Not signed in (Sign In)

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

  • Sign in using OpenID

Site Tag Cloud

2-category 2-category-theory abelian-categories adjoint algebra algebraic algebraic-geometry algebraic-topology analysis analytic-geometry arithmetic arithmetic-geometry bundles calculus categorical categories category category-theory chern-weil-theory cohesion cohesive-homotopy-theory cohesive-homotopy-type-theory cohomology colimits combinatorics complex complex-geometry computable-mathematics computer-science constructive constructive-mathematics cosmology deformation-theory descent diagrams differential differential-cohomology differential-equations differential-geometry digraphs duality elliptic-cohomology enriched fibration foundation foundations functional-analysis functor galois-theory gauge-theory gebra geometric 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 k-theory lie-theory limits linear linear-algebra locale localization logic mathematics measure-theory modal modal-logic model model-category-theory monad monads monoidal monoidal-category-theory morphism motives motivic-cohomology nforum 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 string string-theory superalgebra supergeometry svg symplectic-geometry synthetic-differential-geometry terminology theory topology topos topos-theory type type-theory universal variational-calculus

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

Welcome to nForum
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).
    • New entry universal epimorphism redirectinig also universal monomorphism. It is not among those variants listed in epimorphism. We also do not list absolute epimorphism (epimorphism which stays epimorphism after applying any functor to it). Every split epimorphism stays split after applying a functor hence it is absolute, but is there a counterexample of an absolute epimorphism which is not in fact split ?

      By the way, here is an archived version of the old query from strict epimorphism

      David Roberts: I’m interested in a bicategorical version of this. You haven’t happened to have done this Mike?

      Mike Shulman: Not more than can be extracted from 2-congruence (michaelshulman) and regular 2-category (michaelshulman). What is there called an “eso” is the bicategorical version of a strong epi (which agrees with an extremal epi in the presence of pullbacks), and what is there called “the quotient of a 2-congruence” is the bicategorical version of a regular epi. I’ve never thought about the bicategorical version of a strict epi; since strict epis agree with regular epis in the presence of finite limits I’ve never really had occasion to care about them independently.

    • A graduate student at Johns Hopkins who is being supervised by Jack Morava, named Jon(athan) Beardsley, wrote a short article Bousfield Lattice. More on this in a moment.

    • I created locally regular category and added a corresponding section to allegory.

      Edit: removed some complaints that were due to it being too late at night and my brain not working correctly.

    • I am experimenting with a notion of Heisenberg Lie nn-algebras, for all nn \in \mathbb{N}.

      I have made an experimental note on this here in the entry Heisenberg Lie algebra.

      It’s explicitly marked as “experimental”. If it turns out to be a bad idea, I’ll remove it again. Please try to shoot it down to see if I can rescue it! :-)

      I mean, the definition in itself is elementary and very simple. The question is if this is “the right notion” to consider. The reasoning here is:

      by the arguments as mentioned on the nCafé here we may feel sure that Chris Rogers’s notion of Poisson Lie n-algebra is correct. (Not that there were any particular doubts, but the fact that we can derive it from very general abstract homotopy theoretic constructions reinforces belief in it.)

      But the ordinary Heisenberg Lie algebra is just the sub-Lie algebra of the Poisson Lie algebra on the constant and the linear functions. Therefore it makes sense to look at the sub-Lie nn-algebra of the Poisson Lie nn-alhebra on the constant and linear differential forms That’s what my experimental definition does.

    • Added some relevant bits to connected limit, fiber product, and pushout. I wanted to record the result at connected limit that functors preserve connected limits iff they preserve wide pullbacks, which may be a slightly surprising result if one has never seen it before.

    • I have been expanding and polishing the entry Heisenberg group.

      This had existed in bad shape for quite a while, but now it’s maybe getting into better shape.

      I tried to spend some sentences on issues which I find are rarely highlighted appropriately in the literature. So there is discussion now of the fact that

      • there are different Lie groups for a given Heisenberg Lie algebra,

      • and the appearance of an “ii” in [q,p]=i[q,p] = i may be all understood as not picking the simply conncted ones of these;

      I also added remarks on the relation to Poisson brackets, and symplectomorphisms.

