Separation Proof
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I have been banging my head against this proof for a few days now, as I can visualize why it is true in my head, but don't know how to prove it in words:
Let $A$ and $B$ be nonempty subsets of $mathbb{R}$. Show that if there exist disjoint, open sets $U$ and $V$ with $A subseteq U$ and $B subseteq V$, then $A$ and $B$ are separated.
I've seen two answers to this proof on here, but I don't fully understand either one of them, and the question was asked so long ago that neither of those users are active on here anymore, so I can't even ask them specific questions to help my understanding. I have tried proving it directly, but immediately get bogged down in multiple cases of what $A$ looks like in $U$ while $B$ looks a certain way in $V$, and vice versa. I have also tried assuming that $A$ and $B$ aren't separated, and I can't find a way to reach a contradiction (or contrapositive) from that. I would greatly appreciate any assistance as it is the only proof from my homework that I haven't been able to figure out on my own.
general-topology
$endgroup$
add a comment |
$begingroup$
I have been banging my head against this proof for a few days now, as I can visualize why it is true in my head, but don't know how to prove it in words:
Let $A$ and $B$ be nonempty subsets of $mathbb{R}$. Show that if there exist disjoint, open sets $U$ and $V$ with $A subseteq U$ and $B subseteq V$, then $A$ and $B$ are separated.
I've seen two answers to this proof on here, but I don't fully understand either one of them, and the question was asked so long ago that neither of those users are active on here anymore, so I can't even ask them specific questions to help my understanding. I have tried proving it directly, but immediately get bogged down in multiple cases of what $A$ looks like in $U$ while $B$ looks a certain way in $V$, and vice versa. I have also tried assuming that $A$ and $B$ aren't separated, and I can't find a way to reach a contradiction (or contrapositive) from that. I would greatly appreciate any assistance as it is the only proof from my homework that I haven't been able to figure out on my own.
general-topology
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How do you define “separated”?
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– José Carlos Santos
Dec 9 '18 at 23:47
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The definition I was given is as follows: Two nonempty sets $A,B subseteq mathbb{R}$ are separated if $overline{A} cap B$ and $A cap overline{B}$ are both empty, where $overline{A}$ is the closure of $A$.
$endgroup$
– automattik
Dec 9 '18 at 23:51
add a comment |
$begingroup$
I have been banging my head against this proof for a few days now, as I can visualize why it is true in my head, but don't know how to prove it in words:
Let $A$ and $B$ be nonempty subsets of $mathbb{R}$. Show that if there exist disjoint, open sets $U$ and $V$ with $A subseteq U$ and $B subseteq V$, then $A$ and $B$ are separated.
I've seen two answers to this proof on here, but I don't fully understand either one of them, and the question was asked so long ago that neither of those users are active on here anymore, so I can't even ask them specific questions to help my understanding. I have tried proving it directly, but immediately get bogged down in multiple cases of what $A$ looks like in $U$ while $B$ looks a certain way in $V$, and vice versa. I have also tried assuming that $A$ and $B$ aren't separated, and I can't find a way to reach a contradiction (or contrapositive) from that. I would greatly appreciate any assistance as it is the only proof from my homework that I haven't been able to figure out on my own.
general-topology
$endgroup$
I have been banging my head against this proof for a few days now, as I can visualize why it is true in my head, but don't know how to prove it in words:
Let $A$ and $B$ be nonempty subsets of $mathbb{R}$. Show that if there exist disjoint, open sets $U$ and $V$ with $A subseteq U$ and $B subseteq V$, then $A$ and $B$ are separated.
I've seen two answers to this proof on here, but I don't fully understand either one of them, and the question was asked so long ago that neither of those users are active on here anymore, so I can't even ask them specific questions to help my understanding. I have tried proving it directly, but immediately get bogged down in multiple cases of what $A$ looks like in $U$ while $B$ looks a certain way in $V$, and vice versa. I have also tried assuming that $A$ and $B$ aren't separated, and I can't find a way to reach a contradiction (or contrapositive) from that. I would greatly appreciate any assistance as it is the only proof from my homework that I haven't been able to figure out on my own.
general-topology
general-topology
asked Dec 9 '18 at 23:45
automattikautomattik
517
517
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How do you define “separated”?
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– José Carlos Santos
Dec 9 '18 at 23:47
$begingroup$
The definition I was given is as follows: Two nonempty sets $A,B subseteq mathbb{R}$ are separated if $overline{A} cap B$ and $A cap overline{B}$ are both empty, where $overline{A}$ is the closure of $A$.
$endgroup$
– automattik
Dec 9 '18 at 23:51
add a comment |
$begingroup$
How do you define “separated”?
$endgroup$
– José Carlos Santos
Dec 9 '18 at 23:47
$begingroup$
The definition I was given is as follows: Two nonempty sets $A,B subseteq mathbb{R}$ are separated if $overline{A} cap B$ and $A cap overline{B}$ are both empty, where $overline{A}$ is the closure of $A$.
$endgroup$
– automattik
Dec 9 '18 at 23:51
$begingroup$
How do you define “separated”?
$endgroup$
– José Carlos Santos
Dec 9 '18 at 23:47
$begingroup$
How do you define “separated”?
$endgroup$
– José Carlos Santos
Dec 9 '18 at 23:47
$begingroup$
The definition I was given is as follows: Two nonempty sets $A,B subseteq mathbb{R}$ are separated if $overline{A} cap B$ and $A cap overline{B}$ are both empty, where $overline{A}$ is the closure of $A$.
