
Re: Precompactness
Posted:
Dec 13, 2012 2:52 AM


On Wed, 12 Dec 2012, Kaba wrote: > 12.12.2012 5:24, William Elliot wrote: > > On Tue, 11 Dec 2012, Kaba wrote:
> > > Let X be a locally compact Hausdorff space. Is every open set of X > > > precompact (compact closure)?
> Related, let X be a Hausdorff space. Royden (Real analysis) defines E subset X > to be _bounded_ if it is contained in a compact set. It seems to me that > precompact and bounded are equivalent properties. > It's not for R with the include 0 topology, { U  0 in U } \/ {empty set}. {0} is bounded but not precompact.
> Assume E is precompact. Then cl(E) is a compact set which contains E. > Therefore E is bounded. Assume E is bounded. Then there is a compact set K > such that E subset K. Since X is Hausdorff, K is closed.
Why is X Hausdorff?
> subset K. Since cl(E) is a closed subset of a compact set K, cl(E) is compact. > Therefore E is precompact. > > Unless I am missing something?
Assuming X is Hausdorff.
For Hausdorff spaces, or more general, kc spaces (compact sets are closed), precompact and bounded are equivalent. Otherwise, they're not.

