If $f$ is continuous and with a limit at infinity then $f$ is uniformly continuous












0












$begingroup$



Let $f : [a, +infty) rightarrow mathbb{R}$ be continuous, such that
$lim_{x rightarrow +infty} f(x) = l in mathbb{R}$. Show that f is
uniformly continuous.




I want to split the domain - one in which we have continuity, and the other in which the values of f are approaching $l$. This means there are theree possibilities for our $x$ and $y$ using the uniformly continuous definition: x and y are in the first part, x and y are in the second, x is in the first and y in the second.



How would I do this proof?










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  • 2




    $begingroup$
    To avoid splitting into three cases, you could allow the pieces to overlap and make sure to choose $delta$ less than the width of the overlap. This avoids the case where $x$ is in one piece and $y$ is in the other.
    $endgroup$
    – J. Loreaux
    Nov 7 '16 at 19:06












  • $begingroup$
    @GregordeCillia A uniformly continuous function can have unbounded derivative
    $endgroup$
    – zhw.
    Nov 7 '16 at 19:12












  • $begingroup$
    Yes. That's why I deleted my comment after a few mjnutes. I was thinking anout lipschitz continuouity and wanted to give an osciallator-like counterexample
    $endgroup$
    – Gregor de Cillia
    Nov 7 '16 at 19:21










  • $begingroup$
    Fix epsilon. Let delta(x) be the maximum delta used in the definition of continuity at x. Show delta(x) has a nonzero limit given the conditions of the problem.
    $endgroup$
    – Alephnull
    Nov 7 '16 at 20:35
















0












$begingroup$



Let $f : [a, +infty) rightarrow mathbb{R}$ be continuous, such that
$lim_{x rightarrow +infty} f(x) = l in mathbb{R}$. Show that f is
uniformly continuous.




I want to split the domain - one in which we have continuity, and the other in which the values of f are approaching $l$. This means there are theree possibilities for our $x$ and $y$ using the uniformly continuous definition: x and y are in the first part, x and y are in the second, x is in the first and y in the second.



How would I do this proof?










share|cite|improve this question











$endgroup$








  • 2




    $begingroup$
    To avoid splitting into three cases, you could allow the pieces to overlap and make sure to choose $delta$ less than the width of the overlap. This avoids the case where $x$ is in one piece and $y$ is in the other.
    $endgroup$
    – J. Loreaux
    Nov 7 '16 at 19:06












  • $begingroup$
    @GregordeCillia A uniformly continuous function can have unbounded derivative
    $endgroup$
    – zhw.
    Nov 7 '16 at 19:12












  • $begingroup$
    Yes. That's why I deleted my comment after a few mjnutes. I was thinking anout lipschitz continuouity and wanted to give an osciallator-like counterexample
    $endgroup$
    – Gregor de Cillia
    Nov 7 '16 at 19:21










  • $begingroup$
    Fix epsilon. Let delta(x) be the maximum delta used in the definition of continuity at x. Show delta(x) has a nonzero limit given the conditions of the problem.
    $endgroup$
    – Alephnull
    Nov 7 '16 at 20:35














0












0








0





$begingroup$



Let $f : [a, +infty) rightarrow mathbb{R}$ be continuous, such that
$lim_{x rightarrow +infty} f(x) = l in mathbb{R}$. Show that f is
uniformly continuous.




I want to split the domain - one in which we have continuity, and the other in which the values of f are approaching $l$. This means there are theree possibilities for our $x$ and $y$ using the uniformly continuous definition: x and y are in the first part, x and y are in the second, x is in the first and y in the second.



How would I do this proof?










share|cite|improve this question











$endgroup$





Let $f : [a, +infty) rightarrow mathbb{R}$ be continuous, such that
$lim_{x rightarrow +infty} f(x) = l in mathbb{R}$. Show that f is
uniformly continuous.




I want to split the domain - one in which we have continuity, and the other in which the values of f are approaching $l$. This means there are theree possibilities for our $x$ and $y$ using the uniformly continuous definition: x and y are in the first part, x and y are in the second, x is in the first and y in the second.



