Friday, 1 February 2008

gn.general topology - Paracompact but not Hausdorff

The answer is no.
Take the "classical" example of the line with two origins. This space is non-Hausdorff, paracompact and doesn't admit partitions of unity.



EDTI: I think the question is a kind of "duplicate" .
Ok, but if you have an example for a non-Hausdorff manifold, which doesn't admit partitions of unity, you have an example for a non-Hausdorff paracompact space with the same property.



First the definition:
The line with two origins is the quotient space of two copies of the real line
mathbbRtimesa and mathbbRtimesb.
with equivalence relation given by
(x,a)sim(x,b)textifxneq0.
Since all neighbourhoods of 0a intersect all neighbourhoods of 0b, it is non-Hausdorff.
However, this space is paracompact, since mathbbR is paracompact.



For the non-existence of a partition of unitiy: take the open covering U=(infty,0)cup0acup(0,infty) and tildeU=(infty,0)cup0bcup(0,infty). Assume, there is a partition of unity subordinate to this cover. Then the value of each origin would have to be 1 which cannot be true. (Edit: villemoes was a little faster :-) )

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