# Closed set in an open set

1. Nov 26, 2007

### ehrenfest

1. The problem statement, all variables and given/known data
I have a closed set in an open set in a metric space and I am trying to find an epsilon radius of the closed set that is in the open set. So I want to find some way to take the infimal distance between the boundary of the closed set and the boundary of the open set...but I need to prove that this is not zero...?

2. Relevant equations

3. The attempt at a solution

2. Nov 26, 2007

### CompuChip

Why would the distance have to be infinitesimal? If I have a closed $\epsilon$-ball, then wouldn't an open $\frac{\epsilon}{2}$-ball do?
Now generalize to any closed set.

3. Nov 26, 2007

### HallsofIvy

Staff Emeritus
Compuchip, I don't think "infimal" was meant to be "infinitesmal" but "infimum"- greatest lower bound of distances between the boundary of the closed set and boundary of the open set.

4. Nov 26, 2007

### ehrenfest

So, does anyone know how to do that?

5. Nov 28, 2007

### ehrenfest

anyone?

6. Nov 28, 2007

### Dick

You need more than just closed, is it compact?

7. Nov 28, 2007

### ehrenfest

Yes. It is closed an bounded. How do you use compactness here?

8. Nov 28, 2007

### Dick

The distance between a point in the closed set and the boundary of the open set is a nonzero, continuous function on a compact set. It attains a nonzero minimum.

9. Nov 28, 2007

### ehrenfest

Maybe this will work. Get a nbhd of each of the points on the boundary of the closed set that is in the open set. Then take the interior of the closed set. So now we have an open cover of the closed set and we must have a finite subcover. Let x_i be the finite set of boundary points whose nbhds are in the finite subcover. Take the minimum of the radii of these nbhds. This will serve as the epsilon nbhd.

Does that work?

EDIT: I posted this before I read the post above. But still does this work?

10. Nov 28, 2007

### Dick

You are ok as far as the finite subcover, but then you lose me. There's not a finite set of bdy pts x_i, there is a finite set of open balls. It's easy to draw a picture where the bdy of the open set comes closer to an enclosed closed set than the minimum radius of a certain covering with balls.