear space
- Category Latest Changes
- Started by Urs
- Comments 10
- Last comment by TobyBartels
- Last Active Oct 6th 2009

created microlinear space

One thing I might be mixed up above:

in the literature I have seen it seems to say that

$ X^D x_X X^D \simeq X^{D(2)}$

with

$ D(2) = { (x_1,x_2) \in R \times R | x_i x_j = 0} $.

But shouldn't it be

$ D(2)' = { (x_1,x_2) \in R \times R | x_i^2 = 0} $.

?

- Discussion Type
- discussion topicsynthetic differential geometry - contents
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 6th 2009

started synthetic differential geometry - contents and added it as floating table of contents to the relevant entries

- Discussion Type
- discussion topicinfinitesimal space
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 6th 2009

quick addition of "formal infinitesimal spaces" and Weil algebras to infinitesimal space

but am planning to polish this entry further later, it is a bit of a mess at the moment

- Discussion Type
- discussion topicintegration axiom
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 6th 2009

started integration axiom, but incomplete for the moment

at schreiber:integration of oo-Lie algebroid valued differential forms I am thinking about the higher version of this

- Discussion Type
- discussion topiccyclic things
- Category Latest Changes
- Started by Mike Shulman
- Comments 3
- Last comment by David_Corfield
- Last Active Oct 5th 2009

Looks like maybe Todd is right at cyclic order that the cycle category cannot be defined in that way.

- Discussion Type
- discussion topicAdded detexify to online resources
- Category Latest Changes
- Started by Andrew Stacey
- Comments 1
- Last comment by Andrew Stacey
- Last Active Oct 5th 2009

Just learnt about detexify from the Secret Blogging Seminar and thought it worth adding to the online resources page.

- Discussion Type
- discussion topicNew pages
- Category Latest Changes
- Started by TobyBartels
- Comments 3
- Last comment by TobyBartels
- Last Active Oct 5th 2009

- Discussion Type
- discussion topicnatural isomorphism
- Category Latest Changes
- Started by TobyBartels
- Comments 1
- Last comment by TobyBartels
- Last Active Oct 4th 2009

A little more detail at natural isomorphism, including when one can speak of the functor satisfying certain conditions.

- Discussion Type
- discussion topicsynthetic differential geometry
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 4th 2009

I worked on synthetic differential geometry:

I rearranged slightly and then expanded the "Idea" section, trying to give a more comprehensive discussion and more links to related entries. Also added more (and briefly commented) references. Much more about references can probably be said, I have only a vague idea of the "prehistory" of the subject, before it became enshrined in the textbooks by Kock, Lavendhomme and Moerdijk-Reyes.

Also, does anyone have an electronic copy of that famous 1967 lecture by Lawvere on "categorical dynamics"? It would be nice to have an entry on that, as it seems to be a most visionary and influential text. If I understand right it gave birth to topos theory, to synthetic differential geometry and all that just as a spin-off of a more ambitious program to formalize physics. If I am not mistaken, we are currently at a point where finally also that last bit is finding a full implmenetation as a research program.

- Discussion Type
- discussion topicsmooth topos
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 4th 2009

created smooth topos on the axioms on toposes used in synthetic differential geometry.

- Discussion Type
- discussion topichonoring Dubuc's work on synthetic differential geometry
- Category Latest Changes
- Started by Urs
- Comments 3
- Last comment by Urs
- Last Active Oct 4th 2009

By chance I came across an old CatTheory mailing list post by e. Dubuc, where he complains about how is work on SDG is not sufficiently recognized and asks people to speak of the "Dubuc topos".

I added a remark about this to synthetic differential geometry in the section on "Well adapted models".

- Discussion Type
- discussion topicI found another term for a [[tight relation]].
- Category Latest Changes
- Started by TobyBartels
- Comments 2
- Last comment by TobyBartels
- Last Active Oct 4th 2009

I found another term for a tight relation.

- Discussion Type
- discussion topicon moduli stack of elliptic curves
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 1st 2009

A few more sections at A Survey of Elliptic Cohomology - elliptic curves on

topol. invariants of the moduli stack of ell. curves

the compactified version

the definition of Gromov-Witten invariants

an example.

As before, this is raw material which I am thinking lends itself to be turned into entries.

- Discussion Type
- discussion topicsynthetic differential supergeometry
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 1st 2009

started synthetic differential supergeometry

so far just to record the (three!) references

- Discussion Type
- discussion topicpeople entries. D'Auria, Fre, Castellani, Sati
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 1st 2009

created entries for the following people

Riccardo D'Auria, Pietro Fre, Leonardo Castellani, Hisham Sati

not much there, yet, but these pages serve a purpose as listing the pages that link to them, which is useful I think

- Discussion Type
- discussion topicgroup object
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 1st 2009

Added to group object the Yoneda-embedding-style definition and added supergroup to the list of examples.

- Discussion Type
- discussion topicsupergroup
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Oct 1st 2009

Added the Yoneda-embedding way to talk about group objects and hence supergroups.

- Discussion Type
- discussion topicInstructions
- Category Latest Changes
- Started by TobyBartels
- Comments 5
- Last comment by TobyBartels
- Last Active Oct 1st 2009

A list of all recently edited entries can be seen at Recently Revised. But that list tends to contain lots of minor changes: it's not easy to spot the important ones. So, if you feel people's attention should be drawn to some changes you make, then please mention them

*here*. This way the rest of us can spot them, so we can learn what you know — and maybe make further improvements!- Go to the Latest Changes category in the Forum. If you are reading this on the forum, then you are there right now, or go to http://www.math.ntnu.no/~stacey/Vanilla/nForum/?CategoryID=5.
- Optionally, create an account (if you don't have one) and log in if necessary; this may be helpful but is not required.
- Hit ‘Start new discussion’ at the upper left.
- Choose a meaningful subject title, perhaps the name of the page you've edited or something like that.
- Write a brief message summarising your edit. Most of the formatting is the same as on the Lab if you choose the ‘Markdown+Itex’ radio button beneath the comment box.
- Please include a
`[[wikilink]]`

to the page or pages that you edited. - If you didn't log in, then please also include your name.
- Check Preview Post (or skip it), and then Start your discussion!

Questions? Ask them here!

- Discussion Type
- discussion topicString Lie 2-algebra
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Sep 30th 2009

created String Lie 2-algebra

- Discussion Type
- discussion topicexterior differential system
- Category Latest Changes
- Started by Urs
- Comments 1
- Last comment by Urs
- Last Active Sep 30th 2009

uploaded course notes by Bryant to exterior differential system (linked to from the Reference section)