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.










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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















up vote
1
down vote

favorite
1












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.










share|cite|improve this question




















  • 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













up vote
1
down vote

favorite
1









up vote
1
down vote

favorite
1






1





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.










share|cite|improve this question















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






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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














  • 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










1 Answer
1






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2
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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.






share|cite|improve this answer























  • 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











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1 Answer
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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.






share|cite|improve this answer























  • 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















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.






share|cite|improve this answer























  • 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













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.






share|cite|improve this answer














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.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








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


















  • 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


















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