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Example of a continuous function that don't have a continuous extension

Extending a continuous function defined on the rationalsLet $Asubset X$; let $f:Ato Y$ be continuous; let $Y$ be Hausdorff. Is there an example where there is no continuous function for $g$?Continuity of a product of two real valued continuous function.a counter example of extension of a continuous functionInverse of a continuous functionIf $Asubseteqmathbb R$ is closed and $f:Atomathbb R$ is right-continuous, is there a right-continuous extension of $f$ to $mathbb R$?Proving Topological Equivalence without finding a functionA function that can be continuously extended is continuousContinuous Extension of Densely Defined Continuous (but not Uniformly Continuous) Function.A topological space with the Universal Extension Property which is not homeomorphic to a retract of $mathbb{R}^J$?

5

0

$begingroup$

Give an example of a topological space $(X,tau)$, a subset $Asubset X$ that is dense in $X$ (i.e., $overline{A} = X$), and a continuous function $f:Atomathbb{R}$ that cannot be continually extended to $X$, that is, a $f$ for such do not exist a continuous function $g:Xto mathbb{R}$ such that $f(x) = g(x)$ for all $xin A$.

I just proved that if $f,g:Xtomathbb{R}$ are continuous and agree in a dense subset $Asubset X$ then they're equal.

I thought in $X=mathbb{R}$ with usual topology and $A = mathbb{R}-{0} =:mathbb{R}^* $, so I think $f:mathbb{R}^*tomathbb{R}, f(x) = x^{-1}$ is a continuous function that cannot be continually extended to $mathbb{R}$. I'm quite sure of this, but I'm stuck in proving it using the definition of continuity in general topological spaces.

Also, I'm quite confused on how this asked example is not a counterexample of what I proved.

Thanks in advance.

general-topology continuity

share|cite|improve this question

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The open intervals form a basis for topology on the real line. A set is open if and only if it contains an open interval around each of these points. Using this definition of open sets you can show that the two different definitions of continuity are actually the same in this case. So you're example will work. And to show it will work you can show it using the usual definition of continuity you're used to in the real numbers.
  $endgroup$
  – Melody
  yesterday
  

  
  
  
  

add a comment | 

5

0

$begingroup$

Give an example of a topological space $(X,tau)$, a subset $Asubset X$ that is dense in $X$ (i.e., $overline{A} = X$), and a continuous function $f:Atomathbb{R}$ that cannot be continually extended to $X$, that is, a $f$ for such do not exist a continuous function $g:Xto mathbb{R}$ such that $f(x) = g(x)$ for all $xin A$.

I just proved that if $f,g:Xtomathbb{R}$ are continuous and agree in a dense subset $Asubset X$ then they're equal.

I thought in $X=mathbb{R}$ with usual topology and $A = mathbb{R}-{0} =:mathbb{R}^* $, so I think $f:mathbb{R}^*tomathbb{R}, f(x) = x^{-1}$ is a continuous function that cannot be continually extended to $mathbb{R}$. I'm quite sure of this, but I'm stuck in proving it using the definition of continuity in general topological spaces.

Also, I'm quite confused on how this asked example is not a counterexample of what I proved.

Thanks in advance.

general-topology continuity

share|cite|improve this question

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The open intervals form a basis for topology on the real line. A set is open if and only if it contains an open interval around each of these points. Using this definition of open sets you can show that the two different definitions of continuity are actually the same in this case. So you're example will work. And to show it will work you can show it using the usual definition of continuity you're used to in the real numbers.
  $endgroup$
  – Melody
  yesterday
  

  
  
  
  

add a comment | 

$begingroup$

Give an example of a topological space $(X,tau)$, a subset $Asubset X$ that is dense in $X$ (i.e., $overline{A} = X$), and a continuous function $f:Atomathbb{R}$ that cannot be continually extended to $X$, that is, a $f$ for such do not exist a continuous function $g:Xto mathbb{R}$ such that $f(x) = g(x)$ for all $xin A$.

