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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeNov 3rd 2020

    added pointer to:

    diff, v5, current

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeNov 3rd 2020

    and this one:

    diff, v5, current

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeNov 10th 2020

    added equivariant tautological line bundles to the list (“list”) of examples

    diff, v6, current

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeNov 18th 2020

    added this pointer:

    diff, v7, current

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021
    • (edited Mar 11th 2021)

    added pointer to:

    diff, v9, current

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    added pointer to:

    diff, v9, current

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    added pointer to:

    diff, v10, current

    • CommentRowNumber8.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021
    • (edited Mar 11th 2021)

    It’s interesting to see the back-and-forth in the definition:

    • Lashof 82 takes the structure group to commute with the equivariance group.

    • Lashof-May 86 take the structure group to extend the equivariance group.

    • May 90 takes the structure group to split-extend the equivariance group.

    I am thinking:

    The right definition of equivariant principal bundles is that which corresponds to principal bundles internal to GG-spaces (for GG being the equivariance group).

    With respect to this notion, it seems to me that:

    • Lashof 82 is too restrictive.

    • Lashof-May 86 is too general.

    • May 90 should be just right.

    Does any author make this explicit: GG-equivariant principal bundles in the semi-generalized sense used in May 90 and regarded as principal bundles internal to GG-spaces?

    Sounds like asking for the obvious – but I haven’t yet seen any article taking this point of view.

    • CommentRowNumber9.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    Added pointer to:

    Hm, this already has the right definition! some 15-20 years before Lashof-May. Maybe a language barrier here?

    diff, v10, current

    • CommentRowNumber10.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    Interesting:

    Now I see that Lashof 82 has a “Note added in proof” where it says that the author has meanwhile been made aware of tomDieck 69. The note goes on to credit tomDieck 69 for some theorems, but does not mention tom Dieck’s more general definition.

    When Lashof-May 86 present their generalized definition, Lashof seems to have forgotten about tomDieck 69 again, because it’s not mentioned/cited.

    • CommentRowNumber11.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    added pointer to:

    diff, v11, current

    • CommentRowNumber12.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    added pointer to:

    diff, v12, current

    • CommentRowNumber13.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    I have now written out some paragraphs of an Idea-section, discussing/highlighting these perspectives/issues.

    In compiling this I noticed that we have relevant discussion at category of G-sets – Internal Group Actions.

    I have added pointer to that from here, but I feel like giving that statement a more transparent form and home – maybe in a small entry “semidirect product groups over G are group objects in G-actions”? (small entry with a long title, that would be :-)

    diff, v12, current

    • CommentRowNumber14.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    added pointer to

    (and here they finally cite tom Dieck 69! :-)

    diff, v14, current

    • CommentRowNumber15.
    • CommentAuthorDavid_Corfield
    • CommentTimeMar 11th 2021

    I see P Hořava in Chern-Simons gauge theory on orbifolds: Open strings from three dimensions - Journal of Geometry and Physics, 1996 opts for tom Dieck’s version:

    we are summing in the “exotic” version of gauge theory over the objects classified by tom Dieck’s classifying space

    referring to

    • T. tom Dieck, “Faserb ̈undel mit Gruppenoperation,”Arch. Math.20 (1969) 136 see also: T. tom Dieck, “Transformation Groups,” de Gruyter Series in Math.8(W. de Gruyter, Berlin 1987
    • CommentRowNumber16.
    • CommentAuthorUrs
    • CommentTimeMar 11th 2021

    Ah, that’s interesting! Thanks for the pointer. Will add it…

    • CommentRowNumber17.
    • CommentAuthorUrs
    • CommentTimeMar 13th 2021

    added pointer to the recent

    diff, v19, current

    • CommentRowNumber18.
    • CommentAuthorUrs
    • CommentTimeMar 13th 2021

    added pointer to:

    diff, v20, current

    • CommentRowNumber19.
    • CommentAuthorUrs
    • CommentTimeMar 13th 2021

    added pointer to:

    diff, v20, current

    • CommentRowNumber20.
    • CommentAuthorUrs
    • CommentTimeMar 13th 2021

    Have now used the new material at equivariant group to state a definition of equivariant principal bundles simply as principal bundles internal to GG-spaces, and to prove that this definition is equivalent to the definition in tomDieck1969.

