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

Discussion Tag Cloud

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).
    • CommentRowNumber1.
    • CommentAuthorDavidRoberts
    • CommentTimeAug 6th 2011

    Cleaned up partition of unity and fine sheaf a bit, so I could link to them from this MO answer to the question ’Why are there so many smooth functions?’.

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeAug 6th 2011

    Thanks! That’s the way to go!

    I have edited the formatting ot fine sheaf a bit (sections, floating TOC, etc) and added “Related concepts” cross-references between the entries

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeMay 12th 2017

    I have spelled out the detailed proof that smooth manifolds admit smooth partitions of unity, here

    • CommentRowNumber4.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMay 12th 2017

    The statement of this theorem requires X to be paracompact, but in the proof it says “the smooth manifold X X is a normal topological space because it is a compact Hausdorff space”, i.e., X is compact.

    • CommentRowNumber5.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMay 12th 2017

    A displayed formula in the proof reads:

    Vi⊂Cl(U’i)⊂U’i⊂Ui

    I presume this should really be

    Vi⊂Cl(Vi)⊂U’i⊂Ui?

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeMay 12th 2017

    Thanks, yes, fixed now.

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeMay 30th 2017

    have added a simple example

    • CommentRowNumber8.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMar 19th 2020

    Proposition 4.1 seems to claim that existence of partitions of unity for all open covers implies Hausdorffness. But the antidiscrete topology admits partitions of unity and is not Hausdorff.

    Additionally, the article uses point-finite partitions of unity, but it seems to me that the more restrictive class of locally finite partitions occurs far more often in the literature.

    • CommentRowNumber9.
    • CommentAuthorarsmath
    • CommentTimeMay 17th 2020

    Add reference to Engelking for Mather result.

    diff, v33, current

    • CommentRowNumber10.
    • CommentAuthorDmitri Pavlov
    • CommentTimeJun 16th 2021

    Added:


    The case of non-Hausdorff spaces

    Slightly more generally, a topological space (not necessarily Hausdorff) is fully normal if and only every open cover admits a subordinate partition of unity.

    A T1-space is fully normal if and only if it is paracompact, in which case it is also Hausdorff.

    For topological spaces that are not T1-spaces, the condition of being fully normal is strictly stronger than paracompactness.

    The case of locales

    A regular locale is fully normal if and only if it is paracompact.

    The usual proof of the existence of partitions of unity goes through for such locales since it does not make any use of points.


    diff, v34, current

Add your comments
  • Please log in or leave your comment as a "guest post". If commenting as a "guest", please include your name in the message as a courtesy. Note: only certain categories allow guest posts.
  • To produce a hyperlink to an nLab entry, simply put double square brackets around its name, e.g. [[category]]. To use (La)TeX mathematics in your post, make sure Markdown+Itex is selected below and put your mathematics between dollar signs as usual. Only a subset of the usual TeX math commands are accepted: see here for a list.

  • (Help)