Intermediate Value Theorem in point-set topology.












0












$begingroup$


Theorem: Let $f:X to Y$ be a continuous map, where $X$ is a connected space and Y is an ordered set in the order topology. If $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$, then $exists c in X (f(c)=r)$



Proof Attempt: Let $f:X to Y$ be a continuous map where $X$ is a connected space and $Y$ is an ordered set under the order topology. Suppose $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$. Since $X$ is connected then $f(X)$ is connected. Let $f(X)=(gamma_0, gamma)cup (overline{gamma},gamma')$. Since $f(a),f(b) in f(X) subset Y$, then $f(a),f(b)in (gamma_0, gamma)cup (overline{gamma},gamma')$, then, w.l.o.g., let $f(b)in (overline{gamma},gamma')$, and $f(a)in (overline{gamma},gamma')$. Also, $f(X)$ is connected so $(gamma_0,gamma)cap (gamma,gamma')neqemptyset$. This forces $rin(gamma_0,gamma)cup (overline{gamma},gamma')=f(X)$. Therefore, $rin f(X)$ and $exists cin X(f(c)=r)$.










share|cite|improve this question











$endgroup$












  • $begingroup$
    What is the question?
    $endgroup$
    – Wojowu
    Jan 6 at 12:46










  • $begingroup$
    I want the proof I wrote to be verified.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:46












  • $begingroup$
    Could you at least let us know what you are proving? Please give the complete statement.
    $endgroup$
    – Wojowu
    Jan 6 at 12:47










  • $begingroup$
    I have added the theorem's statement. Thanks.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:52










  • $begingroup$
    why is $f[X]$ of that form? I see no justification. Just a lot of useless $gamma$'s I think.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 13:27


















0












$begingroup$


Theorem: Let $f:X to Y$ be a continuous map, where $X$ is a connected space and Y is an ordered set in the order topology. If $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$, then $exists c in X (f(c)=r)$



Proof Attempt: Let $f:X to Y$ be a continuous map where $X$ is a connected space and $Y$ is an ordered set under the order topology. Suppose $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$. Since $X$ is connected then $f(X)$ is connected. Let $f(X)=(gamma_0, gamma)cup (overline{gamma},gamma')$. Since $f(a),f(b) in f(X) subset Y$, then $f(a),f(b)in (gamma_0, gamma)cup (overline{gamma},gamma')$, then, w.l.o.g., let $f(b)in (overline{gamma},gamma')$, and $f(a)in (overline{gamma},gamma')$. Also, $f(X)$ is connected so $(gamma_0,gamma)cap (gamma,gamma')neqemptyset$. This forces $rin(gamma_0,gamma)cup (overline{gamma},gamma')=f(X)$. Therefore, $rin f(X)$ and $exists cin X(f(c)=r)$.










share|cite|improve this question











$endgroup$












  • $begingroup$
    What is the question?
    $endgroup$
    – Wojowu
    Jan 6 at 12:46










  • $begingroup$
    I want the proof I wrote to be verified.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:46












  • $begingroup$
    Could you at least let us know what you are proving? Please give the complete statement.
    $endgroup$
    – Wojowu
    Jan 6 at 12:47










  • $begingroup$
    I have added the theorem's statement. Thanks.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:52










  • $begingroup$
    why is $f[X]$ of that form? I see no justification. Just a lot of useless $gamma$'s I think.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 13:27
















0












0








0





$begingroup$


Theorem: Let $f:X to Y$ be a continuous map, where $X$ is a connected space and Y is an ordered set in the order topology. If $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$, then $exists c in X (f(c)=r)$



Proof Attempt: Let $f:X to Y$ be a continuous map where $X$ is a connected space and $Y$ is an ordered set under the order topology. Suppose $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$. Since $X$ is connected then $f(X)$ is connected. Let $f(X)=(gamma_0, gamma)cup (overline{gamma},gamma')$. Since $f(a),f(b) in f(X) subset Y$, then $f(a),f(b)in (gamma_0, gamma)cup (overline{gamma},gamma')$, then, w.l.o.g., let $f(b)in (overline{gamma},gamma')$, and $f(a)in (overline{gamma},gamma')$. Also, $f(X)$ is connected so $(gamma_0,gamma)cap (gamma,gamma')neqemptyset$. This forces $rin(gamma_0,gamma)cup (overline{gamma},gamma')=f(X)$. Therefore, $rin f(X)$ and $exists cin X(f(c)=r)$.










