# Regular open sets ,

1. May 24, 2012

### 99butterfly

Regular open sets,,,,

If U is an open set in a topological space (X,τ),is it true that U=〖int〗_X 〖cl〗_X U?Justify.

2. May 24, 2012

### 99butterfly

Re: Regular open sets,,,,

I think this says about regular open sets.
so I need to find an open set which does not satisfy the equality given in the question above.

3. May 24, 2012

### micromass

Re: Regular open sets,,,,

Try to find a counterexample. Take an nice open set in a nice space and remove a point.

4. May 24, 2012

### 99butterfly

Re: Regular open sets,,,,

thank you verymuch micromass.......

I have another question regarding closure axioms.

I know all the axioms but I'm confused with choosing two arbitrary subsets of X since it takes two possibilities for theta.

Please somebody help me with this!!!!

Let θ:P(X)→P(X),where θ(A)={A ;if |A| <|N|
X ; O/W.
Verify that θ satisfy Kuratowski closure axioms.

5. May 24, 2012

### micromass

Re: Regular open sets,,,,

Well, what are the axioms?? Which ones are troubling you??