Applying theorem to disprove uniform convergence
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I recently read this theorem in real analysis:(Actually a corollary to a theorem)
{$f_m$} is a sequence of continuous functions defined on $D$ such that $f_m$$to$$f$ uniformly on $D$ then for every sequence {$x_m$} in $D$ satisfying $x_m$$to$$x$ then $f_m(x)$$to$$f(x)$.
please correct me if I wrote this wrong. The doubt is that I am unable to use it in a problem:
$$f_m(x)=dfrac{mx}{1+mx}qquad xin[0,1].$$
I want to prove that this is not uniformly convergent, but I am not able to understand what is $x_m$. How does it depend on $m$? Does it even depend on $m$?
If yes then how to prove that $f_m(x)$ does not converge to $f(x)$ when $x_m$ tends to $x$(to show that it is not uniformly convergent).(also, is there $x_m$ or $x$ inside $f_m(cdot)$ . Basically, how to approach this problem, please explain stepwise because I need to get the feeling of this theorem.
real-analysis uniform-convergence sequence-of-function
add a comment |
up vote
1
down vote
favorite
I recently read this theorem in real analysis:(Actually a corollary to a theorem)
{$f_m$} is a sequence of continuous functions defined on $D$ such that $f_m$$to$$f$ uniformly on $D$ then for every sequence {$x_m$} in $D$ satisfying $x_m$$to$$x$ then $f_m(x)$$to$$f(x)$.
please correct me if I wrote this wrong. The doubt is that I am unable to use it in a problem:
$$f_m(x)=dfrac{mx}{1+mx}qquad xin[0,1].$$
I want to prove that this is not uniformly convergent, but I am not able to understand what is $x_m$. How does it depend on $m$? Does it even depend on $m$?
If yes then how to prove that $f_m(x)$ does not converge to $f(x)$ when $x_m$ tends to $x$(to show that it is not uniformly convergent).(also, is there $x_m$ or $x$ inside $f_m(cdot)$ . Basically, how to approach this problem, please explain stepwise because I need to get the feeling of this theorem.
real-analysis uniform-convergence sequence-of-function
4
Are you sure the original statement is correct? I suspect that it should be about a sequence {$x_n$} where $x_nto x$ and $f_m(x_n)to f(x)$.
– herb steinberg
Nov 23 at 21:41
I am not really sure about it. You might be right here. I'll change it when someone else points it out too.Thanks!
– Cosmic
Nov 23 at 21:44
@herbsteinberg I believe that n=m, because if they are not, then finding $f_m(x_n)$ will be vague since it will be a sequence of functions and so we will have to vary both $n$ and then $m$ to find out what $f_m(x_n)$ converges to. Correct me if i am wrong. Also take a look at the comment by Yadati.
– Cosmic
Nov 23 at 22:26
1
Since I am not familiar with this theorem, the comment by Yadati may be correct.
– herb steinberg
Nov 23 at 22:36
add a comment |
up vote
1
down vote
favorite
up vote
1
down vote
favorite
I recently read this theorem in real analysis:(Actually a corollary to a theorem)
{$f_m$} is a sequence of continuous functions defined on $D$ such that $f_m$$to$$f$ uniformly on $D$ then for every sequence {$x_m$} in $D$ satisfying $x_m$$to$$x$ then $f_m(x)$$to$$f(x)$.
please correct me if I wrote this wrong. The doubt is that I am unable to use it in a problem:
$$f_m(x)=dfrac{mx}{1+mx}qquad xin[0,1].$$
I want to prove that this is not uniformly convergent, but I am not able to understand what is $x_m$. How does it depend on $m$? Does it even depend on $m$?
If yes then how to prove that $f_m(x)$ does not converge to $f(x)$ when $x_m$ tends to $x$(to show that it is not uniformly convergent).(also, is there $x_m$ or $x$ inside $f_m(cdot)$ . Basically, how to approach this problem, please explain stepwise because I need to get the feeling of this theorem.
real-analysis uniform-convergence sequence-of-function
I recently read this theorem in real analysis:(Actually a corollary to a theorem)
{$f_m$} is a sequence of continuous functions defined on $D$ such that $f_m$$to$$f$ uniformly on $D$ then for every sequence {$x_m$} in $D$ satisfying $x_m$$to$$x$ then $f_m(x)$$to$$f(x)$.
please correct me if I wrote this wrong. The doubt is that I am unable to use it in a problem:
$$f_m(x)=dfrac{mx}{1+mx}qquad xin[0,1].$$
I want to prove that this is not uniformly convergent, but I am not able to understand what is $x_m$. How does it depend on $m$? Does it even depend on $m$?