share|cite|improve this question











$endgroup$




Theorem: Let $f:X to Y$ be a continuous map, where $X$ is a connected space and Y is an ordered set in the order topology. If $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$, then $exists c in X (f(c)=r)$



Proof Attempt: Let $f:X to Y$ be a continuous map where $X$ is a connected space and $Y$ is an ordered set under the order topology. Suppose $a,b in X$ and $r in Y$ such that $f(a) lt r lt f(b)$. Since $X$ is connected then $f(X)$ is connected. Let $f(X)=(gamma_0, gamma)cup (overline{gamma},gamma')$. Since $f(a),f(b) in f(X) subset Y$, then $f(a),f(b)in (gamma_0, gamma)cup (overline{gamma},gamma')$, then, w.l.o.g., let $f(b)in (overline{gamma},gamma')$, and $f(a)in (overline{gamma},gamma')$. Also, $f(X)$ is connected so $(gamma_0,gamma)cap (gamma,gamma')neqemptyset$. This forces $rin(gamma_0,gamma)cup (overline{gamma},gamma')=f(X)$. Therefore, $rin f(X)$ and $exists cin X(f(c)=r)$.







general-topology proof-verification






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share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 6 at 12:52







TheLast Cipher

















asked Jan 6 at 12:44









TheLast CipherTheLast Cipher

715715




715715












  • $begingroup$
    What is the question?
    $endgroup$
    – Wojowu
    Jan 6 at 12:46










  • $begingroup$
    I want the proof I wrote to be verified.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:46












  • $begingroup$
    Could you at least let us know what you are proving? Please give the complete statement.
    $endgroup$
    – Wojowu
    Jan 6 at 12:47










  • $begingroup$
    I have added the theorem's statement. Thanks.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:52










  • $begingroup$
    why is $f[X]$ of that form? I see no justification. Just a lot of useless $gamma$'s I think.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 13:27




















  • $begingroup$
    What is the question?
    $endgroup$
    – Wojowu
    Jan 6 at 12:46










  • $begingroup$
    I want the proof I wrote to be verified.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:46












  • $begingroup$
    Could you at least let us know what you are proving? Please give the complete statement.
    $endgroup$
    – Wojowu
    Jan 6 at 12:47










  • $begingroup$
    I have added the theorem's statement. Thanks.
    $endgroup$
    – TheLast Cipher
    Jan 6 at 12:52










  • $begingroup$
    why is $f[X]$ of that form? I see no justification. Just a lot of useless $gamma$'s I think.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 13:27


















$begingroup$
What is the question?
$endgroup$
– Wojowu
Jan 6 at 12:46




$begingroup$
What is the question?
$endgroup$
– Wojowu
Jan 6 at 12:46












$begingroup$
I want the proof I wrote to be verified.
$endgroup$
– TheLast Cipher
Jan 6 at 12:46






$begingroup$
I want the proof I wrote to be verified.
$endgroup$
– TheLast Cipher
Jan 6 at 12:46














$begingroup$
Could you at least let us know what you are proving? Please give the complete statement.
$endgroup$
– Wojowu
Jan 6 at 12:47




$begingroup$
Could you at least let us know what you are proving? Please give the complete statement.
$endgroup$
– Wojowu
Jan 6 at 12:47












$begingroup$
I have added the theorem's statement. Thanks.
$endgroup$
– TheLast Cipher
Jan 6 at 12:52




$begingroup$
I have added the theorem's statement. Thanks.
$endgroup$
– TheLast Cipher
Jan 6 at 12:52












$begingroup$
why is $f[X]$ of that form? I see no justification. Just a lot of useless $gamma$'s I think.
$endgroup$
– Henno Brandsma
Jan 6 at 13:27






$begingroup$
why is $f[X]$ of that form? I see no justification. Just a lot of useless $gamma$'s I think.
$endgroup$
– Henno Brandsma
Jan 6 at 13:27












1 Answer
1






active

oldest

votes


















2












$begingroup$

Better: suppose we have $(Y,<)$ in the order topology and $f: X to Y$ continuous with $X$ connected and for some points $a,b in X$ we have $f(a) < r < f(b)$ with $r in Y$.