How would I do this proof?







real-analysis limits continuity uniform-continuity






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share|cite|improve this question













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share|cite|improve this question








edited Nov 7 '16 at 20:16









Did

248k23224462




248k23224462










asked Nov 7 '16 at 19:01









user381164user381164

724




724








  • 2




    $begingroup$
    To avoid splitting into three cases, you could allow the pieces to overlap and make sure to choose $delta$ less than the width of the overlap. This avoids the case where $x$ is in one piece and $y$ is in the other.
    $endgroup$
    – J. Loreaux
    Nov 7 '16 at 19:06












  • $begingroup$
    @GregordeCillia A uniformly continuous function can have unbounded derivative
    $endgroup$
    – zhw.
    Nov 7 '16 at 19:12












  • $begingroup$
    Yes. That's why I deleted my comment after a few mjnutes. I was thinking anout lipschitz continuouity and wanted to give an osciallator-like counterexample
    $endgroup$
    – Gregor de Cillia
    Nov 7 '16 at 19:21










  • $begingroup$
    Fix epsilon. Let delta(x) be the maximum delta used in the definition of continuity at x. Show delta(x) has a nonzero limit given the conditions of the problem.
    $endgroup$
    – Alephnull
    Nov 7 '16 at 20:35














  • 2




    $begingroup$
    To avoid splitting into three cases, you could allow the pieces to overlap and make sure to choose $delta$ less than the width of the overlap. This avoids the case where $x$ is in one piece and $y$ is in the other.
    $endgroup$
    – J. Loreaux
    Nov 7 '16 at 19:06












  • $begingroup$
    @GregordeCillia A uniformly continuous function can have unbounded derivative
    $endgroup$
    – zhw.
    Nov 7 '16 at 19:12












  • $begingroup$
    Yes. That's why I deleted my comment after a few mjnutes. I was thinking anout lipschitz continuouity and wanted to give an osciallator-like counterexample
    $endgroup$
    – Gregor de Cillia
    Nov 7 '16 at 19:21










  • $begingroup$
    Fix epsilon. Let delta(x) be the maximum delta used in the definition of continuity at x. Show delta(x) has a nonzero limit given the conditions of the problem.
    $endgroup$
    – Alephnull
    Nov 7 '16 at 20:35








2




2




$begingroup$
To avoid splitting into three cases, you could allow the pieces to overlap and make sure to choose $delta$ less than the width of the overlap. This avoids the case where $x$ is in one piece and $y$ is in the other.
$endgroup$
– J. Loreaux
Nov 7 '16 at 19:06






$begingroup$
To avoid splitting into three cases, you could allow the pieces to overlap and make sure to choose $delta$ less than the width of the overlap. This avoids the case where $x$ is in one piece and $y$ is in the other.
$endgroup$
– J. Loreaux
Nov 7 '16 at 19:06














$begingroup$
@GregordeCillia A uniformly continuous function can have unbounded derivative
$endgroup$
– zhw.
Nov 7 '16 at 19:12






$begingroup$
@GregordeCillia A uniformly continuous function can have unbounded derivative
$endgroup$
– zhw.
Nov 7 '16 at 19:12














$begingroup$
Yes. That's why I deleted my comment after a few mjnutes. I was thinking anout lipschitz continuouity and wanted to give an osciallator-like counterexample
$endgroup$
– Gregor de Cillia
Nov 7 '16 at 19:21




$begingroup$
Yes. That's why I deleted my comment after a few mjnutes. I was thinking anout lipschitz continuouity and wanted to give an osciallator-like counterexample
$endgroup$
– Gregor de Cillia
Nov 7 '16 at 19:21












$begingroup$
Fix epsilon. Let delta(x) be the maximum delta used in the definition of continuity at x. Show delta(x) has a nonzero limit given the conditions of the problem.
$endgroup$
– Alephnull
Nov 7 '16 at 20:35




$begingroup$
Fix epsilon. Let delta(x) be the maximum delta used in the definition of continuity at x. Show delta(x) has a nonzero limit given the conditions of the problem.
$endgroup$
– Alephnull
Nov 7 '16 at 20:35










1 Answer
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$begingroup$

It suffices you do the following. Given $epsilon>0$, take $N$ large so that, for $x>N$,



$$|f(x)- l|<varepsilon/2$$



It follows that if $x,y>N$, then $|f(x)-f(y)|<varepsilon$. Now consider the interval $[a,N+1]$, which is compact. Here $f$ is uniformly continuous, so there is some $delta >0$ such that if $|x-y|<delta$ then $|f(x)-f(y)|<varepsilon$. We can assume that $delta<1$.