    There is room left to beautify this further. But I’ll call it quits for tonight.

    diff, v21, current

    • CommentRowNumber21.
    • CommentAuthorUrs
    • CommentTimeMar 15th 2021
    • CommentRowNumber22.
    • CommentAuthorzskoda
    • CommentTimeMar 16th 2021
    • (edited Mar 16th 2021)

    It is very interesting to read all this history of the notion of equivariant bundle in topology where the bundle is in the sense of Steenrod.

    On the other hand, in algebraic geometry there is in one sense a more general notion of equivariant sheaf or a structure on a sheaf which Mumford called the G-linearization on his geometric invariant theory. The book of Mumford on GIT is from around 1965. Now it may be not general enough as it does not talk at first glance about the structure group but in fact the idea of Mumford to do the definition in the descent picture (cocycle) works at the (possibly larger) generality of fibered categories. Namely if you have a group object GG acting on an object XX in the base of a fibered category FBF\to B where the base has fibered products, you can look at the simplicial object corresponding to the action of GG on XX (Borel construction) and look at cartesian sections of FF over that simplicial object. They form a category, the equivariant fiber over XX and its objects are GG-equivariant objects over XX in the fibered category. This is, in modern language, what Mumford called the GG-linearization in the case of the fibered category of quasicoherent sheaves (and soon used in other analogous situations). This is in a sense more general than the case of equivariant bundles.

    This language has been used for example in Vistoli’s lectures on descent theory (and later in my note on some equivariant notions in noncommutative geometry in Georgian Math. J., where I point out that 2-categorical version is straightforward). Vistoli also proves the descent along torsors in this clean generality, where of course one assumes a Grothendieck topology in base and that π:FC\pi: F\to C is a stack. Vistoli also shows that the equivariant object can be seen as simply an object ξ\xi in FF together with a natural transformation which is an action (in the category of presheaves of sets on FF) of a group presheaf h Gπh_G\circ \pi (which is obtained by taking the representing functor for GG on BB precomposed by the projection π\pi of the fibered category) on the representing functor h ξh_\xi. This generalizes to dimension 2 by working with pseudonatural 2-transformations and actions of categorical group objects in 2-category of 2-presheaves and corresponding Yoneda.

    • CommentRowNumber23.
    • CommentAuthorUrs
    • CommentTimeMar 17th 2021

    added pointer to:

    (only discovering now that tom Dieck has this beautiful and comprehensive textbook account of his notion of equivariant bundles)

    diff, v25, current

    • CommentRowNumber24.
    • CommentAuthorUrs
    • CommentTimeMar 20th 2021
    • (edited Mar 20th 2021)

    the entry still has ellipses (…) where it refers to the local triviality condition on equivariant bundles.

    All existing literature on equivariant bundles seems to require (just) that a GG-equivariant 𝒢\mathcal{G}-principal bundle be locally trivial as a 𝒢\mathcal{G}-principal bundle, i.e. ignoring the action of the equivariance group GG.

    However, for a principal bundle internal to GG-spaces the local triviality condition would be not just that the bundle is trivial on some open cover, but on some equivariant open cover (i.e. one that is compatible with the equivariance group action).

    Does any author discuss this seemingly stronger condition? Is it actually equivalent to the seemingly weaker condition that everyone uses?

    diff, v26, current

    • CommentRowNumber25.
    • CommentAuthorUrs
    • CommentTimeMar 20th 2021

    Oh, I see. This is the remark on p. 374 of

    • CommentRowNumber26.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 20th 2021
    Re #25: Is it clear how Atiyah's argument for the special case of the group Z/2 continues to work for arbitrary groups?
    • CommentRowNumber27.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    So he appeals to the equivariant Tietze extension theorem to find equivariant local sections around any orbit in a cover over which the bundle has already been trivialized non-equivariantly.

    The equivariant extension theorem applies, when it applies, for equivariance groups being compact Lie, eg finite groups.