Suppose there is no $x in X$ with $f(x)=r$.



Then define $O_1={y in Y: y < r }$ and $O_2={y in Y: y > r}$ which are open in $Y$ in the order topology.



Firstly $$f^{-1}[O_1] cup f^{-1}[O_2] = X$$



because we assumed $r$ is not assumed as a value, and $Y$ has a linear order, so always $f(x) > r$ or $f(x) < r$ must hold, but not both, so the sets are disjoint. We also know that $a in f^{-1}[O_1]$ and $b in f^{-1}[O_2]$, so both these sets are open (by continity of $f$), disjoint, non-empty and cover $X$. This is a contradiction with the connectedness of $X$. So we must have some $x$ with $f(x)=r$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:08












  • $begingroup$
    @TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:19










  • $begingroup$
    I don't understand why it would not matter, especially when we use it as our starting assumption. :(
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:23










  • $begingroup$
    @TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:27










  • $begingroup$
    ahh yes. It's the contrapositive. Thanks!
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:30











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1 Answer
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active

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









2












$begingroup$

Better: suppose we have $(Y,<)$ in the order topology and $f: X to Y$ continuous with $X$ connected and for some points $a,b in X$ we have $f(a) < r < f(b)$ with $r in Y$.



Suppose there is no $x in X$ with $f(x)=r$.



Then define $O_1={y in Y: y < r }$ and $O_2={y in Y: y > r}$ which are open in $Y$ in the order topology.



Firstly $$f^{-1}[O_1] cup f^{-1}[O_2] = X$$



because we assumed $r$ is not assumed as a value, and $Y$ has a linear order, so always $f(x) > r$ or $f(x) < r$ must hold, but not both, so the sets are disjoint. We also know that $a in f^{-1}[O_1]$ and $b in f^{-1}[O_2]$, so both these sets are open (by continity of $f$), disjoint, non-empty and cover $X$. This is a contradiction with the connectedness of $X$. So we must have some $x$ with $f(x)=r$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:08












  • $begingroup$
    @TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:19










  • $begingroup$
    I don't understand why it would not matter, especially when we use it as our starting assumption. :(
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:23










  • $begingroup$
    @TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:27










  • $begingroup$
    ahh yes. It's the contrapositive. Thanks!
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:30
















2












$begingroup$

Better: suppose we have $(Y,<)$ in the order topology and $f: X to Y$ continuous with $X$ connected and for some points $a,b in X$ we have $f(a) < r < f(b)$ with $r in Y$.



Suppose there is no $x in X$ with $f(x)=r$.



Then define $O_1={y in Y: y < r }$ and $O_2={y in Y: y > r}$ which are open in $Y$ in the order topology.



Firstly $$f^{-1}[O_1] cup f^{-1}[O_2] = X$$



because we assumed $r$ is not assumed as a value, and $Y$ has a linear order, so always $f(x) > r$ or $f(x) < r$ must hold, but not both, so the sets are disjoint. We also know that $a in f^{-1}[O_1]$ and $b in f^{-1}[O_2]$, so both these sets are open (by continity of $f$), disjoint, non-empty and cover $X$. This is a contradiction with the connectedness of $X$. So we must have some $x$ with $f(x)=r$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:08












  • $begingroup$
    @TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:19










  • $begingroup$
    I don't understand why it would not matter, especially when we use it as our starting assumption. :(
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:23










  • $begingroup$
    @TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:27










  • $begingroup$
    ahh yes. It's the contrapositive. Thanks!
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:30














2












2








2





$begingroup$

Better: suppose we have $(Y,<)$ in the order topology and $f: X to Y$ continuous with $X$ connected and for some points $a,b in X$ we have $f(a) < r < f(b)$ with $r in Y$.



Suppose there is no $x in X$ with $f(x)=r$.



Then define $O_1={y in Y: y < r }$ and $O_2={y in Y: y > r}$ which are open in $Y$ in the order topology.



