Tube lemma generalization proof

radou

1. The problem statement, all variables and given/known data

Let A and B be compact subspaces of X and Y, respectively. Let N be an open set in X x Y containing A x B. One needs to show that there exist open sets U in X and V in Y such that A x B [itex]\subseteq[/itex] U x V [itex]\subseteq[/itex] N.

3. The attempt at a solution

Here's my try:

First of all, since N is open, it can be written as a union of basis elements in X x Y, i.e. let N = [itex]\cup U_{i} \times V_{i}[/itex].

Then we cover A x B with basis elements contained in N, so that [itex]A \times B \subseteq \cup U_{i}' \times V_{i}'[/itex]. Since A and B are compact, so is A x B, and for this cover, we have a finite subcover, so that [itex]A \times B \subseteq \cup_{i=1}^n U_{i}' \times V_{i}'[/itex].

Now we have the following relation:

[itex]A \times B \subseteq \cup_{i=1}^n U_{i}' \times V_{i}' \subseteq \cup U_{i} \times V_{i} = N[/itex].

Now, I'm not sure if this relation holds:

[tex]\cup_{i=1}^n (U_{i}' \times V_{i}') \cap (\cup U_{i} \times V_{i}) \subseteq \cup_{i=1}^n (U_{i}' \cap (\cup U_{i})) \times \cup_{i=1}^n (V_{i}' \cap (\cup V_{i})) \subseteq N[/tex]. If it does, then [tex]U = \cup_{i=1}^n (U_{i}' \cap (\cup U_{i}))[/tex] and [tex]V = \cup_{i=1}^n (V_{i}' \cap (\cup V_{i}))[/tex] are the sets we were looking for.

radou

If x = (a, b) is in [tex](\cup_{i=1}^n (U_{i}' \times V_{i}')) \cap (\cup U_{i} \times V_{i})[/tex] then a is in Ui, b is in Vi, for some i, and a is in Ui' and b is in Vi'. So, a is in the intersection of Ui and Ui', for some i, and b is in the intersection of Vi and Vi', for some i, i.e. in their unions, so a is in [tex](\cup_{i=1}^n (U_{i}' \cap (\cup U_{i}))) \times (\cup_{i=1}^n (V_{i}' \cap (\cup V_{i})))[/tex].

Any comments?

Edit: just corrected some LaTeX errors and added some brackets, now it should be OK.

radou

Still no ideas? I'd like to know if this is correct, so I can finalize the exercise section I'm working on.