    So Atiyah’s argument should readily generalize to all finite groups. Not so easily to compact Lie groups: While the equivariant extension theorem would still apply, we would need an argument now that the bundle still trivializes non-equivariantly over all orbits.

    The other assumption hidden in Atiyah’s argument is that the equivariant extension theorem applies at all. This generally requires that our typical fiber is an absolute neighbourhood retract.

    I started to make a note on examples of ANRs last night, but the my wifi collapsed. I gather examples include all finite-dimensional topological manifolds and/or all finite-dimensional locally finite CW-complexes and or all finite-dimensional (?) locally compact (?) locally contractible spaces – though the literature is incredibly shy about admitting these statements as theorems with proofs.

    • CommentRowNumber28.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 21st 2021
    Re #27: I think Hanner's theorem (see Theorem III.8.1 in Sze-Tsen Hu's Theory of Retracts), being an ANR is a local property, i.e., admitting an open cover by ANRs means that the space itself is an ANR.

    This immediately settles the case of topological manifolds, for example.

    The original reference is

    O. Hanner, Retraction and extension of mappings of metric and non-metric spaces, Arkiv Mat., Svenska Vetens. Akad., 2(1952), 315-360.

    Hu also derives the following consequences:

    COROLLARY 8.2. If a metrizable space Υ is locally finitely triangulable, then Υ is an ANR.

    COROLLARY 8.3. If a metrizable space Υ is locally Euclidean, then Υ is an ANR.

    COROLLARY 8.4. Every locally finite simplicial polytope is an ANR.
    • CommentRowNumber29.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    Thanks! I’ll be adding material to ANR now.

    • CommentRowNumber30.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 21st 2021
    And polyhedra are ANRs by Theorem III.11.3 in Hu's book.
    • CommentRowNumber31.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    I feel silly for asking, but:

    What’s the definition of “finite-dimensional”, used freely (without definition) by Lashof 81 (right at the beginning), also Borsuk 32 (towards the end).

    This is referring to spaces that don’t seem to be assumed to be cell complexes. So what’s the tacit notion of dimension? Is it covering dimension?

    • CommentRowNumber32.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 21st 2021
    Locally finite CW complexes are ANRs by these two (independent) results:

    J. Dugundji, Note on CW polytopes, Portugaliae Math. 11 (1952), 7-10.

    Y. Kodama, Note on an absolute neighborhood extensor for metric spaces, J. Math. Soc. Japan 8 (1956), 206-215.

    A useful survey of ANRs: Sibe Mardešić, Absolute Neighborhood Retracts and Shape Theory,
    in History of Topology (Elsevier, 1999).
    • CommentRowNumber33.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    Thanks for all this! I’ll try to move it all into ANR. Unless you do it first.

    A useful survey

    I was looking into that last night, but found it hard to spot definite results. Maybe I was too tired.

    • CommentRowNumber34.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 21st 2021
    • (edited Mar 21st 2021)
    Re #32: For separable metric spaces, the three dimensions (small inductive, large inductive, covering) all coincide. Thus, one simply talks about “the” dimension.

    The large inductive and covering dimensions coincide for all metric spaces, in fact.
    • CommentRowNumber35.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    I see. Thanks!! We should record that somewhere…

    • CommentRowNumber36.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 21st 2021
    Do we have any articles on the large/small inductive dimension of topological spaces?
    • CommentRowNumber37.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    No, I don’t think so.

    Also, it sounds like first of all we need an entry separable metric space, so that it can host the statement of dimension of such spaces.

    • CommentRowNumber38.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    I see there is a comprehensive account Engelking: Dimension theory of separable metric spaces (pdf).

    • CommentRowNumber39.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 21st 2021
    • (edited Mar 21st 2021)
    Actually, this book has a new (and considerably updated) version/rewriting, published as

    Ryszard Engelking.
    Theory of dimensions finite and infinite.
    Sigma Series in Pure Mathematics, 10.
    Heldermann Verlag, Lemgo, 1995.
    viii+401 pp.
    ISBN: 3-88538-010-2

    Also, inductive dimensions are defined for arbitrary topological spaces, just like Lebesgue's covering dimension.
    • CommentRowNumber40.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021

    Thanks, very useful. So I have started separable metric space to record all this….

