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• I am moving the following old query box exchange from orbifold to here.

old query box discussion:

I am confused by this page. It starts out by boldly declaring that “An orbifold is a differentiable stack which may be presented by a proper étale Lie groupoid” but then it goes on to talk about the “traditional” definition. The traditional definition definitely does not view orbifolds as stacks. Neither does Moerdijk’s paper referenced below — there orbifolds form a 1-category.

Personally I am not completely convinced that orbifolds are differentiable stacks. Would it not be better to start out by saying that there is no consensus on what orbifolds “really are” and lay out three points of view: traditional, Moerdijk’s “orbifolds as groupoids” (called “modern” by Adem and Ruan in their book) and orbifolds as stacks?

end of old query box discussion

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• apparently we didn’t have this. Am adding also a diambiguation page

• I’ve created a page for quaternionic manifolds, linked from quaternion-Kähler page. Basic references and discussion of main definition re Cauchy-Feuter calculus. Comparison to hypercomplex structure.

• starting something here. For the moment this is just the list of references which I had previously recorded at RR-field tadpole cancellation, now joined by a quick Idea-sentence and a minimal statement of the actual cancellation condition

• created Einstein manifold

(for the moment only to record the example of weak $G_2$-manifolds…)

• some minimum, just so to have a canonical place for linking references jointly from LHC and flavour anomaly

• I corrected a couple of wrong claims and added the link to a counter-example

AG

• Edit to: GUT by Urs Schreiber at 2018-04-01 01:21:13 UTC.