      In this context: either I am dreaming, or there is a mistake in the Wikipedia entry Poisson bracket - Lie algebra.

      There it says that the Poisson bracket is the Lie algebra of the group of symplectomorphisms. But instead, it is the Lie algebra of a central extension of the group of Hamiltonian symplectomorphisms.

    • I have started an entry canonical extension.

      But I am only learning about this myself right now. Expert input would be most welcome.

    • I am fiddling with an entry table - models for (infinity,1)-operads meant to allow to see 10+ different model categories and their main Quillen equivalences at one glance.

      I guess there are better ways to typeset this. (Volunteers please feel free to lend a hand!) But for the time being I’ll settle with what I have so far.

    • Added a section arithmetic D-modules. This is the optimal theory for p-adic cohomology of varieties over finite fields, since it has the six operations. This section is complementary to rigid cohomology.

    • In, at

      “… the differential respects the brackets: for all xg 0 x \in g_0 and hg 1h \in g_1 we have

      δ[x,h]=[x,δh] \delta [x,h]=[x,\delta h]


      is wrong. The equation should be:

      δα(x,h)=[x,δh]\delta \alpha(x,h) = [x,\delta h]

      Since I don’t know if I have the right to change an nLab entry,I post this here as an suggestion.

    • Created a new entry on "special $\Delta$-spaces". Does anyone know of a better name?

      The entry is poorly edited since I'm not fluent with the iTex syntax.
    • I made some much-needed corrections at simplicial complex, directed mostly at errors which had been introduced by yours truly. I also created quasi-topological space (the notion due to Spanier).

      I haven’t thought this through, but regarding the process of turning a simplicial complex into a simplicial set, the usual sequence of words seems to involve putting a non-canonical ordering on the set of vertices and then getting ordered simplices from that. But is there anything “wrong” with taking the composite

      SimpCompSet Fin + opSet i opSet Δ opSimpComp \hookrightarrow Set^{Fin_{+}^{op}} \stackrel{Set^{i^{op}}}{\to} Set^{\Delta^{op}}

      where the inclusion is the realization of simplicial complexes as concrete presheaves on nonempty finite sets, and the second arrow is pulling back along the forgetful functor ii from nonempty totally ordered finite sets to nonempty finite sets? This looks much more canonical.

    • I hope Urs doesn’t mind my inserting a not-too-serious but nevertheless amusing example at symplectomorphism.

    • Someone set up lawvere theory, but did not add anything to it. They had previously done an edit to FinSet. The new page has a redirect from Lawvere+theory, so I don’t see what is doing. Can someone check the edit at [[FinSet]. It looks as if the person knows some things and so has added a bit, but it is so long since I knew that stuff well so I cannot tell if it is a valid edit or not.

    • I have created a stub on Volodin. I have been unable to find out more on him. Can anyone help?

    • Started presentation of a category by generators and relations. This is probably an evil definition (there was an old discussion on this in the context of quotient category), and there is perhaps a more modern way to do this, so feel free to change the entry. I used “quotient category” as in CWM and mentioned that this is not the definition in the nLab.

    • The last two days Stephan Spahn was visiting me, and we chatted a lot about étaleness in cohesive ∞-toposes.

      We found proofs that

      • for every notion of infinitesimal cohesive neighbourhood

        i:HH th i : \mathbf{H} \hookrightarrow \mathbf{H}_{th}

        the total space projections of locally constant \infty-stacks are formally étale;

      • the formally étale morphisms with respect to any choice of infinitesimal cohesion satisfy all the axioms of axiomatic open maps (or rather their \infty-version, of course).

      (These are to be written up. Requires plenty of 3d iterated \infty-pullback diagrams which are hard to typeset).

      Recall – from synthetic differential infinity-groupoid – that for the infinitesimal cohesive neighbourhood

      i:i : Smooth∞Grpd \hookrightarrow SynthDiff∞Grpd

      the axiomatically formally étale morphisms between smooth manifolds are precisely the étale maps in the traditional sense.