    • CommentRowNumber41.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2021
    • (edited Mar 21st 2021)

    made explicit that principal bundles internal to GG-spaces will have GG-equivariant local trivialization, and added a remark (here) on sufficient conditions for plain local trivializability to imply equivariant local trivializability, following Atiyah66.

    (There is room left to polish-up the remark further, but now my battery is dying…)

    diff, v27, current

    • CommentRowNumber42.
    • CommentAuthorDavidRoberts
    • CommentTimeMar 21st 2021
    • (edited Mar 21st 2021)

    I’ve tweaked the phrasing for that statement, since there were some singular vs plural issues that made it unclear on a casual reading if the listed conditions were meant to be taken together, or were all individually all sufficient.

    diff, v28, current

    • CommentRowNumber43.
    • CommentAuthorUrs
    • CommentTimeMar 22nd 2021

    slightly re-worded the internal definition to cross-link explicitly with action object

    diff, v29, current

    • CommentRowNumber44.
    • CommentAuthorUrs
    • CommentTimeApr 2nd 2021
    • (edited Apr 2nd 2021)

    On p. 28 of

    appears a notion of equivariant local trivialization which only requires G xG_x-equivariant trivialization on any G xG_x-neighbourhood of xx, for all xXx \in X.

    It is not hard to show that, under standard assumptions, this is equivalent to asking for an actual GG-equivariant local trivialization. While not hard, it’s not trivial: this needs a slice theorem, as far as I can see.

    Is there any reference that would discuss this equivalence? Or just one that would state the Atiyah-Segal notion as an explicit condition one might contemplate for equivariant bundles?

    • CommentRowNumber45.
    • CommentAuthorUrs
    • CommentTimeApr 2nd 2021
    • (edited Apr 2nd 2021)

    added pointer to:

    They build the existence of slices essentially into their definition of equivariant local trivialization.

    diff, v30, current

    • CommentRowNumber46.
    • CommentAuthorUrs
    • CommentTimeApr 2nd 2021

    Oh, I see now that the definition that Atiyah-Segal use is that given in Section 4 of Bierstone 78.

    I’ll start a section on equivariant local trivializations, to sort this out…

    • CommentRowNumber47.
    • CommentAuthorDmitri Pavlov
    • CommentTimeApr 2nd 2021

    Corrected DOI for Lashof’s paper.

    diff, v33, current

    • CommentRowNumber48.
    • CommentAuthorUrs
    • CommentTimeApr 3rd 2021
    • (edited Apr 3rd 2021)

    Ah, I see, thanks.

    Okay, I am on the entry now, straightening out the issue with the equivariant local trivializability.

    So I guess the internalized notion of local trivialization as currently stated in the entry is actually no good, it’s too restrictive. Externally it comes out as tom Dieck’s/Bierstone’s condition but with trivial action on the direct product fiber factor.

    Indeed, what actually matters for the purpose of using equivariant topological bundles as presentations for equivariant \infty-bundles is that the their image under passing to fixed loci and applying SingSing is a weakly-simplicial principal bundle (in the sense of NSS12b) in simplicial presheaves over the orbit category.

    The weak-principality is automatic from the internal principality of the original bundle (that’s the point of working internally), but the remaining condition is that these bundles in simplicial presheaves be projectively Kan fibrations, and that’s the (only) aspect that any local triviality on the original topological principal bundles is needed for.

    But for that we just need that all the fixed loci of the underlying topological bundle are locally trivial. And that’s what tomDieck/Bierstone’s condition ensures.

    Their condition still gives a kind of internal local trivialization, but with respect to possibly more than one typical fiber (the underlying fiber is the same, but its GG-action may change.)

    Anyway, I’ll try to bring that out in the entry now.

    • CommentRowNumber49.
    • CommentAuthorUrs
    • CommentTimeApr 3rd 2021
    • (edited Apr 3rd 2021)

    kept being distracted today, but at least I have now typed out (here) three definitions of equivariant local trivializability (1. tom Dieck, 2. Bierstone, 3. Lashoff).

