Locally compact and hausdorff proof

In summary, to find a compact neighborhood of x contained in U, we can take the set D=Fr(N\cap\bar U) which is closed and compact, and then find disjoint neighborhoods N_y and N_y' of y and x respectively for each y in D. The set \{N_y:y\in D\} is an open cover of D and has a finite subcover \{N_{y_n}:y_n\in D\}. The set \cap \bar N_{y_n}'} is a closed neighborhood of x and is contained in U.
  • #1
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Homework Statement


Let [tex]X[/tex] be a locally compact, Hausdorff topological space. If [tex]x[/tex] is an element of [tex]X[/tex] and [tex]U[/tex] is a neighborhood of [tex]x[/tex], find a compact neighborhood of [tex]x[/tex] contained in [tex]U[/tex].

Homework Equations


The Attempt at a Solution


Let [tex]N[/tex] be a compact neighborhood of [tex]x_[/tex]. The set [tex]D=Fr(N\cap\bar U)[/tex] is closed, hence compact. For each [tex]y\in D[/tex], there exist disjoint neighborhoods [tex]N_y[/tex] and [tex]N_y'[/tex] of [tex]y[/tex] and [tex]x[/tex], respectively. The set [tex]\{N_y:y\in D\}[/tex] is an open cover of [tex]D[/tex], hence it has a finite subcover [tex]\{N_{y_n}:y_n\in D\}[/tex]. The set [tex]\cap \bar N_{y_n}'}\subset N\cap U[/tex] is a closed neighborhood of [tex]x[/tex], hence it is compact.

Is this correct?
 
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  • #2
How can you be certain that [tex]\bigcap \overline{N_{y_n}^\prime}[/tex] is a subset of [tex]N\cap U[/tex]?? I think you still need to take the intersection with [tex]\overline{U}^\prime[/tex]...
 
  • #3
I think so. The N'_y's are contained in U^0, and that every neighborhood of D is disjoint from the intersection of the N'_y's shows that their closure is also disjoint from it, and contained in U^0. Closed neighborhood contained in N => compact neighborhood.

This was my thinking, and its the part I'm worried about. If you've got any hints to an easier way...

^0 denotes interior.
 

1. What does it mean for a space to be locally compact and Hausdorff?

A locally compact and Hausdorff space is a topological space that satisfies two important properties: local compactness and Hausdorffness. Local compactness means that every point in the space has a compact neighborhood, while Hausdorffness means that every two distinct points in the space have disjoint open neighborhoods.

2. Why are locally compact and Hausdorff spaces important in mathematics?

Locally compact and Hausdorff spaces are important because they provide a framework for studying topological spaces that have nice properties, such as being well-behaved under continuous maps and allowing for the existence of useful constructions, like partitions of unity. They also appear in many areas of mathematics, such as geometry, analysis, and algebraic topology.

3. Can you give an example of a locally compact and Hausdorff space?

One example of a locally compact and Hausdorff space is the real line, equipped with the usual Euclidean topology. Every point in the real line has a compact neighborhood (for example, the interval [0,1]), and any two distinct points have disjoint open neighborhoods (for example, the open intervals (0,1/2) and (1/2,1)).

4. How can you prove that a space is locally compact and Hausdorff?

There are several ways to prove that a space is locally compact and Hausdorff. One approach is to use the definition directly and show that the space satisfies the properties of local compactness and Hausdorffness. Another approach is to use known theorems and properties to deduce that the space is locally compact and Hausdorff. For example, a Hausdorff space that is also paracompact is locally compact.

5. Is every locally compact and Hausdorff space also metrizable?

No, not every locally compact and Hausdorff space is metrizable. While all metric spaces are locally compact and Hausdorff, the converse is not necessarily true. For example, the space of continuous functions on a compact interval is locally compact and Hausdorff, but it is not metrizable.

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