If yes then how to prove that $f_m(x)$ does not converge to $f(x)$ when $x_m$ tends to $x$(to show that it is not uniformly convergent).(also, is there $x_m$ or $x$ inside $f_m(cdot)$ . Basically, how to approach this problem, please explain stepwise because I need to get the feeling of this theorem.
real-analysis uniform-convergence sequence-of-function
real-analysis uniform-convergence sequence-of-function
edited Nov 23 at 21:41
asked Nov 23 at 21:29
Cosmic
448
448
4
Are you sure the original statement is correct? I suspect that it should be about a sequence {$x_n$} where $x_nto x$ and $f_m(x_n)to f(x)$.
– herb steinberg
Nov 23 at 21:41
I am not really sure about it. You might be right here. I'll change it when someone else points it out too.Thanks!
– Cosmic
Nov 23 at 21:44
@herbsteinberg I believe that n=m, because if they are not, then finding $f_m(x_n)$ will be vague since it will be a sequence of functions and so we will have to vary both $n$ and then $m$ to find out what $f_m(x_n)$ converges to. Correct me if i am wrong. Also take a look at the comment by Yadati.
– Cosmic
Nov 23 at 22:26
1
Since I am not familiar with this theorem, the comment by Yadati may be correct.
– herb steinberg
Nov 23 at 22:36
add a comment |
4
Are you sure the original statement is correct? I suspect that it should be about a sequence {$x_n$} where $x_nto x$ and $f_m(x_n)to f(x)$.
– herb steinberg
Nov 23 at 21:41
I am not really sure about it. You might be right here. I'll change it when someone else points it out too.Thanks!
– Cosmic
Nov 23 at 21:44
@herbsteinberg I believe that n=m, because if they are not, then finding $f_m(x_n)$ will be vague since it will be a sequence of functions and so we will have to vary both $n$ and then $m$ to find out what $f_m(x_n)$ converges to. Correct me if i am wrong. Also take a look at the comment by Yadati.
– Cosmic
Nov 23 at 22:26
1
Since I am not familiar with this theorem, the comment by Yadati may be correct.
– herb steinberg
Nov 23 at 22:36
4
4
Are you sure the original statement is correct? I suspect that it should be about a sequence {$x_n$} where $x_nto x$ and $f_m(x_n)to f(x)$.
– herb steinberg
Nov 23 at 21:41
Are you sure the original statement is correct? I suspect that it should be about a sequence {$x_n$} where $x_nto x$ and $f_m(x_n)to f(x)$.
– herb steinberg
Nov 23 at 21:41
I am not really sure about it. You might be right here. I'll change it when someone else points it out too.Thanks!
– Cosmic
Nov 23 at 21:44
I am not really sure about it. You might be right here. I'll change it when someone else points it out too.Thanks!
– Cosmic
Nov 23 at 21:44
@herbsteinberg I believe that n=m, because if they are not, then finding $f_m(x_n)$ will be vague since it will be a sequence of functions and so we will have to vary both $n$ and then $m$ to find out what $f_m(x_n)$ converges to. Correct me if i am wrong. Also take a look at the comment by Yadati.
– Cosmic
Nov 23 at 22:26
@herbsteinberg I believe that n=m, because if they are not, then finding $f_m(x_n)$ will be vague since it will be a sequence of functions and so we will have to vary both $n$ and then $m$ to find out what $f_m(x_n)$ converges to. Correct me if i am wrong. Also take a look at the comment by Yadati.
– Cosmic
Nov 23 at 22:26
1
1
Since I am not familiar with this theorem, the comment by Yadati may be correct.
– herb steinberg
Nov 23 at 22:36
Since I am not familiar with this theorem, the comment by Yadati may be correct.
– herb steinberg
Nov 23 at 22:36
add a comment |
1 Answer
1
active
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up vote
2
down vote
accepted
Another way to show $f$ is not uniformly convergent.
$$f_m=begin{cases}0 &x=0\dfrac{mx}{1+mx} &xneq0end{cases}$$ $:$For $xneq0:$ $f_m(x)=dfrac{mx}{1+mx}=dfrac{x}{frac1m+x}$. $:$ Now if $mtoinftyimplies f_m(x)to1$.
So $$f_mtobegin{cases}0 &x=0\1 &xneq0end{cases}=f$$
and $f$ is clearly not continuous. Hence the convergence is not uniform.
$rule{17cm}{1pt}$
$0neq x_m=dfrac1mto0implies:forall:mquad f_m(x_m)=dfrac12notto f(0)=0$. So the convergence is not uniform.
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
1
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
1
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
accepted
Another way to show $f$ is not uniformly convergent.
$$f_m=begin{cases}0 &x=0\dfrac{mx}{1+mx} &xneq0end{cases}$$ $:$For $xneq0:$ $f_m(x)=dfrac{mx}{1+mx}=dfrac{x}{frac1m+x}$. $:$ Now if $mtoinftyimplies f_m(x)to1$.
