# Thread: Interior, Closure and Boundary of sets

1. ## Interior, Closure and Boundary of sets

A little topology (I think) problem here:

For A:= [0,1) ∪ ([2,3] ∩ ) ∪ {5} ⊂ R (real numbers) find the closure, boundary and interior of A

I know the definitions but I don't know how to actually calculate them

2. ## Re: Interior, Closure and Boundary of sets

To compute the closure, just notice that the closure of a finite union is the union of the closures.
For the interior, can the points of $\left[2,3\right]\cap \mathbb Q$ be in it? And what about $\left\{5\right\}$?

3. ## Re: Interior, Closure and Boundary of sets

Originally Posted by fylth
For A:= [0,1) ∪ ([2,3] ∩ ) ∪ {5} ⊂ R (real numbers) find the closure, boundary and interior of A
Do you know that $\overline{A\cup B}=\overline{A}\cup\overline{B}~?$

$\overline{[0,1)}=[0,1],~\overline{[2,3]\cap\mathbb{Q}}=[2,3],~\&~\overline{\{5\}}=\{5\}$

4. ## Re: Interior, Closure and Boundary of sets

The boundary is the closure toss (set difference) the set...
The interior is...