Real numbers equipped with the metric $ d (x,y) = | arctan(x) - arctan(y)| $ is an incomplete metric space











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20
down vote

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I have to show that the real numbers equipped with the metric
$ d (x,y) = | arctan(x) - arctan(y)| $ is an incomplete metric space.



Certainly, I have to search for a Cauchy sequence of real numbers with respect to given metric that must not be convergent. But I am unable to figure out that.
Can anybody help me with this.



Thanks for helping me.










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  • 5




    The simplest way to do this is to notice that $x mapsto arctan(x)$ is an isometry for your space to the open interval $(-pi/2,pi/2)$ with its usual metric.
    – Chris Eagle
    May 31 '12 at 22:54






  • 4




    Try the sequence $x_n = n$. Draw a picture.
    – copper.hat
    May 31 '12 at 23:00








  • 1




    You can show easily that it does not converge to any number, ie, for any $y$, show $d(n,y)$ does not converge to zero.
    – copper.hat
    May 31 '12 at 23:10








  • 1




    $d(n,y) = | arctan(n) - arctan(y)|$. So $d(n,y) to (frac{pi}{2}-y)$. $y$ is a fixed number, so $arctan(y) < frac{pi}{2}$.
    – copper.hat
    May 31 '12 at 23:26








  • 2




    I am showing that $x_n$ does not converge to any fixed y. $arctan y$ is a fixed number, it doesn't tend towards anything but itself.
    – copper.hat
    May 31 '12 at 23:37

















up vote
20
down vote

favorite
12












I have to show that the real numbers equipped with the metric
$ d (x,y) = | arctan(x) - arctan(y)| $ is an incomplete metric space.



Certainly, I have to search for a Cauchy sequence of real numbers with respect to given metric that must not be convergent. But I am unable to figure out that.
Can anybody help me with this.



Thanks for helping me.










share|cite|improve this question




















  • 5




    The simplest way to do this is to notice that $x mapsto arctan(x)$ is an isometry for your space to the open interval $(-pi/2,pi/2)$ with its usual metric.
    – Chris Eagle
    May 31 '12 at 22:54






  • 4




    Try the sequence $x_n = n$. Draw a picture.
    – copper.hat
    May 31 '12 at 23:00








  • 1




    You can show easily that it does not converge to any number, ie, for any $y$, show $d(n,y)$ does not converge to zero.
    – copper.hat
    May 31 '12 at 23:10








  • 1




    $d(n,y) = | arctan(n) - arctan(y)|$. So $d(n,y) to (frac{pi}{2}-y)$. $y$ is a fixed number, so $arctan(y) < frac{pi}{2}$.
    – copper.hat
    May 31 '12 at 23:26








  • 2




    I am showing that $x_n$ does not converge to any fixed y. $arctan y$ is a fixed number, it doesn't tend towards anything but itself.
    – copper.hat
    May 31 '12 at 23:37















up vote
20
down vote

favorite
12









up vote
20
down vote

favorite
12






12





I have to show that the real numbers equipped with the metric
$ d (x,y) = | arctan(x) - arctan(y)| $ is an incomplete metric space.



Certainly, I have to search for a Cauchy sequence of real numbers with respect to given metric that must not be convergent. But I am unable to figure out that.
Can anybody help me with this.



Thanks for helping me.










share|cite|improve this question















I have to show that the real numbers equipped with the metric
$ d (x,y) = | arctan(x) - arctan(y)| $ is an incomplete metric space.



Certainly, I have to search for a Cauchy sequence of real numbers with respect to given metric that must not be convergent. But I am unable to figure out that.
Can anybody help me with this.