    There are probably typos left, but I have to call it quits for tonight.

    diff, v37, current

    • CommentRowNumber50.
    • CommentAuthorDmitri Pavlov
    • CommentTimeApr 3rd 2021

    “the following two conditions are satisfied:

    (principality) the shear map”

    The second condition is missing, though.

    • CommentRowNumber51.
    • CommentAuthorUrs
    • CommentTimeApr 4th 2021

    Thanks for catching this! That was a remnant of me removing the local triviality condition from the definition, yesterday.

    Have just made it read “the following condition” for now. But even though it follows tradition to define equivariant principal bundles without having equivariant local triviality be part of the definition (but instead be relegated to an add-on condition later on), I find this weird and might change it back later.

    I am once again inclined to think that it is tom Dieck 69 who gives the “right” definition, now of equivariant local trivialization, with all other authors ignoring that reference and incrementally rediscovering special cases of it.

    But it’s a little fiddly to relate it all.

    diff, v38, current

    • CommentRowNumber52.
    • CommentAuthorUrs
    • CommentTimeApr 4th 2021

    I have added a section Properties – Over coset spaces ([here](https://ncatlab.org/nlab/show/equivariant bundle#OverCosetSpaces)) and spelled out statement and proof of the characterization of equivariant principal bundles over coset spaces G/HG/H for closed subgroups.

    (Following section 2.1 in tom Dieck 69, but all written out a little differently.)

    diff, v41, current

    • CommentRowNumber53.
    • CommentAuthorUrs
    • CommentTimeApr 4th 2021
    • (edited Apr 4th 2021)

    I suspect now that the equivariant local triviality condition used by (at least) tom Dieck is really the following:

    These are in fact internally locally trivial, but not as internal principal bunldes but as internal groupoid principal bundles, where the groupoid is that of topological groups with GG-actions by automorphisms whose morphisms may change the automorphisms bu inner automorphisms.

    Just a suspicion, still need to write out a proof.

    • CommentRowNumber54.
    • CommentAuthorUrs
    • CommentTimeApr 4th 2021
    • (edited Apr 4th 2021)

    I have added statement and proof (here) that equivariant local trivializability in the sense of Lashof implies that in the sense of tom Dieck, for α=1\alpha = 1 (i.e. when the equivariance group and the structure group commute, as Lashof assumes).

    (Not claiming the writeup is optimal yet. Lots of moving parts here, notation-wise. But we’ll get there.)

    [Same idea should work for general α\alpha and comparing then to Lashof-May 86. But i’ll call it quits for tonight.]

    diff, v43, current

    • CommentRowNumber55.
    • CommentAuthorUrs
    • CommentTimeApr 7th 2021
    • (edited Apr 7th 2021)

    added the statement (here) that the HH-fixed point functor takes GG-equivariant Γ\Gamma-principal bundles to N(H)/HN(H)/H-equivariant Γ H\Gamma^H-principal bundles – as is immediate from the internal perspective and the fact that () H(-)^H on the ambient category is a right adjoint

    diff, v46, current

    • CommentRowNumber56.
    • CommentAuthorUrs
    • CommentTimeApr 7th 2021

    finally added the pertinent diagrams to the definition of the internal principal bundles (here)

    diff, v46, current

    • CommentRowNumber57.
    • CommentAuthorUrs
    • CommentTimeApr 8th 2021

    Added statement and proof (here) that Lashof’s local models are locally trivial as ordinary fiber bundles.

    (This is inside Lemma 1.1 in Lashof 82, I have just tried to isolate it for emphasis and expand out the argument a little for clarity)

    diff, v48, current

    • CommentRowNumber58.
    • CommentAuthorUrs
    • CommentTimeApr 12th 2021

    added this pointer:

    diff, v49, current

    • CommentRowNumber59.
    • CommentAuthorUrs
    • CommentTimeApr 12th 2021
    • (edited Apr 12th 2021)

    added this pointer as “precursor discussion” (has essentially all the ingedients, but doesn’t quite articulate a definition of equivariant bundles as such):