I just proved that if $f,g:Xtomathbb{R}$ are continuous and agree in a dense subset $Asubset X$ then they're equal.

I thought in $X=mathbb{R}$ with usual topology and $A = mathbb{R}-{0} =:mathbb{R}^* $, so I think $f:mathbb{R}^*tomathbb{R}, f(x) = x^{-1}$ is a continuous function that cannot be continually extended to $mathbb{R}$. I'm quite sure of this, but I'm stuck in proving it using the definition of continuity in general topological spaces.

Also, I'm quite confused on how this asked example is not a counterexample of what I proved.

Thanks in advance.

general-topology continuity

share|cite|improve this question

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

$endgroup$

share|cite|improve this question

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

share|cite|improve this question

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

asked yesterday

AnalyticHarmonyAnalyticHarmony

699313

3 Answers
3

active

oldest

votes

4

$begingroup$

Define $f(x)=1/x$ like you did, and assume you can find a continuous extension $g : mathbb{R}tomathbb{R}$. Well this $g $ takes a real numbered value at $0$, namely $-infty < g (0) < infty $, and it agrees with $f $ at non-zero values.

One definition of continuity is that given a net of points in $X $ converging to $x_0$ and a function $g $, then the images converge to $g(x_0) $. Since $mathbb{R}$ is a metric space, we can use sequences instead of nets. But given a sequence of real numbers $(x_n )_{n=1}^{infty} $ converging to $0$, the sequence $(g (x_n))_{n=1}^{infty} $ converges to either positive or negative $infty $. So it does not converge to $g (0) $. So $g $ is not continuous

---

BTW regarding your question on the results you proved. You proved a result about two functions that were continuous on the entire space, who agree on a dense subset. But the main question of your post is regarding a function who is not assumed to be continuous on the entire space, and comparing it to one that is continuous on the entire space. So the main example is not countering your original result

share|cite|improve this answer

edited yesterday

answered yesterday

NazimJNazimJ

79019

$endgroup$

add a comment | 

2

$begingroup$

Using sequences is the easiest way to go, but for a more "topological" proof, to show that no extension of $f$ is continuous at $x=0,$ we suppose there is one (we still call it $f$ for convenience), and we show that there is an $epsilon>0$ so that for any $delta >0$, there is an $xin (-delta,delta$), such that$f(x)>f(0)+epsilon$ (or that $f(x)<f(0)-epsilon$). Let's do the former.

Now, drawing a picture will make the following obvious:

Take $epsilon=1.$ Then, if $f(0)+1le 0$, then $text{any} xin (0,delta)$ will do because $f(x)=1/x>0.$

If $f(0)+1> 0$, all we need do is choose $x$ small enough so that $f(x)=1/x>f(0)+1,$ which is to say, choose $x<min{delta, frac{1}{f(0)+1}}$

share|cite|improve this answer

answered 23 hours ago

MatematletaMatematleta

12.1k21020

$endgroup$

add a comment | 

1

$begingroup$

Another reason: continuous in the whole line implies locally bounded near every point.

And another counterexample based in a different idea: $Bbb Q$ is dense in $Bbb R$ with the usual topology. The function
$$f:Bbb QlongrightarrowBbb R$$
$$
f(x) =
begin{cases}
0:& x < sqrt2,\
1:& x > sqrt2,
end{cases}
$$
is continuous (check it) and can't be extended continuously to $Bbb R$.

share|cite|improve this answer

edited 18 hours ago

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972

$endgroup$

add a comment | 

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4

$begingroup$

Define $f(x)=1/x$ like you did, and assume you can find a continuous extension $g : mathbb{R}tomathbb{R}$. Well this $g $ takes a real numbered value at $0$, namely $-infty < g (0) < infty $, and it agrees with $f $ at non-zero values.