So $$f_mtobegin{cases}0 &x=0\1 &xneq0end{cases}=f$$
and $f$ is clearly not continuous. Hence the convergence is not uniform.
$rule{17cm}{1pt}$
$0neq x_m=dfrac1mto0implies:forall:mquad f_m(x_m)=dfrac12notto f(0)=0$. So the convergence is not uniform.
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
1
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
1
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
add a comment |
up vote
2
down vote
accepted
Another way to show $f$ is not uniformly convergent.
$$f_m=begin{cases}0 &x=0\dfrac{mx}{1+mx} &xneq0end{cases}$$ $:$For $xneq0:$ $f_m(x)=dfrac{mx}{1+mx}=dfrac{x}{frac1m+x}$. $:$ Now if $mtoinftyimplies f_m(x)to1$.
So $$f_mtobegin{cases}0 &x=0\1 &xneq0end{cases}=f$$
and $f$ is clearly not continuous. Hence the convergence is not uniform.
$rule{17cm}{1pt}$
$0neq x_m=dfrac1mto0implies:forall:mquad f_m(x_m)=dfrac12notto f(0)=0$. So the convergence is not uniform.
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
1
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
1
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
add a comment |
up vote
2
down vote
accepted
up vote
2
down vote
accepted
Another way to show $f$ is not uniformly convergent.
$$f_m=begin{cases}0 &x=0\dfrac{mx}{1+mx} &xneq0end{cases}$$ $:$For $xneq0:$ $f_m(x)=dfrac{mx}{1+mx}=dfrac{x}{frac1m+x}$. $:$ Now if $mtoinftyimplies f_m(x)to1$.
So $$f_mtobegin{cases}0 &x=0\1 &xneq0end{cases}=f$$
and $f$ is clearly not continuous. Hence the convergence is not uniform.
$rule{17cm}{1pt}$
$0neq x_m=dfrac1mto0implies:forall:mquad f_m(x_m)=dfrac12notto f(0)=0$. So the convergence is not uniform.
Another way to show $f$ is not uniformly convergent.
$$f_m=begin{cases}0 &x=0\dfrac{mx}{1+mx} &xneq0end{cases}$$ $:$For $xneq0:$ $f_m(x)=dfrac{mx}{1+mx}=dfrac{x}{frac1m+x}$. $:$ Now if $mtoinftyimplies f_m(x)to1$.
So $$f_mtobegin{cases}0 &x=0\1 &xneq0end{cases}=f$$
and $f$ is clearly not continuous. Hence the convergence is not uniform.
$rule{17cm}{1pt}$
$0neq x_m=dfrac1mto0implies:forall:mquad f_m(x_m)=dfrac12notto f(0)=0$. So the convergence is not uniform.
edited Nov 23 at 22:03
answered Nov 23 at 21:39
Yadati Kiran
1,237417
1,237417
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
1
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
1
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
add a comment |
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
1
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
1
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
Your explanation is right, but I specifically want to use the above corollary to prove it.
– Cosmic
Nov 23 at 21:46
1
1
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
Consider $x_n=frac1n$
– Yadati Kiran
Nov 23 at 21:49
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
According to you Is the mistake pointed out above by herb correct?
– Cosmic
Nov 23 at 21:52
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
Yes the theorem should be " Suppose $f_nrightarrow f$ uniformly, $f_n$ are continuous, and $x_nrightarrow x$ then $displaystylelim_{nto infty} f_n(x_n) =f(x) $"
– Yadati Kiran
Nov 23 at 21:57
1
1
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
@Cosmic: $f_m(x_m)=dfrac{mcdotfrac1m}{1+mcdotfrac1m}=dfrac12$
– Yadati Kiran
Nov 23 at 22:14
add a comment |
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4
Are you sure the original statement is correct? I suspect that it should be about a sequence {$x_n$} where $x_nto x$ and $f_m(x_n)to f(x)$.
– herb steinberg
Nov 23 at 21:41
I am not really sure about it. You might be right here. I'll change it when someone else points it out too.Thanks!
– Cosmic
Nov 23 at 21:44
@herbsteinberg I believe that n=m, because if they are not, then finding $f_m(x_n)$ will be vague since it will be a sequence of functions and so we will have to vary both $n$ and then $m$ to find out what $f_m(x_n)$ converges to. Correct me if i am wrong. Also take a look at the comment by Yadati.
– Cosmic
Nov 23 at 22:26
1
Since I am not familiar with this theorem, the comment by Yadati may be correct.
– herb steinberg
Nov 23 at 22:36