      Motivated by all this, I finally see, I think, what the correct definition of cohesive étale ∞-groupoid is:

      simply: XHX \in \mathbf{H} is an étale cohesive \infty-groupoid if it admits an atlas X 0XX_0 \to X by a formally étale morphism in H\mathbf{H}.

      I have spelled out the proof now here that with this definition a Lie groupoid 𝒢\mathcal{G} is an étale groupoid in the traditional sense, precisely if it is cohesively étale when regarded as an object of the infinitesimal cohesive neighbourhood SmoothGrpdSynthDiffGrpdSmooth \infty Grpd \hookrightarrow SynthDiff \infty Grpd.

      I hope to further expand on all this with Stephan. But I may be absorbed with other things. Next week I am in Goettingen, busy with a seminar on L L_\infty-connections.

    • I found in the disambiguation page basis an entry basis of a vector space missing, and so I created one. I have then cross-linked this with relevant existing entries, such as basis theorem. Also created a brief entry orthogonal basis in order to get rid of grayish links is some entries.

    • In context of size issues on admissible structures (in the sense of DAG V) I wondered which closure properties (e.g. obviously its closed under pullbacks along relatively k-compact morphisms) the class of relatively k-compact (for a regular cardinal k) morphisms in a (∞,1)-category satisfies. Is there any reference concerning this?

    • I needed an entry that lists references on twisted K-homology, so I created one. This made me notice that we currently lack an entry K-homology. I can try to create a stub for that a little later…

    • the link to the picture in the entry Charles Wells is broken. Does anyone know how to fix it or have an alternative picture?

    • Perhaps we need a page on Jean-Louis Koszul, and possibly an agreement on terminology / names for entries. Ben has created a page called Koszul, but usually single names like that would be used for the ’person’ page of that mathematician, so … The term in in any case (as adjective) is also used in various other contexts e.g. for operads, so possibly there needs to be some rationalisation. My thought would be to combine a page on Koszul (and the Wikipedia (English) page on him is poor, and includes some very poor translation from the French ) with a certain amount of disambiguation, however I am not an expert on things ’Koszul’ and this may not be the most efficient way to go.

    • I created orbifold groupoids with some classes of groupoids whose elements I like to think of as ‘‘orbifold groupoids‘‘. It would be nice to have a discussion of the interelation of these classes there, too.

    • I thought it would be useful to supplement the entry operad with entries symmetric operad and planar operad, that amplify a bit more on the specifics of these respective flavors of the general notion, and that will allow us in other entries to link specifically to one of the two notions, when the choice is to be made explicit.

      So far I have written (only) an Idea-section at symmetric operad with some comments.

      To be expanded.

    • I have created a bunch of stub entries such as iterated loop space object with little non-redundant content for the moment. I am filling the k-monoidal table. Please bear with me for the time being, while I add stuff.

    • have created an entry k-monoidal table to be used for inclusion into the entries that it organizes (see for instance at infinite loop space).

      Will now create at least stubs for the missing links.

    • I have created an entry on the Steinberg group St(R)St(R) of a ring RR. The entry includes the Whitehead lemma.

    • I have half-heartedly started adding something to Kac-Moody algebra. Mostly refrences so far. But I don’t have the time right now to do any more.

    • Is the notion of local Kan extension in weak or infinity n-categories well defined. I know there is the infinity,1-case done by Lurie, but it is not local. I would define a Kan extension as the datum of a 1-morphism filling the usual diagram, with a 2-morphism that induces an equivalence of (n-2)-categories of morphisms between morphisms. You may do the same in the infinity,n case, if needed.

      Is there a better definition?

      The point is to define limits in weak n-categories using this. It is not simpler to define limit than to define Kan extensions.

      Are there finer notions of local extensions, that use more explicitely the higher category structure?
    • A new contribution
    • There is a strange glitch on this page: the geometric realization of a cubical set (see geometric realizationealization) below) tends to have the wrong homotopy type:

      That is what appears but is not a t all what the source looks like:

      the geometric realization of a cubical set (see [geometric realization](#geometric realization) below) tends to have the wrong homotopy type:

      What is going wrong and how can it be fixed?

      Another point : does anyone know anything about symmetric cubical sets?