One definition of continuity is that given a net of points in $X $ converging to $x_0$ and a function $g $, then the images converge to $g(x_0) $. Since $mathbb{R}$ is a metric space, we can use sequences instead of nets. But given a sequence of real numbers $(x_n )_{n=1}^{infty} $ converging to $0$, the sequence $(g (x_n))_{n=1}^{infty} $ converges to either positive or negative $infty $. So it does not converge to $g (0) $. So $g $ is not continuous

---

BTW regarding your question on the results you proved. You proved a result about two functions that were continuous on the entire space, who agree on a dense subset. But the main question of your post is regarding a function who is not assumed to be continuous on the entire space, and comparing it to one that is continuous on the entire space. So the main example is not countering your original result

share|cite|improve this answer

edited yesterday

answered yesterday

NazimJNazimJ

79019

$endgroup$

add a comment | 

4

$begingroup$

Define $f(x)=1/x$ like you did, and assume you can find a continuous extension $g : mathbb{R}tomathbb{R}$. Well this $g $ takes a real numbered value at $0$, namely $-infty < g (0) < infty $, and it agrees with $f $ at non-zero values.

One definition of continuity is that given a net of points in $X $ converging to $x_0$ and a function $g $, then the images converge to $g(x_0) $. Since $mathbb{R}$ is a metric space, we can use sequences instead of nets. But given a sequence of real numbers $(x_n )_{n=1}^{infty} $ converging to $0$, the sequence $(g (x_n))_{n=1}^{infty} $ converges to either positive or negative $infty $. So it does not converge to $g (0) $. So $g $ is not continuous

---

BTW regarding your question on the results you proved. You proved a result about two functions that were continuous on the entire space, who agree on a dense subset. But the main question of your post is regarding a function who is not assumed to be continuous on the entire space, and comparing it to one that is continuous on the entire space. So the main example is not countering your original result

share|cite|improve this answer

edited yesterday

answered yesterday

NazimJNazimJ

79019

$endgroup$

add a comment | 

$begingroup$

Define $f(x)=1/x$ like you did, and assume you can find a continuous extension $g : mathbb{R}tomathbb{R}$. Well this $g $ takes a real numbered value at $0$, namely $-infty < g (0) < infty $, and it agrees with $f $ at non-zero values.

One definition of continuity is that given a net of points in $X $ converging to $x_0$ and a function $g $, then the images converge to $g(x_0) $. Since $mathbb{R}$ is a metric space, we can use sequences instead of nets. But given a sequence of real numbers $(x_n )_{n=1}^{infty} $ converging to $0$, the sequence $(g (x_n))_{n=1}^{infty} $ converges to either positive or negative $infty $. So it does not converge to $g (0) $. So $g $ is not continuous

---

BTW regarding your question on the results you proved. You proved a result about two functions that were continuous on the entire space, who agree on a dense subset. But the main question of your post is regarding a function who is not assumed to be continuous on the entire space, and comparing it to one that is continuous on the entire space. So the main example is not countering your original result

share|cite|improve this answer

edited yesterday

answered yesterday

NazimJNazimJ

79019

$endgroup$

share|cite|improve this answer

edited yesterday

answered yesterday

NazimJNazimJ

79019

answered yesterday

NazimJNazimJ

79019

answered yesterday

NazimJNazimJ

79019

2

$begingroup$

Using sequences is the easiest way to go, but for a more "topological" proof, to show that no extension of $f$ is continuous at $x=0,$ we suppose there is one (we still call it $f$ for convenience), and we show that there is an $epsilon>0$ so that for any $delta >0$, there is an $xin (-delta,delta$), such that$f(x)>f(0)+epsilon$ (or that $f(x)<f(0)-epsilon$). Let's do the former.

