# "Show B is closed if and only if...." (More Topology....)

• sa1988
In summary: B## is closed.As we have assumed that ##B## shall be closed in ##A##, we can use the same argument as before and define ##\tau_A## to be the set of all open sets in ##A##. This is indeed the desired topology, as it defines "closed in ##A##".
sa1988

## Homework Statement

Let ##A## be a subspace of a topological space ##X##, and let ##B\subset A##.

Show that ##B## is closed if and only if there exists a closed subset ##C \subset X## such that ##B = C \cap A##

## The Attempt at a Solution

So I've started by just drawing the two cases in which ##B## is said to exist. It doesn't really help with much but it's a way for me to see clearly what I'm supposed to be working with.

I believe the right hand image demonstrates the 'only if' part of the statement, where we have ##B \subset A \subset X## which is open by definition. (Since the complement of ##A## is ##X## which is open because it's a topological space, thus ##A## is closed, and then the same applies to show that ##B## is open since the complement of ##B## is closed).

So ##B## can only possibly be closed in the other case, shown in the left hand image, which is the 'if' part. From this we can see that ##\exists## neighbourhoods ##N## of ##n \in B## where ##N \subset B## and ##N \subset C##, so ##N \subset B \cap C ##

But if ##N \subset B \cap C ##, this means ##B## contains elements with neighbourhoods that go beyond its boundaries into the closed set ##C##.

Hence ##B## must be closed too, in the case where ##B = A \cap C##, and only this case.

Is this proof heading in the right direction? I fear I may have been a bit too hand-wavy with the 'only if' part.

Thanks.

sa1988 said:

## Homework Statement

Let ##A## be a subspace of a topological space ##X##, and let ##B\subset A##.

Show that ##B## is closed if and only if there exists a closed subset ##C \subset X## such that ##B = C \cap A##

## The Attempt at a Solution

So I've started by just drawing the two cases in which ##B## is said to exist. It doesn't really help with much but it's a way for me to see clearly what I'm supposed to be working with.

I believe the right hand image demonstrates the 'only if' part of the statement, where we have ##B \subset A \subset X## which is open by definition. (Since the complement of ##A## is ##X## which is open because it's a topological space, thus ##A## is closed, and then the same applies to show that ##B## is open since the complement of ##B## is closed).

So ##B## can only possibly be closed in the other case, shown in the left hand image, which is the 'if' part. From this we can see that ##\exists## neighbourhoods ##N## of ##n \in B## where ##N \subset B## and ##N \subset C##, so ##N \subset B \cap C ##

But if ##N \subset B \cap C ##, this means ##B## contains elements with neighbourhoods that go beyond its boundaries into the closed set ##C##.

Hence ##B## must be closed too, in the case where ##B = A \cap C##, and only this case.

Is this proof heading in the right direction? I fear I may have been a bit too hand-wavy with the 'only if' part.

Thanks.
I'm not quite sure what this problem is about. Since it is true, it is important to separate what is known from what has to be shown.
Let ##A## be a subspace of a topological space ##X##, and let ##B\subset A##.
So far we have only sets: ##B \subseteq A \subseteq X = (X,\tau)##. One of them with a topology, and only one, namely ##X##.
Show that ##B## is closed ...
Here is the first sloppy notation. It makes a difference, whether ##B## is closed in ##(X,\tau)## or closed in ##A##, for which we have strictly speaking no topology yet. Thus we cannot decide, what is meant by ##B## is closed. Remember that ##[0,\frac{1}{2})## is a open set in ##[0,1] \subseteq \mathbb{R}## with the (induced) subset topology in ##[0,1]##, whereas it is not open in ##\mathbb{R}##. This shows that it is crucial to define where a set is regarded as open. In our case as we regard closed sets, we might change the example a little bit and define ##(0,1) =: A \subseteq \mathbb{R}## and ##B=(0,\frac{1}{2}]##. Then ##B \subseteq A## is closed and ##B \subseteq \mathbb{R}## is not!

To proceed, let's make as two assumptions:
1. ##B## shall be closed in ##A##.
2. But therefore we need a topology of ##A##, which defines "closed in ##A##". As ##A \subseteq (X,\tau)## is a subset of a topological space, it's natural to assume that it carries the (from ##\tau##) induced topology ##\tau_A##.
Now here we face the next difficulty: What is ##\tau_A##? As I have mentioned yesterday, a set ##B \subseteq A## is said to be open in ##\tau_A##, if and only if there is a open set ##C \subseteq (X,\tau)## of ##X##, such that ##B=C \cap A##. However, this is very close to the statement that we want to prove.
... if and only if there exists a closed subset ##C \subset X## such that ##B = C \cap A##
So the question is: which definition of the topology ##\tau_A## of a subset ##A## do you actually use? It is always a good start to list what is given, i.e. which definitions, theorems and so on. It is the reason why we require part 2 of our template to be filled out. And in this case it is even more important, as the statement which has to be proven can as well be taken as the definition of ##\tau_A## and nothing would be left to show. So the only difference between the definition of ##(A,\tau_A)## and the statement is the fact, that one deals with open and the other one with closed sets? In this case we have to deal with the complements.

