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    • Expanded Vassiliev invariant, started Kontsevich integral, did a bit of reorganisation on knot theory (in particular, linking to more pages).

      In case anyone’s wondering, there was a book put on the arXiv a couple of days ago touting itself as an introduction to Vassiliev invariants. I’m reading through it and taking notes as I go. I left in a bit of a rush today so the formatting of the Kontsevich integral went a bit haywire, and I made a statement on the Vassiliev invariant page that I know I didn’t say quite right.

      In the arXiv book, Vassiliev invariants are introduced first using the Vassiliev skein relations, not their “proper” way (which I haven’t gotten to yet so I don’t know it). The formula looked very like a boundary map on a complex, but I think it has to be a cubical complex rather than a simplicial one. Only it isn’t the full boundary map, rather a partial boundary map (going to opposite faces), but I didn’t get it straight in my head until later. But now I think I’m going to wait until I read the bit about the true definition - which I guess will be something like this - before correcting it (unless anyone gets there before me, of course).

      Drew a few more SVGs relevant for knots as well. The code for producing the trefoil knot is very nice now, though I say so myself!

    • Bas Spitters has kindly pointed out to me that the proof by Banaschewski and Mulvey of Gelfand duality is not actually constructive, as it invokes Barr’s theorem, and that he has a genuine constructive and also simpler proof with Coquand. I have added that to the refrences at constructive Gelfand duality theorem

    • I have created a (stubby) entry for Turaev. It needs more links.

    • created cohomology operation, just to record the two references that they are discussing curently on the ALG-TOP list

    • Couple of minor knot changes: writhe is new, and I added the missing diagram (and some redirects) to framed link.

    • I need to be looking again into the subject of the Gelfand-Naimark theorem for noncommutative C *C^*-algebras AA regarded as commutative C *C^*-algebras in the copresheaf topos on the poset of commutative subalgebras of AA, as described in

      Heunen, Landsman, Spitters, A topos for algebraic quantum theory.

      While it seems clear that something relevant is going on in these constructions, I am still trying to connect all this better to other topos-theoretic descriptions of physics that I know of.

      Here is just one little observation in this direction. Not sure how far it carries.

      If I understand correctly, we have in particular the following construction: for \mathcal{H} a Hilbert space and B()B(\mathcal{H}) its algebra of bounded operators, let A:𝒪(X)CStarA : \mathcal{O}(X) \to CStar be a local net of algebras on some Minkowski space XX. landing (without restriction of generality) in subalgebras of B()B(\mathcal{H}).

      By the internal/noncommutative Gelfand-Naimark theorem we have that each noncommutative C *C^*-algebra that AA assigns to an open subset corresponds bijectively to a locale internal to the topos 𝒯 B()\mathcal{T}_{B(\mathcal{H})} of copresheaves on the commutative subalgebras of B()B(\mathcal{H}).

      So using this, our Haag-Kastler local net becomes an internal-locale-valued presheaf

      A:𝒪(X) opLoc(𝒯 B()). A : \mathcal{O}(X)^{op} \to Loc(\mathcal{T}_{B(\mathcal{H})}) \,.

      So over the base topos B()B(\mathcal{H}) this is a “space-valued presheaf”. we could think about generalizing this to \infty-presheaves, probably (though I’d need to think about if we really get there given that the locales need not come from actual spaces). The we could think about if this generalization dually corresponds indeed to the “higher order local nets” such as factorization algebras.

      Just a very vague thought. Have to run now.

    • do we already have this in nLab? it seems that the long exact sequence in cohomology

      H n(X,Y;A)H n(X,A)H n(Y,A)H n+1(X,Y;A) \cdots \to H^n(X,Y;A)\to H^n(X,A)\to H^n(Y,A) \to H^{n+1}(X,Y;A)\to \cdots

      for an inclusion YXY\hookrightarrow X should have the following very simple and natural interpretation: for a morphism f:YXf:Y\to X in a (oo,1)-topos H\mathbf{H} and a coefficient object AA together with a fixed morphism φ:YA\varphi:Y\to A, consider the induced morphism f *:H(X,A)H(Y,A)f^*:\mathbf{H}(X,A)\to \mathbf{H}(Y,A) and take its (homotopy) fiber over the point *φH(Y,A)*\stackrel{\varphi}{\to}\mathbf{H}(Y,A). In particular, when the coefficient object AA is pointed, we can consider the case where φ:YA\varphi:Y\to A is the distinguished point of H(Y,A)\mathbf{H}(Y,A). In this case the homotopy fiber one is considering should be denoted H(X,Y;A)\mathbf{H}(X,Y;A) and is the hom-space for the cohomology of the pair (X,Y)(X,Y) with coefficients in AA (here one should actually make an explicit reference to the morphism f:YXf:Y\to X in the notation, unless it is “clear” as in the case of the inclusion of the classical cohomology of a pair). then, for a deloopable coefficients object AA, the long exact sequence in cohomology should immediately follow from the fiber sequence

      H(X,Y;A) H(X,A) * H(Y,A) \array{ \mathbf{H}(X,Y;A) &\to& \mathbf{H}(X,A) \\ \downarrow && \downarrow \\ * &\to& \mathbf{H}(Y,A) }
    • Can someone with more access than I have do a search and replace for Phyics. I have changed two entries to Physics (which I assume is correct :-)) but as it is not an important typo and there are five or six other occurrences a block replace is probably easy to do.