Now, drawing a picture will make the following obvious:

Take $epsilon=1.$ Then, if $f(0)+1le 0$, then $text{any} xin (0,delta)$ will do because $f(x)=1/x>0.$

If $f(0)+1> 0$, all we need do is choose $x$ small enough so that $f(x)=1/x>f(0)+1,$ which is to say, choose $x<min{delta, frac{1}{f(0)+1}}$

share|cite|improve this answer

answered 23 hours ago

MatematletaMatematleta

12.1k21020

$endgroup$

add a comment | 

2

$begingroup$

Using sequences is the easiest way to go, but for a more "topological" proof, to show that no extension of $f$ is continuous at $x=0,$ we suppose there is one (we still call it $f$ for convenience), and we show that there is an $epsilon>0$ so that for any $delta >0$, there is an $xin (-delta,delta$), such that$f(x)>f(0)+epsilon$ (or that $f(x)<f(0)-epsilon$). Let's do the former.

Now, drawing a picture will make the following obvious:

Take $epsilon=1.$ Then, if $f(0)+1le 0$, then $text{any} xin (0,delta)$ will do because $f(x)=1/x>0.$

If $f(0)+1> 0$, all we need do is choose $x$ small enough so that $f(x)=1/x>f(0)+1,$ which is to say, choose $x<min{delta, frac{1}{f(0)+1}}$

share|cite|improve this answer

answered 23 hours ago

MatematletaMatematleta

12.1k21020

$endgroup$

add a comment | 

$begingroup$

Using sequences is the easiest way to go, but for a more "topological" proof, to show that no extension of $f$ is continuous at $x=0,$ we suppose there is one (we still call it $f$ for convenience), and we show that there is an $epsilon>0$ so that for any $delta >0$, there is an $xin (-delta,delta$), such that$f(x)>f(0)+epsilon$ (or that $f(x)<f(0)-epsilon$). Let's do the former.

Now, drawing a picture will make the following obvious:

Take $epsilon=1.$ Then, if $f(0)+1le 0$, then $text{any} xin (0,delta)$ will do because $f(x)=1/x>0.$

If $f(0)+1> 0$, all we need do is choose $x$ small enough so that $f(x)=1/x>f(0)+1,$ which is to say, choose $x<min{delta, frac{1}{f(0)+1}}$

share|cite|improve this answer

answered 23 hours ago

MatematletaMatematleta

12.1k21020

$endgroup$

share|cite|improve this answer

answered 23 hours ago

MatematletaMatematleta

12.1k21020

answered 23 hours ago

MatematletaMatematleta

12.1k21020

answered 23 hours ago

MatematletaMatematleta

12.1k21020

1

$begingroup$

Another reason: continuous in the whole line implies locally bounded near every point.

And another counterexample based in a different idea: $Bbb Q$ is dense in $Bbb R$ with the usual topology. The function
$$f:Bbb QlongrightarrowBbb R$$
$$
f(x) =
begin{cases}
0:& x < sqrt2,\
1:& x > sqrt2,
end{cases}
$$
is continuous (check it) and can't be extended continuously to $Bbb R$.

share|cite|improve this answer

edited 18 hours ago

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972

$endgroup$

add a comment | 

1

$begingroup$

Another reason: continuous in the whole line implies locally bounded near every point.

And another counterexample based in a different idea: $Bbb Q$ is dense in $Bbb R$ with the usual topology. The function
$$f:Bbb QlongrightarrowBbb R$$
$$
f(x) =
begin{cases}
0:& x < sqrt2,\
1:& x > sqrt2,
end{cases}
$$
is continuous (check it) and can't be extended continuously to $Bbb R$.

share|cite|improve this answer

edited 18 hours ago

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972

$endgroup$

add a comment | 

$begingroup$

Another reason: continuous in the whole line implies locally bounded near every point.

And another counterexample based in a different idea: $Bbb Q$ is dense in $Bbb R$ with the usual topology. The function
$$f:Bbb QlongrightarrowBbb R$$
$$
f(x) =
begin{cases}
0:& x < sqrt2,\
1:& x > sqrt2,
end{cases}
$$
is continuous (check it) and can't be extended continuously to $Bbb R$.

share|cite|improve this answer

edited 18 hours ago

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972

$endgroup$

share|cite|improve this answer

edited 18 hours ago

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972

answered 21 hours ago

Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

35.4k42972