Let ##B \subseteq (A,\tau_A)## be a closed set, i.e. ##A-B \subseteq (A,\tau_A)## is open and by definition ##A-B = C \cap A## for a open set ##C \subseteq (X,\tau)##. ...

Now you have to find a closed set ##D \subseteq (X,\tau)## for which ##B=D \cap A## to prove ##"\Longrightarrow "## and in a further step ##"\Longleftarrow "##.

sa1988
In which case I best add the little bit in brackets that I thought wasn't vital to the question!

"Let ##A## be a subspace of a topological space ##X## (that is, with the subspace topology), and let ... etc etc..." <-- note the part in brackets which I missed out in the original question wording. Sorry!

I'm a little confused by what you said. Why does it need this extra set ##D## ?

Is it not possible to use open neighbourhoods and show that they overlap on boundaries where they shouldn't?

Thanks.

sa1988 said:
I'm a little confused by what you said. Why does it need this extra set ##D## ?
I ran out of letters. You may switch ##C## and ##D## if you like or use ##C_1## and ##C_2##. Since we only have the definition of "open", I used up ##C## for the open set ##A-B=C \cap A## and had to take a new letter for the set in the statement ##B=D \cap A## which is supposed to be closed. Of course, ##D## and ##C## are closely related, since ##C## is our only candidate to define ##D##, but it cannot be equal, because on is open (##C## in my notation) and one must be constructed closed (##D## in my notation).
Is it not possible to use open neighbourhoods and show that they overlap on boundaries where they shouldn't?
Maybe, but then you bring in additional sets, namely boundaries (which you haven't defined, yet) and plenty of neighborhoods (one for each point in the boundary). I don't think this complication is necessary as we only have to switch closed to open and back. This can easily be done by only considering the complements. Just stay close at the definition, because that's all we have.

sa1988
I think it's this concept of complements which is killing me now. Everything seems to be continually flipping between open and closed (or 'clopen' in other cases!).

It's interesting to see that this problem can be overcome with just these concepts, however. Certainly shows the importance of openness and closedness - which I don't think I've properly grasped yet.

So if we're looking at the original definition and the scenario in the question, we have ##A \subset (X, \tau)## , with the subspace topology.

So ##A## is a subset equipped with a topology and is therefore open.

Then introduce ##C \subset X## which is defined as closed, and create ##B = A \cap C##.

So ##B## is the set created by the intersection of ##A \subset X## (open) and ##C \subset X## (closed), and therefore ##B## is closed.

Now to show the 'only if' part:

Construct ##B## with the intersection of ##A## (open) and ##D## (open), then ##B## is open, because the intersection of any open sets it open.

Hence ##B## is closed if and only if ##B = A \cap C## where ##C \subset X## is closed.

This is honestly all I can fathom right now, and I'm pretty sure I'm still missing something.

## 1. What does it mean for a show to be "closed" in topology?

In topology, a set is considered closed if it contains all of its limit points. This means that if you take any sequence of points within the set, and those points converge to a point outside the set, then the set is not considered closed.

## 2. Can you explain the concept of "if and only if" in relation to closed shows in topology?

"If and only if" is a logical statement that means both conditions must be true for a statement to be true. In the case of a closed show in topology, it means that the show is closed if and only if it contains all of its limit points. So, if a show is closed, it must contain all of its limit points, and if it contains all of its limit points, it is considered closed.

## 3. How does the concept of "closed" shows relate to open shows in topology?

In topology, a set is considered open if it does not contain its boundary points. This is the opposite of a closed set, which contains all of its limit points. In terms of shows, a closed show would have all of its performances sold out, whereas an open show would still have tickets available for purchase.

## 4. What are some examples of closed shows in topology?

One example of a closed show in topology is a completely sold out concert, where there are no more tickets available for purchase. Another example could be a closed art exhibit, where all of the pieces have been sold and there are no more on display.

## 5. How does the concept of closed shows in topology apply to other fields of science?

The concept of closed shows in topology can be applied to other fields of science, such as biology and physics. In biology, a closed system would be one where no matter (such as energy or nutrients) can enter or leave the system. In physics, a closed system would be one where there is no exchange of matter or energy with the surroundings. This concept is important in understanding the behavior of systems and their interactions with the environment.

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