    • T. E. Stewart, Lifting Group Actions in Fibre Bundles, Annals of Mathematics Second Series, Vol. 74, No. 1 (1961), pp. 192-198 (jstor:1970310)

    It’s interesting though, because if we grant that this is the origin of equivariant bundles, then the general tomDieck-definition-rediscovered-by-Lashof-May is already right there on the first page, if we agree that by “invariant subgroup” the author must mean “normal subgroup” (clearly).

    diff, v50, current

    • CommentRowNumber60.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2021

    Just emerged out of a little paradox crisis, with the following insight (unless I am still confused):

    If we define “principal bundle” internally by just demanding the principality condition

    Γ×P(g,p)(p,gp)P× XP \Gamma \times P \underoverset{\simeq}{ (g,p) \mapsto ( p, g \cdot p ) }{\longrightarrow} P \times_X P

    (which is a limit-theory condition)

    and not explicitly demanding that XP/GX \simeq P/G – since that is implied by the principality condition IF PXP \to X is an effective epi — then the empty bundle is principal.

    Γ×(g,p)(p,gp)× X \Gamma \times \varnothing \underoverset{\simeq}{ (g,p) \mapsto ( p, g \cdot p ) }{\longrightarrow} \varnothing \times_X \varnothing

    !!

    This is actually relevant – and resolves an apparent paradox – when thinking about fixed loci of equivariant bundles:

    The fixed locus functor is right adjoint and hence preserves internal limit theories such as the above flavour of internal principal bundle. But it also frequently keeps the structure group intact while producing an empty underlying bundle.

    First I thought I had discovered a flaw in mathematics and was about to call the Fields Institute (or who you’re gonna call in that case?) but now I see that all is good: The empty bundle is principal.

    • CommentRowNumber61.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2021

    I have added a remark to this effect (here).

    diff, v53, current

    • CommentRowNumber62.
    • CommentAuthorDavidRoberts
    • CommentTimeApr 15th 2021

    Hmm, interesting! I guess this is the difference between a torsor on the nLab (which doesn’t seem to need to be inhabited), and the more usual definition (which requires the underlying set to be inhabited).

    • CommentRowNumber63.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2021

    Ah; what you call “a torsor on the nLab” was introduced just days ago by Richard, in rev 40! Before that, from Todd’s rev 13 on, “torsor on the nLab” was assumed to be inhabited. :-)

    • CommentRowNumber64.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2021

    And, luckily, the empty bundle is also a fibration…

    • CommentRowNumber65.
    • CommentAuthorDavidRoberts
    • CommentTimeApr 15th 2021
    • (edited Apr 15th 2021)

    Yes, i was very surprised to see that at torsor. I had remembered that we had material on empty heaps, and was going to make that comparison, but then double checked…

    • CommentRowNumber66.
    • CommentAuthorUrs
    • CommentTimeApr 27th 2021
    • (edited Apr 27th 2021)

    I am puzzled by a statement on universal equivariant principal bundles. Maybe somebody can help me:

    A neat explicit construction of the universal equivariant principal bundle is given in Murayama-Shimakawa 95: if the equivariance group is discrete, then (using their remark on the bottom of p. 6) the base of the universal GG-equivariant Γ\Gamma-principal bundle is the realization of the GG-topological groupoid

    ΓTopGroupoids(G×GG,Γ*) \mathcal{B} \Gamma \;\coloneqq\; TopGroupoids \big( G \times G \rightrightarrows G,\; \Gamma \rightrightarrows \ast \big)

    whose GG-action on functors FF and natural transformations η\eta is

    (gF)(g 1,g 2)F(g 1g,g 2g),(gη)(g 1)η(g 1g). (g \cdot F)(g_1, g_2) \;\coloneqq\; F(g_1 g, g_2 g) \,, \;\;\;\;\;\; (g \cdot \eta)(g_1) \;\coloneqq\; \eta( g_1 g ) \,.

    It’s a fun fact (which these authors don’t mention, but which one can check) that for HGH \subset G any subgroup, the HH-fixed groupoid of this is equivalent, as a topological groupoid (no stackification anywhere), to

    (Γ) HTopGroupoids(G*,Γ*). (\mathcal{B} \Gamma)^H \;\;\simeq\;\; TopGroupoids \big( G \rightrightarrows \ast,\, \Gamma \rightrightarrows \ast \big) \,.

