# Start a new discussion

## Not signed in

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

## Site Tag Cloud

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

• Just in case you see me editing in the Recently Revised list and are wondering:

I have created and have started to fill some content into semiclassical state. But I am not done yet and the entry is not in good shape yet. So don’t look at yet it unless in a mood for fiddling and editing.

• I have started something at Bohr-Sommerfeld leaf, but need to continue later when I have more time and energy

• I started an entry classical-to-quantum notions - table for inclusion in “Related concepts”-sections in the relevant entries.

This is meant to clean up the existing such “Related concepts”-lists. But I am not done yet with the cleaning-up…

• New entry semiclassical approximation. It requires a careful choice of references. The ones at the wikipedia article are catastrophically particular, 1-dimensional, old and non-geometric and hide the story more than reveal. Stub Maslov index containing the main references for Maslov index.

• started stub for Chow group

hoping I got this right...

• I created Galois topos following Dubuc’s article.

But I must be missing something about the notation: does it really mean to say that $A$ is an $\Delta \mathrm{Aut}\left(A\right)$-torsor, as opposed to saying that it is associated to an $\Delta \mathrm{Aut}\left(A\right)$-torsor?

• Many more references and a couple of new sentences in Idea section of logarithmic CFT stub.

• There have been two empty pages created lately, both anonymous. They are at Riemann sphere and quasi inverse. It looks as if both were attempts to add something that was aborted.

• I have edited and expanded wall crossing a little

One question to Zoran:

you have designed the entry to cover the notion in great generality. But most of the references that you already had, and now also all that I have added, concern wall crossing of BPS states. Eventually we need to do something to make the entry more systematic on this point. Should we split off an “wall crossing of BPS states”, maybe?

• This page, wall crossing in Aarhus, refers (in the future tense) to a course in 2010. The webpage link is broken as well. Does anyone have a link that could replace that one?

• I have started modal logic, and relational structure. Here my eventual aim will be to see if there is an n-POV version of some of the modal logics. (This was discussed some time ago on the Café I seem to remember.) For that I am trying link it to higher transition systems and various other things. I will probably do some exploration on my personal pages a bit later on but for the moment am just getting basic stuff down on the Lab.

• Thought I would flag up that there have been two of these lately, ideal in semigroups and liars paradox. I waited to see if their ‘authors’ were going to come back and correct them, but so far they have not.

The entry Mochizuki's proof of abc is non-standard in form but has been updated by someone called Daniel.

• created Poisson tensor just for completeness, to be able to point to it from related entries.

• added a little bit to foliation: a brief list of equivalent alternative definitions and and Idea-section with some general remarks.

• Added a little to the Idea-section of holonomy groupoid. But this deserves to be further expanded upon.

• seeing Eric create diffeology I became annoyed by the poor state that the entry diffeological space was in. So I spent some minutes expanding and editing it. Still far from perfect, but a step in the right direction, I think.

(One day I should add details on how the various sites in use are equivalent to using CartSp)

• Mentions of the category $\mathrm{Set}$ occur all over the nLab, but with quite a bit of plasticity of meaning. I thought it might be good to have another look at the entry Set and try to describe this plasticity as considered along various axes, to help readers who might be puzzled by “just what does the nLab think the category of sets is?” For example, one reads that the category of sets has marvelous properties such as being a well-pointed topos, and then a little further down one sees that $\mathrm{Set}$ is not a topos according to predicative mathematics. This could be very confusing. Similarly, there are some pages in the nLab that assume $\mathrm{Set}$ satisfies AC without batting an eye, while others discuss arcane weaker choice principles that $\mathrm{Set}$ might satisfy. I think we need to be a just a bit more up-front about this, right on the page Set.

In the definition section on Set, I made a meager start on this by declaring that the nLab adopts a ’pluralist’ position on the matter of sets and $\mathrm{Set}$, and jotted down a few of the possible axes (“axises”, if I were James Dolan) of meaning and interpretation that guide how one thinks of $\mathrm{Set}$, e.g., predicative vs. impredicative, classical vs. intuitionist, selection of choice principles, and others. I didn’t think really hard about this, but it might suggest useful ways of organizing the page.

I left out other axes such as “structural vs. material”, and said nothing about type theory. The page set talked more about this; I envision Set as concentrating more on properties of the category of sets.

I got to thinking about this when I began to wonder how Toby thinks about $\mathrm{Set}$, which is maybe different from how I usually think about it. (Usually it feels slightly alien to me to posit say WISC as a possible choice principle for the category of sets, which for me usually connotes a model of ETCS – normally I’d think of WISC instead as a possible axiom for a topos or a pretopos.) I was wondering whether Toby had a kind of “bottom line” for $\mathrm{Set}$, say for example “$\mathrm{Set}$ for me means at least a well-pointed topos with NNO, unless I choose to adopt a predicative mode”, or something like that. Anyway, discussion is invited.