Firstly $$f^{-1}[O_1] cup f^{-1}[O_2] = X$$



because we assumed $r$ is not assumed as a value, and $Y$ has a linear order, so always $f(x) > r$ or $f(x) < r$ must hold, but not both, so the sets are disjoint. We also know that $a in f^{-1}[O_1]$ and $b in f^{-1}[O_2]$, so both these sets are open (by continity of $f$), disjoint, non-empty and cover $X$. This is a contradiction with the connectedness of $X$. So we must have some $x$ with $f(x)=r$.






share|cite|improve this answer









$endgroup$



Better: suppose we have $(Y,<)$ in the order topology and $f: X to Y$ continuous with $X$ connected and for some points $a,b in X$ we have $f(a) < r < f(b)$ with $r in Y$.



Suppose there is no $x in X$ with $f(x)=r$.



Then define $O_1={y in Y: y < r }$ and $O_2={y in Y: y > r}$ which are open in $Y$ in the order topology.



Firstly $$f^{-1}[O_1] cup f^{-1}[O_2] = X$$



because we assumed $r$ is not assumed as a value, and $Y$ has a linear order, so always $f(x) > r$ or $f(x) < r$ must hold, but not both, so the sets are disjoint. We also know that $a in f^{-1}[O_1]$ and $b in f^{-1}[O_2]$, so both these sets are open (by continity of $f$), disjoint, non-empty and cover $X$. This is a contradiction with the connectedness of $X$. So we must have some $x$ with $f(x)=r$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 6 at 13:26









Henno BrandsmaHenno Brandsma

113k348123




113k348123












  • $begingroup$
    I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:08












  • $begingroup$
    @TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:19










  • $begingroup$
    I don't understand why it would not matter, especially when we use it as our starting assumption. :(
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:23










  • $begingroup$
    @TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:27










  • $begingroup$
    ahh yes. It's the contrapositive. Thanks!
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:30


















  • $begingroup$
    I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:08












  • $begingroup$
    @TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:19










  • $begingroup$
    I don't understand why it would not matter, especially when we use it as our starting assumption. :(
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:23










  • $begingroup$
    @TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
    $endgroup$
    – Henno Brandsma
    Jan 6 at 14:27










  • $begingroup$
    ahh yes. It's the contrapositive. Thanks!
    $endgroup$
    – TheLast Cipher
    Jan 6 at 14:30
















$begingroup$
I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
$endgroup$
– TheLast Cipher
Jan 6 at 14:08






$begingroup$
I initially thought of starting with the $f^{-1}[O_1]$ and $f^{-1}[O_2]$ like how you have defined it. But I think that would make $f(X)$ not connected, since $X$ is connected, would it not?
$endgroup$
– TheLast Cipher
Jan 6 at 14:08














$begingroup$
@TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
$endgroup$
– Henno Brandsma
Jan 6 at 14:19




$begingroup$
@TheLastCipher it doesn’t matter really if we get a contradiction from $X$ connected or $f[X]$ connected. The eventual idea will be the same.
$endgroup$
– Henno Brandsma
Jan 6 at 14:19












$begingroup$
I don't understand why it would not matter, especially when we use it as our starting assumption. :(
$endgroup$
– TheLast Cipher
Jan 6 at 14:23




$begingroup$
I don't understand why it would not matter, especially when we use it as our starting assumption. :(
$endgroup$
– TheLast Cipher
Jan 6 at 14:23












$begingroup$
@TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
$endgroup$
– Henno Brandsma
Jan 6 at 14:27




$begingroup$
@TheLastCipher I chose to use the assumption "$X$ connected" directly. $O_1$ and $O_2$ form a direct disconnection of $f[X]$ which also gives a contradiction with a known theorem. Both are OK. My proof can be seen as a partial reproof of $X$ connected implies $f[X]$ connected in a way.
$endgroup$
– Henno Brandsma
Jan 6 at 14:27












$begingroup$
ahh yes. It's the contrapositive. Thanks!
$endgroup$
– TheLast Cipher
Jan 6 at 14:30




$begingroup$
ahh yes. It's the contrapositive. Thanks!
$endgroup$
– TheLast Cipher
Jan 6 at 14:30


















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