# Thread: Topology Open/Closed Sets

1. ## Topology Open/Closed Sets

Hey there, I don't know if anyone can help with this or not but what the heck.
Y=(1,5]. Which of the following subsets of Y are open and which are closed in Y, in the standard topology?

(0,1), (0,1], [1,2]

Thanks

2. Originally Posted by reagan3nc
Hey there, I don't know if anyone can help with this or not but what the heck.
Y=(1,5]. Which of the following subsets of Y are open and which are closed in Y, in the standard topology?

(0,1), (0,1], [1,2]

Thanks

None of these sets are subsets of Y! Did you mean Y = (0, 5] ?

-Dan

3. Yes I meant Y=(0,5] thanks for catching that.

4. The only open interval of real numbers have one of these forms: $\displaystyle \left( { - \infty ,a} \right),\quad \left( {a,b} \right)\quad or\quad \left( {b,\infty } \right)$

5. Originally Posted by Plato
The only open interval of real numbers have one of these forms: $\displaystyle \left( { - \infty ,a} \right),\quad \left( {a,b} \right)\quad or\quad \left( {b,\infty } \right)$
now here's one of those things that confuse me to no end in topology. (i never took topology officially, and the text i have is like a crash course in topology, so it's not good to learn from). but anyway, somewhere in the text they said [0,1] was an open set

here's the context:

they were considering the subspace $\displaystyle X = [0,1] \cup (2,3) \subset \mathbb{R}$. The claim was, this subspace is not connected since we can express it as the union of two dis-joint, open sets, namely [0,1] and (2,3). Now instead of explaining to a dunce like me why [0,1] is open in the topology of X, the book simply warns, "remember, folks, we're dealing with a topology of X and not that of $\displaystyle \mathbb{R}! \cdots$"

i tried looking back through the text to see what the author had said to make him believe this should not be a surprise to me...to no avail.

the best i could come up with is the definition, "Let $\displaystyle X$ be a topological space. The set $\displaystyle A \subset X$ is called closed when $\displaystyle X \backslash A$ is open"

now i could not reconcile this with the claim [0,1] is open. is not $\displaystyle X \backslash [0,1] = (2,3)$, which is open, so $\displaystyle [0,1]$ should be closed?