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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeMay 25th 2010
• (edited May 25th 2010)

created nerve theorem

linked to it from homotopy groups in an (infinity,1)-topos, where it had implicitly been mentioned before, but not made explicit.

Apart from stating the theorem, I wrote a section that explains what’s going on from the nPOV. As far as I can see, at least.

• CommentRowNumber2.
• CommentAuthorTodd_Trimble
• CommentTimeMay 25th 2010

For which classes of paracompact spaces is one assured of the existence of a good cover?

Manifolds, I presume, and CW complexes. What else?

The nerve theorem is obviously a wonderful result in view of hoped-for higher-dimensional van Kampen theorems.

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeMay 26th 2010
• (edited May 26th 2010)

For which classes of paracompact spaces is one assured of the existence of a good cover?

I was hoping my local expert on good covers would have helped me with that by now, but he didn’t yet. But let’s try to sort this out.

At least meanwhile I have polished and expanded the proof on my personal web that relates the nerve theorem to the left adjoint of the constant $\infty$-stack functor here.

• CommentRowNumber4.
• CommentAuthorTodd_Trimble
• CommentTimeMay 26th 2010

Just to expand on the case of manifolds for a moment: any paracompact manifold admits a Riemannian metric, and for any point in a Riemannian manifold there is a geodesically convex neighborhood (any two points in the neighborhood are connected by a geodesic in the neighborhood; see for example the remark after lemma 10.3 in Milnor’s Morse Theory, page 59). That should do the trick, since it is immediate that a nonempty intersection of two geodesically convex regions is also geodesically convex, hence contractible.

I’d have to think a bit to convince myself of the case for CW complexes.

• CommentRowNumber5.
• CommentAuthorUrs
• CommentTimeMay 26th 2010

Thanks, Todd. I moved that remark to here for the moment. But will have to call it quits now. See you tomorrow!

• CommentRowNumber6.
• CommentAuthorTodd_Trimble
• CommentTimeMay 26th 2010

I added what is hopefully a more or less correct proof that CW complexes admit good covers to good open cover (under Properties).

• CommentRowNumber7.
• CommentAuthorUrs
• CommentTimeMay 26th 2010

Thanks, Todd. Very nice!

For completeness, I also added the statement of the corollary that hence the category of paracompact manifolds and various of its subcategories admit coverages by good open covers.

• CommentRowNumber8.
• CommentAuthorDmitri Pavlov
• CommentTimeJul 12th 2015

I added several references to the nerve theorem article.

• CommentRowNumber9.
• CommentAuthorUrs
• CommentTimeMar 22nd 2021

• Karol Borsuk, On the imbedding of systems of compacta in simplicial complexes , Fund. Math. 35, (1948) 217–234 (dml:213158)
• CommentRowNumber10.
• CommentAuthorUrs
• CommentTimeJun 30th 2021
• (edited Jun 30th 2021)

Added more explicit pointer to McCord 67, Thm. 2.

• CommentRowNumber11.
• CommentAuthorUrs
• CommentTimeJun 30th 2021

also to the page number in Weil 52, p. 141

• CommentRowNumber12.
• CommentAuthorDmitri Pavlov
• CommentTimeJun 30th 2021
• (edited Jun 30th 2021)

I fixed the section number in a reference to Weil’s paper that I added once: it’s Section 5, not 6. Weil also has a relative version formulated as a Corollary: if two maps f,g send every x∈X to some U_i⊂Y, then f and g are homotopic.