    • I wanted to archive a pointer to Isbells Generic algebras somewhere on the nLab, and now did so in algebra over a monad. But it is sitting a bit lonesomely there now by itself in the References-section…

    • created a stub for normal operator and noticed/remembered that Tim van Beek had once created the beginning of an entry spectral theorem that he ended with an empty section on the version for normal operators. If we are lucky he will come back some day and complete this, but it looks like he won’t. Maybe somebody else feels inspired to work on this entry.

    • I have created an entry model structure on dg-modules in order to record some references and facts.

      I think using this I now have one version of the statement at derived critical locus (schreiber) that is fully precise. But I am still trying to see a better way. This is fiddly, because

      1. contrary to what one might expect, thre is not much at all in the literature on general properties homotopy limits/colimits in dg-geometry;

      2. and large parts of the standard toolset of homotopy theory of oo-algebras does not apply:

        • the fact that we are dealing with commutative dg-algebras makes all Schwede-Shipley theory not applicable,

        • the fact that we are dealing with oo-algebras in chain complexes makes all Berger-Moerdijk theory not apply;

        • and finally the fact that we are dealing with dg-algebras under another dg-algebra makes Hinich’s theory not apply!

      That doesn’t leave many tools to fall back to.

    • I have created an entry on Phil Ehlers since Stephen Gaito has kindly scanned Phil’s MSc Thesis from 1991. (Phil’s PhD thesis was already on the Lab. The MSc is also there now.)

    • I’ve started writing the notes of the talk I’ll be giving in Utrecht next week. They are here

    • I have added to variational calculus a definition of critical loci of functionals, hence a definition of Euler-Lagrange equations, in terms of diffeological spaces. It’s a very natural definition which is almost explicit in Patrick Iglesias-Zemmour’s book, only that he cannot make it fully explicit since the natural formulation involves the sheaf of forms Ω cl 1()\Omega^1_{cl}(-) which is not concrete and hence not considered in that book.

      I was hoping I would find in his book the proof that the critical locus of a function on a diffeological space defined this was coincides with the “EL-locus” – it certainly contains it, but maybe there is some discussion necessary to show that it is not any larger – but on second reading it seems to me that the book also only observes the inclusion.

    • This is a ’latest changes’, but for the Café rather than the backroom! Can David C (or someone) fix the link that does not work to Steve Awodey’s paper (It should be

    • I wanted to understand Milnor’s paper on Link Groups, so I basically rewrote the main bits in to Milnor mu-bar invariants. (I don’t understand the difference between μ\mu-invariants and μ¯\bar{\mu}-invariants, but I was only working on the original paper so presumably haven’t gotten that far yet.)

      I even put a TOC in so Urs will be happy!

    • I have just added a link to the notes that I prepared for the Lisbon meeting on my personal page. I would love to have some feedback, and in particular suggestions for incorporating some more of this in the nLab. The new material also forms part of the extended version of the Menagerie (which is now topping 800 pages.)

    • At string 2-group it is claimed that the sequence of classifying spaces ending --> BSO(n) --> BO(n) is the Whitehead tower of O(n). Also mentioned is the version for smooth infinity groupoids (so I assume it is Urs who put that there). It is certainly not true that the sequence of classifying spaces so stated is the Whitehead tower for O(n), but the details for groups considered as one-object infinity groupoids are open to interpretation, so I haven't changed anything. Just a heads up.

      -David Roberts
    • I have added a brief note about type-theoretic polymorphism to the list of impredicative axioms at predicative mathematics.

    • At effects of foundations on “real” mathematics I’ve put in the example of Fermat’s last theorem as being potentially derivable from PA, and pointed to two articles by McLarty on this topic.

      (Edit: the naive wikilink to the given page breaks, due to the ” ” pair)

    • You may or may not recall the observation, recorded at Lie group cohomology, that there is a natural map from the Segal-Blanc-Brylinski refinement of Lie group cohomology to the intrinsic cohomology of Lie groups when regarded as smooth infinity-groupoids.

      For a while i did not know how to see whether this natural map is an equivalence, as one would hope it is. The subtlety is that the Cech-formula that Brylinski gives for refined Lie group cohomology corresponds to making a degreewise cofibrant replacement of BG\mathbf{B}G in SmoothGrpdSmooth \infty Grpd and then taking the diagonal, and there is no reason that this diagonal is itself still cofibrant (and I don’t think it is). So while Segal-Brylinski Lie group cohomology is finer and less naive than naive Lie group cohomology, it wasn’t clear (to me) that it is fine enough and reproduces the “correct” cohomology.