Thanks for helping me.







functional-analysis metric-spaces






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edited Dec 9 '14 at 13:37









Martin Sleziak

44.6k7115269




44.6k7115269










asked May 31 '12 at 22:49









srijan

6,31563979




6,31563979








  • 5




    The simplest way to do this is to notice that $x mapsto arctan(x)$ is an isometry for your space to the open interval $(-pi/2,pi/2)$ with its usual metric.
    – Chris Eagle
    May 31 '12 at 22:54






  • 4




    Try the sequence $x_n = n$. Draw a picture.
    – copper.hat
    May 31 '12 at 23:00








  • 1




    You can show easily that it does not converge to any number, ie, for any $y$, show $d(n,y)$ does not converge to zero.
    – copper.hat
    May 31 '12 at 23:10








  • 1




    $d(n,y) = | arctan(n) - arctan(y)|$. So $d(n,y) to (frac{pi}{2}-y)$. $y$ is a fixed number, so $arctan(y) < frac{pi}{2}$.
    – copper.hat
    May 31 '12 at 23:26








  • 2




    I am showing that $x_n$ does not converge to any fixed y. $arctan y$ is a fixed number, it doesn't tend towards anything but itself.
    – copper.hat
    May 31 '12 at 23:37
















  • 5




    The simplest way to do this is to notice that $x mapsto arctan(x)$ is an isometry for your space to the open interval $(-pi/2,pi/2)$ with its usual metric.
    – Chris Eagle
    May 31 '12 at 22:54






  • 4




    Try the sequence $x_n = n$. Draw a picture.
    – copper.hat
    May 31 '12 at 23:00








  • 1




    You can show easily that it does not converge to any number, ie, for any $y$, show $d(n,y)$ does not converge to zero.
    – copper.hat
    May 31 '12 at 23:10








  • 1




    $d(n,y) = | arctan(n) - arctan(y)|$. So $d(n,y) to (frac{pi}{2}-y)$. $y$ is a fixed number, so $arctan(y) < frac{pi}{2}$.
    – copper.hat
    May 31 '12 at 23:26








  • 2




    I am showing that $x_n$ does not converge to any fixed y. $arctan y$ is a fixed number, it doesn't tend towards anything but itself.
    – copper.hat
    May 31 '12 at 23:37










5




5




The simplest way to do this is to notice that $x mapsto arctan(x)$ is an isometry for your space to the open interval $(-pi/2,pi/2)$ with its usual metric.
– Chris Eagle
May 31 '12 at 22:54




The simplest way to do this is to notice that $x mapsto arctan(x)$ is an isometry for your space to the open interval $(-pi/2,pi/2)$ with its usual metric.
– Chris Eagle
May 31 '12 at 22:54




4




4




Try the sequence $x_n = n$. Draw a picture.
– copper.hat
May 31 '12 at 23:00






Try the sequence $x_n = n$. Draw a picture.
– copper.hat
May 31 '12 at 23:00






1




1




You can show easily that it does not converge to any number, ie, for any $y$, show $d(n,y)$ does not converge to zero.
– copper.hat
May 31 '12 at 23:10






You can show easily that it does not converge to any number, ie, for any $y$, show $d(n,y)$ does not converge to zero.
– copper.hat
May 31 '12 at 23:10






1




1




$d(n,y) = | arctan(n) - arctan(y)|$. So $d(n,y) to (frac{pi}{2}-y)$. $y$ is a fixed number, so $arctan(y) < frac{pi}{2}$.
– copper.hat
May 31 '12 at 23:26






$d(n,y) = | arctan(n) - arctan(y)|$. So $d(n,y) to (frac{pi}{2}-y)$. $y$ is a fixed number, so $arctan(y) < frac{pi}{2}$.
– copper.hat
May 31 '12 at 23:26






2




2




I am showing that $x_n$ does not converge to any fixed y. $arctan y$ is a fixed number, it doesn't tend towards anything but itself.
– copper.hat
May 31 '12 at 23:37






I am showing that $x_n$ does not converge to any fixed y. $arctan y$ is a fixed number, it doesn't tend towards anything but itself.
– copper.hat
May 31 '12 at 23:37












3 Answers
3






active

oldest

votes

















up vote
9
down vote



accepted










Sorry for reviving such an old problem...