    This implies at once that the HH-fixed subspaces of the classifying space Γ\left\Vert \mathcal{B}\Gamma\right\Vert are homotopy equivalent to the disjoint union over conjugacy classes of group homomorphisms ρ:GΓ\rho : G \to \Gamma of the classifying spaces of the centralizer subgroups Γ ρ\Gamma^\rho

    (Γ) H[ρ]BΓ ρ (\mathcal{B} \Gamma)^H \;\;\simeq\;\; \underset{ [\rho] }{\sqcup} B \Gamma^\rho

    That this should be the case is Theorem 2.17 in Lashof 82, where this is derived not from inspection of a concrete model, but from more abstract criteria for universal equivariant bundles.

    There is a subtlety here in that Lashof 82 considers equivariant bundles where the equivariance group GG commutes with the structure group Γ\Gamma, while Murayama-Shimakawa 95 mean to consider the case where both jointly act as a semidirect product group.

    But in the special case where they do commute, the model of Murayama-Shimakawa 95 makes nicely manifest the fixed point structure of the classifying space for equivariant principal bundles according to Theorem 2.17 in Lashof 82.

    So far so good.

    But implicit in Murayama-Shimakawa 95 is that a more general action of GG on Γ\Gamma (to a direct product group structure) does not affect the underlying GG-space of the universal equivariant bundle which they build.

    So their result says – unless I am mixed up, but it seems clear – that the above formula for the fixed point structure actually holds generally.

    Now, Lashof-May 86 generalize Lashof 82 to these more general group actions. Their Theorem 10 seems to contradict this conclusion from Murayama-Shimakawa 95:

    Namely their Theorem 10 says that for (in particular) semidirect product group action G αΓG \rtimes_\alpha \Gamma, the HH-fixed point subspace of the classifying space has connected components not indexed by conjugacy classes of group homomorphisms GΓG \to \Gamma, as above, but by conjugacy classes of lifts of HH to G αΓG \rtimes_\alpha \Gamma (slightly paraphrasing here).

    That sounds plausible, because such subgroups are exactly what labels the “local models”, namely the equivariant bundles over G/HG/H. But how is this compatible with Murayama-Shimakawa 95?

    • CommentRowNumber67.
    • CommentAuthorUrs
    • CommentTimeApr 27th 2021

    Oh, I see. I was misreading the definition of the action in Murayama-Shimakawa, p. 1293. So never mind.

    • CommentRowNumber68.
    • CommentAuthorUrs
    • CommentTimeApr 28th 2021
    • (edited Apr 28th 2021)

    starting a section on universal equivariant principal bundles (here):

    added the definition of the Murayama-Shiwakawa groupoid (for discrete GG)

    ΓGroupoids(TopSpaces)(G×Gpr 2pr 1G,Γ*) \mathcal{B}\Gamma \;\coloneqq\; Groupoids(TopSpaces) \big( G \times G \underoverset{pr_2}{pr_1}{\rightrightarrows} G, \; \Gamma \rightrightarrows \ast \big)

    with its GG-action

    (gF)(g 1,g 2)α(g)(F(g 1g,g 2g)),AAA(gη)(g 1)α(g)(η(g 1)) (g \cdot F) (g_1, g_2) \;\coloneqq\; \alpha(g) \big( F(g_1 g, g_2 g) \big) \,, {\phantom{AAA}} (g \cdot \eta) (g_1) \;\coloneqq\; \alpha(g)(\eta(g_1))

    and then statement of its HH-fixed loci (as topological groupoids)

    (Γ) H(Groups(TopSpaces) /G(G,Γ αG))Γ. \big( \mathcal{B}\Gamma \big)^H \;\; \simeq \;\; \Big( Groups(TopSpaces)_{/G} \big( G, \, \Gamma \rtimes_\alpha G \big) \Big) \sslash \Gamma \,.