      So one had to argue that for certain coefficients the degreewise cofibrant resolution in [CartSp op,sSet] proj,loc[CartSp^{op}, sSet]_{proj,loc} is already sufficient for computing the derived hom space. It was only yesterday that I realized that this is a corollary of the general result at function algebras on infinity-stacks once we embed smooth infinity-groupoid into synthetic differential infinity-groupoids.

      So I believe I have a proof now. I have written it out in synthetic differential infinity-groupoid in the section Cohomology and principal \infty-bundles.

    • in reply to Jim's question over on the blog, I was looking for a free spot on the nLab where I could write some general motivating remarks on the point of "derived geometry".

      I then noticed that the entry higher geometry had been effectively empty. So I wrote there now an "Idea"-section and then another section specifically devoted to the idea of derived geometry.

      (@Zoran: in similar previous cases we used to have a quarrel afterwards on to which extent Lurie's perspective incorporates or not other people's approaches. I tied to comment on that and make it clear as far as I understand it, but please feel free to add more of a different point of view.)

    • I hadd added a little bit of this and that to category of cobordisms earlier today in a prolonged coffee break.

      This was in reaction to learning about the work by Ayala, now referenced there, whou considers categories of cobordisms equipped with geometric structure given by morphisms into an \infty-stack \mathcal{F}.

    • A while back I had a discussion here with Domenico on how the framed cobordim (,n)(\infty,n)-category Bord n fr(X)Bord^{fr}_n(X) of cobordisns in a topological space XX should be essentially the free symmetric monoidal (,n)(\infty,n)-category on the fundamental \infty-groupoid of XX.

      This can be read as saying

      Every flat \infty-parallel transport of fully dualizable objects has a unique \infty-holonomy.


      Some helpful discussion with Chris Schommer-Pries tonight revealed that this is (unsurprisingly) already a special case of what Jacob Lurie proves. He proves it in more generality, which makes the statement easy to miss on casual reading. So I made it explicit now at cobordism hypothesis in the new section For cobordisms in a manifold.

    • At synthetic differential infinity-groupoid I have entered statement and detailed proof that flat and infinitesimally flat real coefficients are equivalent in SynthDiffGrpdSynthDiff\infty Grpd

      infB nB n. \mathbf{\flat}_{inf} \mathbf{B}^n \mathbb{R} \simeq \mathbf{\flat} \mathbf{B}^n \mathbb{R} \,.

      The proof proceeds by presentation of infB n\mathbf{\flat}_{inf} \mathbf{B}^n \mathbb{R} by essentially (a cofibrant resolution of) Anders Kocks’ s infinitesimal singular simplicial complex. In this presentation cohomology with coefficients in this object is manifestly computed as in de Rham space/Grothendieck descent-technology for oo-stacks.

      But we also have an intrinsic notion of de Rham cohomology in cohesive \infty-toposes, and the above implies that in degree n2n \geq 2 this coincides with the de Rham space presentation as well as the intrinsic real cohomoloy.

      All in all, this proves what Simpson-Teleman called the “de Rham theorem for \infty-stacks” in a note that is linked in the above entry. They consider a slightly different site of which I don’t know if it is cohesive, but apart from that their model category theoretic setup is pretty much exactly that which goes into the above proof. They don’t actually give a proof in this unpublished and sketchy note and they work (or at least speak) only in homotopy categories. But it’s all “morally the same”. For some value of “morally”.

    • A manifold has

      • a set of orientations;

      • an xyz of topological spin structures

      • a 3-groupoid of topological string structures;

      • a 7-groupoid of topological fivebrane stuctures, etc.

      and for some reason it is common in the literature (which of course is small in the last cases) to speak of these nn-groupoids, but not so common to speak of the xyz here:

      • A manifold has a groupoid of spin structures.

      Namely the homotopy fiber of the second Stiefel-Whitney class

      Spin(X)Top(X,BSO)(w 2) *Top(X,B 2 2). Spin(X) \to Top(X,B SO) \stackrel{(w_2)_*}{\to} Top(X, B^2 \mathbb{Z}_2) \,.

      I have added one reference that explicitly discusses the groupoid of spin structures to spin structure.

      Do you have further references?

    • I had created line Lie n-algebra, just for the sake of completeness and so that I know where to link to when I mention it

    • I have created an entry differential characteristic class.

      I felt need for this as the traditional term secondary characteristic class first of all has (as discussed there) quite a bit of variance in convention of meaning in the established literature, and secondly it is unfortunately undescriptive (which is probably the reason for the first problem, I guess!).

      Moreover, I felt the need for a place to discuss the concept “differential characteristic class” in the fully general abstract way in the spirit of our entry on cohomology, whereas “secondary characteristic class” has a certain association with concrete models. Some people use it almost synonymously with “Cheeger-Simons differential character”.

      Anyway, so I created a new entry. So far it contains just the “unrefined” definition. I’ll try to expand on it later,

    • I notice that the entry essential image is in a bad state:

      it starts out making two statements, the first of which is then doubted by Mike in a query box, the second doubted by Zoran in a query box.

      If there is really no agreement on what should go there, we should maybe better clear the entry, and discuss the matter here until we have a minimum of consensus.

      But I guess the problems can easily be dealt with and somebody should try to polish this entry right away.