Anyways, what is important here is that $text{arctan}$ is a bijection from $mathbb{R}$ to $( -pi/2, pi/2 )$, and it is an isometry if we give $left(-pi/2, pi/2right)$ the metric it carries as a subspace of $mathbb{R}$ with the usual metric. If $f: X to Y$ is a surjective isometry then the Cauchy sequences in $Y$ are the images of Cauchy sequences under $f$, and the convergent sequences in $Y$ are the images of convergent sequences under $f$, so $Y$ is complete if and only if $X$ is. Thus $(mathbb{R}, d)$ is complete if and only if $(-pi/2, pi/2)$ with the metric $text{dist}(x, y) = |x - y|$ is complete. But $(-pi/2, pi/2)$ is not complete since ${pi/2 - 1/n}_{n=1}^infty$ is Cauchy and does not converge in $(-pi/2, pi/2)$.






share|cite|improve this answer






























    up vote
    9
    down vote













    Consider $x_n = n$. Let $varepsilon > 0$ and choose $displaystyle N > tanbigg(fracpi2 - varepsilonbigg)$. If $m, n > N$ then $displaystyle {arctan m, arctan m} subseteq bigg(arctan N, fracpi2bigg)$. Thus $$d(x_m, x_n) = vert arctan m - arctan n vert leq bigg vert fracpi2 - arctan N biggvert < bigg vert fracpi2 - fracpi2 + varepsilon biggvert= varepsilon$$ Thus $(x_n)$ is a Cauchy sequence. Observe that as $n to infty$, $arctan x_n to pi/2$. But $(x_n)$ does not converge to any element in $mathbb R$ since there is no $x in mathbb R$ such that $arctan x = pi/2$.






    share|cite|improve this answer



















    • 1




      I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
      – Jonathan
      Sep 29 '17 at 17:11




















    up vote
    6
    down vote













    Consider a sequence that grows without bound. Such a sequence isn't Cauchy in the usual metric on $mathbb{R}$, but will be under this metric.






    share|cite|improve this answer





















    • thanks i took sequence $x_n = n$ as suggested by copper.hat
      – srijan
      May 31 '12 at 23:33











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    3 Answers
    3






    active

    oldest

    votes








    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    9
    down vote



    accepted










    Sorry for reviving such an old problem...



    Anyways, what is important here is that $text{arctan}$ is a bijection from $mathbb{R}$ to $( -pi/2, pi/2 )$, and it is an isometry if we give $left(-pi/2, pi/2right)$ the metric it carries as a subspace of $mathbb{R}$ with the usual metric. If $f: X to Y$ is a surjective isometry then the Cauchy sequences in $Y$ are the images of Cauchy sequences under $f$, and the convergent sequences in $Y$ are the images of convergent sequences under $f$, so $Y$ is complete if and only if $X$ is. Thus $(mathbb{R}, d)$ is complete if and only if $(-pi/2, pi/2)$ with the metric $text{dist}(x, y) = |x - y|$ is complete. But $(-pi/2, pi/2)$ is not complete since ${pi/2 - 1/n}_{n=1}^infty$ is Cauchy and does not converge in $(-pi/2, pi/2)$.






    share|cite|improve this answer



























      up vote
      9
      down vote



      accepted










      Sorry for reviving such an old problem...



      Anyways, what is important here is that $text{arctan}$ is a bijection from $mathbb{R}$ to $( -pi/2, pi/2 )$, and it is an isometry if we give $left(-pi/2, pi/2right)$ the metric it carries as a subspace of $mathbb{R}$ with the usual metric. If $f: X to Y$ is a surjective isometry then the Cauchy sequences in $Y$ are the images of Cauchy sequences under $f$, and the convergent sequences in $Y$ are the images of convergent sequences under $f$, so $Y$ is complete if and only if $X$ is. Thus $(mathbb{R}, d)$ is complete if and only if $(-pi/2, pi/2)$ with the metric $text{dist}(x, y) = |x - y|$ is complete. But $(-pi/2, pi/2)$ is not complete since ${pi/2 - 1/n}_{n=1}^infty$ is Cauchy and does not converge in $(-pi/2, pi/2)$.






      share|cite|improve this answer

























        up vote
        9
        down vote



        accepted







        up vote
        9
        down vote



        accepted






        Sorry for reviving such an old problem...