$endgroup$
– automattik
Dec 9 '18 at 23:51
$begingroup$
The definition I was given is as follows: Two nonempty sets $A,B subseteq mathbb{R}$ are separated if $overline{A} cap B$ and $A cap overline{B}$ are both empty, where $overline{A}$ is the closure of $A$.
$endgroup$
– automattik
Dec 9 '18 at 23:51
add a comment |
2 Answers
2
active
oldest
votes
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I will prove that $overline Acap B=emptyset$. Since $Asubset U$ and $U$ and $V$ are disjoint, then $Asubset V^complement$. But $V$ is open and so $V^complement$ is closed. So, and since $Asubset V^complement$, $overline Asubset V^complement$. But then, since $Bsubset V$, $overline Acap B=emptyset$. For the same reason, $Acapoverline B=emptyset$.
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$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
1
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
add a comment |
$begingroup$
Hint: what can you say about the complements of $U$ and $V$ and their relationships with the closures of $A$ and $B$.
$endgroup$
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
add a comment |
Your Answer
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2 Answers
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$begingroup$
I will prove that $overline Acap B=emptyset$. Since $Asubset U$ and $U$ and $V$ are disjoint, then $Asubset V^complement$. But $V$ is open and so $V^complement$ is closed. So, and since $Asubset V^complement$, $overline Asubset V^complement$. But then, since $Bsubset V$, $overline Acap B=emptyset$. For the same reason, $Acapoverline B=emptyset$.
$endgroup$
$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
1
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
add a comment |
$begingroup$
I will prove that $overline Acap B=emptyset$. Since $Asubset U$ and $U$ and $V$ are disjoint, then $Asubset V^complement$. But $V$ is open and so $V^complement$ is closed. So, and since $Asubset V^complement$, $overline Asubset V^complement$. But then, since $Bsubset V$, $overline Acap B=emptyset$. For the same reason, $Acapoverline B=emptyset$.
$endgroup$
$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
1
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
add a comment |
$begingroup$
I will prove that $overline Acap B=emptyset$. Since $Asubset U$ and $U$ and $V$ are disjoint, then $Asubset V^complement$. But $V$ is open and so $V^complement$ is closed. So, and since $Asubset V^complement$, $overline Asubset V^complement$. But then, since $Bsubset V$, $overline Acap B=emptyset$. For the same reason, $Acapoverline B=emptyset$.
$endgroup$
I will prove that $overline Acap B=emptyset$. Since $Asubset U$ and $U$ and $V$ are disjoint, then $Asubset V^complement$. But $V$ is open and so $V^complement$ is closed. So, and since $Asubset V^complement$, $overline Asubset V^complement$. But then, since $Bsubset V$, $overline Acap B=emptyset$. For the same reason, $Acapoverline B=emptyset$.
edited Dec 10 '18 at 1:00
answered Dec 9 '18 at 23:57
José Carlos SantosJosé Carlos Santos
169k23132237
169k23132237
$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
1
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
add a comment |
$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
1
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
So by assuming $x in overline{A} cap B$, we are assuming that $A$ and $B$ are not separated and then arriving at a contradiction?
$endgroup$
– automattik
Dec 10 '18 at 0:07
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
$begingroup$
Also, what happened to $x$ in your proof? You start out with it, and then it never shows up again.
$endgroup$
– automattik
Dec 10 '18 at 0:56
1
1
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
That sentenc with the $x$ use useless and I've deleted it. What do you think now?
$endgroup$
– José Carlos Santos
Dec 10 '18 at 1:01
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
$begingroup$
I actually really like it. And I suppose you could've left the $x$ sentence in there and just made another sentence at the end saying, "This contradicts that $x in overline{A} cap B$," but either way your proof helped me to make sense of another proof I saw elsewhere on this site, so I greatly appreciate your efforts!
$endgroup$
– automattik
Dec 10 '18 at 1:04
add a comment |
$begingroup$
Hint: what can you say about the complements of $U$ and $V$ and their relationships with the closures of $A$ and $B$.
$endgroup$
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
add a comment |
$begingroup$
Hint: what can you say about the complements of $U$ and $V$ and their relationships with the closures of $A$ and $B$.
$endgroup$
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
add a comment |
$begingroup$
Hint: what can you say about the complements of $U$ and $V$ and their relationships with the closures of $A$ and $B$.
$endgroup$
Hint: what can you say about the complements of $U$ and $V$ and their relationships with the closures of $A$ and $B$.
answered Dec 9 '18 at 23:56
Rob ArthanRob Arthan
29.5k42967
29.5k42967
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
add a comment |
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
I believe this is what one of the other proofs I mentioned was getting at, which I did not quite follow. Since $U$ and $V$ are both open, the complement of $U$ must be closed. We know that $B$ must be a proper subset of this since it is not a subset of $U$.
$endgroup$
– automattik
Dec 10 '18 at 0:02
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
$begingroup$
Try drawing a picture showing $A$, $B$, $U$, $V$, the complements of $U$ and $V$ and the closures of $A$ and $B$. Remember that if $X$ is closed and $Y subseteq X$ then $mbox{cl}(Y) subseteq X$ and think about what this means if $X$ is given as $overline{Z}$ for some open set $Z$.
$endgroup$
– Rob Arthan
Dec 10 '18 at 0:10
add a comment |
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$begingroup$
How do you define “separated”?
$endgroup$
– José Carlos Santos
Dec 9 '18 at 23:47
$begingroup$
The definition I was given is as follows: Two nonempty sets $A,B subseteq mathbb{R}$ are separated if $overline{A} cap B$ and $A cap overline{B}$ are both empty, where $overline{A}$ is the closure of $A$.
$endgroup$
– automattik
Dec 9 '18 at 23:51