# Topology Open/Closed Sets

• Feb 27th 2008, 03:24 PM
reagan3nc
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

(Worried)
• Feb 27th 2008, 04:20 PM
topsquark
Quote:

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

(Worried)

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

-Dan
• Feb 27th 2008, 06:31 PM
reagan3nc
Yes I meant Y=(0,5] thanks for catching that.
• Feb 27th 2008, 06:39 PM
Plato
The only open interval of real numbers have one of these forms: $\left( { - \infty ,a} \right),\quad \left( {a,b} \right)\quad or\quad \left( {b,\infty } \right)$
• Feb 27th 2008, 06:52 PM
Jhevon
Quote:

Originally Posted by Plato
The only open interval of real numbers have one of these forms: $\left( { - \infty ,a} \right),\quad \left( {a,b} \right)\quad or\quad \left( {b,\infty } \right)$

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 :eek:

here's the context:

they were considering the subspace $X = [0,1] \cup (2,3) \subset \mathbb{R}$. 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 $\mathbb{R}! \cdots$" :confused:

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 $X$ be a topological space. The set $A \subset X$ is called closed when $X \backslash A$ is open"

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