        Anyways, what is important here is that $text{arctan}$ is a bijection from $mathbb{R}$ to $( -pi/2, pi/2 )$, and it is an isometry if we give $left(-pi/2, pi/2right)$ the metric it carries as a subspace of $mathbb{R}$ with the usual metric. If $f: X to Y$ is a surjective isometry then the Cauchy sequences in $Y$ are the images of Cauchy sequences under $f$, and the convergent sequences in $Y$ are the images of convergent sequences under $f$, so $Y$ is complete if and only if $X$ is. Thus $(mathbb{R}, d)$ is complete if and only if $(-pi/2, pi/2)$ with the metric $text{dist}(x, y) = |x - y|$ is complete. But $(-pi/2, pi/2)$ is not complete since ${pi/2 - 1/n}_{n=1}^infty$ is Cauchy and does not converge in $(-pi/2, pi/2)$.






        share|cite|improve this answer














        Sorry for reviving such an old problem...



        Anyways, what is important here is that $text{arctan}$ is a bijection from $mathbb{R}$ to $( -pi/2, pi/2 )$, and it is an isometry if we give $left(-pi/2, pi/2right)$ the metric it carries as a subspace of $mathbb{R}$ with the usual metric. If $f: X to Y$ is a surjective isometry then the Cauchy sequences in $Y$ are the images of Cauchy sequences under $f$, and the convergent sequences in $Y$ are the images of convergent sequences under $f$, so $Y$ is complete if and only if $X$ is. Thus $(mathbb{R}, d)$ is complete if and only if $(-pi/2, pi/2)$ with the metric $text{dist}(x, y) = |x - y|$ is complete. But $(-pi/2, pi/2)$ is not complete since ${pi/2 - 1/n}_{n=1}^infty$ is Cauchy and does not converge in $(-pi/2, pi/2)$.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Nov 4 at 23:11









        Adam

        32




        32










        answered Dec 9 '14 at 12:50









        user149792

        4,06842467




        4,06842467






















            up vote
            9
            down vote













            Consider $x_n = n$. Let $varepsilon > 0$ and choose $displaystyle N > tanbigg(fracpi2 - varepsilonbigg)$. If $m, n > N$ then $displaystyle {arctan m, arctan m} subseteq bigg(arctan N, fracpi2bigg)$. Thus $$d(x_m, x_n) = vert arctan m - arctan n vert leq bigg vert fracpi2 - arctan N biggvert < bigg vert fracpi2 - fracpi2 + varepsilon biggvert= varepsilon$$ Thus $(x_n)$ is a Cauchy sequence. Observe that as $n to infty$, $arctan x_n to pi/2$. But $(x_n)$ does not converge to any element in $mathbb R$ since there is no $x in mathbb R$ such that $arctan x = pi/2$.






            share|cite|improve this answer



















            • 1




              I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
              – Jonathan
              Sep 29 '17 at 17:11

















            up vote
            9
            down vote













            Consider $x_n = n$. Let $varepsilon > 0$ and choose $displaystyle N > tanbigg(fracpi2 - varepsilonbigg)$. If $m, n > N$ then $displaystyle {arctan m, arctan m} subseteq bigg(arctan N, fracpi2bigg)$. Thus $$d(x_m, x_n) = vert arctan m - arctan n vert leq bigg vert fracpi2 - arctan N biggvert < bigg vert fracpi2 - fracpi2 + varepsilon biggvert= varepsilon$$ Thus $(x_n)$ is a Cauchy sequence. Observe that as $n to infty$, $arctan x_n to pi/2$. But $(x_n)$ does not converge to any element in $mathbb R$ since there is no $x in mathbb R$ such that $arctan x = pi/2$.