Consider now arbitrary $x,ygeqslant a$ that are at most $delta$ units apart. If both $x,yleqslant N+1$, then you're done. Our choice of $delta$ is such that if $x> N+1$ then also $y>N$, and you're also done.






share|cite|improve this answer









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    1 Answer
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    1 Answer
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    4












    $begingroup$

    It suffices you do the following. Given $epsilon>0$, take $N$ large so that, for $x>N$,



    $$|f(x)- l|<varepsilon/2$$



    It follows that if $x,y>N$, then $|f(x)-f(y)|<varepsilon$. Now consider the interval $[a,N+1]$, which is compact. Here $f$ is uniformly continuous, so there is some $delta >0$ such that if $|x-y|<delta$ then $|f(x)-f(y)|<varepsilon$. We can assume that $delta<1$.



    Consider now arbitrary $x,ygeqslant a$ that are at most $delta$ units apart. If both $x,yleqslant N+1$, then you're done. Our choice of $delta$ is such that if $x> N+1$ then also $y>N$, and you're also done.






    share|cite|improve this answer









    $endgroup$


















      4












      $begingroup$

      It suffices you do the following. Given $epsilon>0$, take $N$ large so that, for $x>N$,



      $$|f(x)- l|<varepsilon/2$$



      It follows that if $x,y>N$, then $|f(x)-f(y)|<varepsilon$. Now consider the interval $[a,N+1]$, which is compact. Here $f$ is uniformly continuous, so there is some $delta >0$ such that if $|x-y|<delta$ then $|f(x)-f(y)|<varepsilon$. We can assume that $delta<1$.



      Consider now arbitrary $x,ygeqslant a$ that are at most $delta$ units apart. If both $x,yleqslant N+1$, then you're done. Our choice of $delta$ is such that if $x> N+1$ then also $y>N$, and you're also done.






      share|cite|improve this answer









      $endgroup$
















        4












        4








        4





        $begingroup$

        It suffices you do the following. Given $epsilon>0$, take $N$ large so that, for $x>N$,



        $$|f(x)- l|<varepsilon/2$$



        It follows that if $x,y>N$, then $|f(x)-f(y)|<varepsilon$. Now consider the interval $[a,N+1]$, which is compact. Here $f$ is uniformly continuous, so there is some $delta >0$ such that if $|x-y|<delta$ then $|f(x)-f(y)|<varepsilon$. We can assume that $delta<1$.



        Consider now arbitrary $x,ygeqslant a$ that are at most $delta$ units apart. If both $x,yleqslant N+1$, then you're done. Our choice of $delta$ is such that if $x> N+1$ then also $y>N$, and you're also done.






        share|cite|improve this answer









        $endgroup$



        It suffices you do the following. Given $epsilon>0$, take $N$ large so that, for $x>N$,



        $$|f(x)- l|<varepsilon/2$$



        It follows that if $x,y>N$, then $|f(x)-f(y)|<varepsilon$. Now consider the interval $[a,N+1]$, which is compact. Here $f$ is uniformly continuous, so there is some $delta >0$ such that if $|x-y|<delta$ then $|f(x)-f(y)|<varepsilon$. We can assume that $delta<1$.



        Consider now arbitrary $x,ygeqslant a$ that are at most $delta$ units apart. If both $x,yleqslant N+1$, then you're done. Our choice of $delta$ is such that if $x> N+1$ then also $y>N$, and you're also done.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Nov 7 '16 at 19:13









        Pedro TamaroffPedro Tamaroff

        96.9k10153297




        96.9k10153297






























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