    I have written some words indicating the proof, which is essentially an elementary inspection (though one best uses some diagrammatic notation which I haven’t tried to Instikify here, my local version uses equations between tikcz diagrams, which cannot be imported here – and tikzcd itself cannot be nested, unfortunately)

    The point is that this gives right away the fixed point behaviour of the classifying space for equivariant principal bundles according to Lashof82 Thm 2.17 and Lashof&May86 Theorem 10 (if we grant that they mean “centralizer” instead of “normalizer” in the first slot!?) – IF we can assume that passage to fixed points commutes with realization.

    Now Murayama-Shiwakawa use fat realization, but comment that they could use ordinary realization (which would commute so) at least if both GG and Γ\Gamma are compact Lie and possibly more generally, which however they leave open.

    diff, v57, current

    • CommentRowNumber69.
    • CommentAuthorUrs
    • CommentTimeApr 28th 2021

    Oh, I see that Guillou, May & Merling 17, pp. 15 has analogous discussion.

    Hm, but so they can’t get around assuming Γ\Gamma to be compact Lie, either? That would be too bad.

    diff, v58, current

    • CommentRowNumber70.
    • CommentAuthorDavidRoberts
    • CommentTimeApr 28th 2021

    So not even a proper action of an arbitrary Lie group?

    • CommentRowNumber71.
    • CommentAuthorUrs
    • CommentTimeApr 28th 2021

    Γ\Gamma here is the structure group, not the equivariance group.

    That’s why we’d rather not have much conditions on this at all, because in practice this needs to allow for choices like PU()PU(\mathcal{H}).

    • CommentRowNumber72.
    • CommentAuthorUrs
    • CommentTimeApr 30th 2021
    • (edited Apr 30th 2021)

    Hm, on the other hand the construction for Γ=PU()\Gamma = PU(\mathcal{H}) in

    • Noe Barcenas, Jesus Espinoza, Michael Joachim, Bernardo Uribe, Universal twist in Equivariant K-theory for proper and discrete actions (arXiv:1202.1880)

    looks just like the Murayama-Shiwakawa construction (not cited as such) but with group homomorphisms GΓ G \to \Gamma restricted to “stable” maps.

    • CommentRowNumber73.
    • CommentAuthorUrs
    • CommentTimeJun 3rd 2021
    • (edited Jun 3rd 2021)

    added a brief remark (here) that the Murayama-Shimakawa equivariant classifying space Γ\mathcal{B}\Gamma has as HH-fixed points the HH-homotopy fixed points of BΓB \Gamma.

    diff, v60, current

    • CommentRowNumber74.
    • CommentAuthorUrs
    • CommentTimeJun 4th 2021

    Oh, I think I finally see the abstract story here:

    For ΓGGroups((SingularSmoothGroupoids ) /BG)\Gamma \!\sslash\! G \;\in\; Groups\big( (SingularSmoothGroupoids_\infty)_{/\mathbf{B}G} \big), the equivariant classifying space GΓ\mathcal{B}_G \Gamma should simply be taken to be (the shape of) the right derived base change of the plain classifying stack along the unit map of the orbi-singular modality \prec, hence:

    GΓ(η BG ) *(B(ΓG)) \mathcal{B}_G \Gamma \;\coloneqq\; \big( \eta^{\prec}_{\mathbf{B}G} \big)_{\ast} \big( \mathbf{B} (\Gamma \!\sslash\! G) \big)

    It then follows by the right base change adjunction that the geometric HH-fixed points of GΓ\mathcal{B}_G \Gamma are the homotopy HH-fixed points of BΓ\mathbf{B} \Gamma, which identifies the equivariant homotopy type of GΓ\mathcal{B}_G \Gamma with the Murayama-Shimakawa-style equivariant classifying space, by the observation in #73 above.

    More generally, it follows by the same right base change adjunction that GG-equivariant Γ\Gamma-principal bundles classified by GΓ\mathcal{B}_G \Gamma on a GG-space XX are equivalently ΓG\Gamma \sslash G-principal bundles on the corresponding orbispace, this being the stack XGX \!\sslash\! G in the slice over BG\mathbf{B}G – thus identifying the the traditional theory of equivariant principal bundles with the evident stacky formulation.

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