            share|cite|improve this answer



















            • 1




              I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
              – Jonathan
              Sep 29 '17 at 17:11















            up vote
            9
            down vote










            up vote
            9
            down vote









            Consider $x_n = n$. Let $varepsilon > 0$ and choose $displaystyle N > tanbigg(fracpi2 - varepsilonbigg)$. If $m, n > N$ then $displaystyle {arctan m, arctan m} subseteq bigg(arctan N, fracpi2bigg)$. Thus $$d(x_m, x_n) = vert arctan m - arctan n vert leq bigg vert fracpi2 - arctan N biggvert < bigg vert fracpi2 - fracpi2 + varepsilon biggvert= varepsilon$$ Thus $(x_n)$ is a Cauchy sequence. Observe that as $n to infty$, $arctan x_n to pi/2$. But $(x_n)$ does not converge to any element in $mathbb R$ since there is no $x in mathbb R$ such that $arctan x = pi/2$.






            share|cite|improve this answer














            Consider $x_n = n$. Let $varepsilon > 0$ and choose $displaystyle N > tanbigg(fracpi2 - varepsilonbigg)$. If $m, n > N$ then $displaystyle {arctan m, arctan m} subseteq bigg(arctan N, fracpi2bigg)$. Thus $$d(x_m, x_n) = vert arctan m - arctan n vert leq bigg vert fracpi2 - arctan N biggvert < bigg vert fracpi2 - fracpi2 + varepsilon biggvert= varepsilon$$ Thus $(x_n)$ is a Cauchy sequence. Observe that as $n to infty$, $arctan x_n to pi/2$. But $(x_n)$ does not converge to any element in $mathbb R$ since there is no $x in mathbb R$ such that $arctan x = pi/2$.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Nov 21 at 19:05









            Yadati Kiran

            1,327418




            1,327418










            answered Feb 3 '13 at 10:48









            Robert Cardona

            5,20023398




            5,20023398








            • 1




              I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
              – Jonathan
              Sep 29 '17 at 17:11
















            • 1




              I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
              – Jonathan
              Sep 29 '17 at 17:11










            1




            1




            I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
            – Jonathan
            Sep 29 '17 at 17:11






            I think you meant : Observe that as $n to infty$, $arctan(x_n) to pi/2$.
            – Jonathan
            Sep 29 '17 at 17:11












            up vote
            6
            down vote













            Consider a sequence that grows without bound. Such a sequence isn't Cauchy in the usual metric on $mathbb{R}$, but will be under this metric.






            share|cite|improve this answer





















            • thanks i took sequence $x_n = n$ as suggested by copper.hat
              – srijan
              May 31 '12 at 23:33















            up vote
            6
            down vote













            Consider a sequence that grows without bound. Such a sequence isn't Cauchy in the usual metric on $mathbb{R}$, but will be under this metric.






            share|cite|improve this answer





















            • thanks i took sequence $x_n = n$ as suggested by copper.hat
              – srijan
              May 31 '12 at 23:33













            up vote
            6
            down vote










            up vote
            6
            down vote









            Consider a sequence that grows without bound. Such a sequence isn't Cauchy in the usual metric on $mathbb{R}$, but will be under this metric.






            share|cite|improve this answer












            Consider a sequence that grows without bound. Such a sequence isn't Cauchy in the usual metric on $mathbb{R}$, but will be under this metric.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered May 31 '12 at 22:58









            Cameron Buie

            84.6k771155




            84.6k771155












            • thanks i took sequence $x_n = n$ as suggested by copper.hat
              – srijan
              May 31 '12 at 23:33


















            • thanks i took sequence $x_n = n$ as suggested by copper.hat
              – srijan
              May 31 '12 at 23:33
















            thanks i took sequence $x_n = n$ as suggested by copper.hat
            – srijan
            May 31 '12 at 23:33




            thanks i took sequence $x_n = n$ as suggested by copper.hat
            – srijan
            May 31 '12 at 23:33


















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