Function satisfying $limlimits_{xto 0}(f(x)-f(2x))$ but doesn't have $limlimits_{xto 0}f(x)$












0












$begingroup$


I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.



Any direction or hint would be appreciated.










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  • 1




    $begingroup$
    I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
    $endgroup$
    – Alejandro Nasif Salum
    Dec 25 '17 at 8:38












  • $begingroup$
    Does $pminfty$ count as existence of the limit?
    $endgroup$
    – Shashi
    Dec 25 '17 at 8:44
















0












$begingroup$


I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.



Any direction or hint would be appreciated.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
    $endgroup$
    – Alejandro Nasif Salum
    Dec 25 '17 at 8:38












  • $begingroup$
    Does $pminfty$ count as existence of the limit?
    $endgroup$
    – Shashi
    Dec 25 '17 at 8:44














0












0








0


0



$begingroup$


I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.



Any direction or hint would be appreciated.










share|cite|improve this question











$endgroup$




I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.



Any direction or hint would be appreciated.







calculus






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edited Dec 21 '18 at 8:58









Saad

20.2k92352




20.2k92352










asked Dec 25 '17 at 8:34









Lin TanakaLin Tanaka

265




265








  • 1




    $begingroup$
    I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
    $endgroup$
    – Alejandro Nasif Salum
    Dec 25 '17 at 8:38












  • $begingroup$
    Does $pminfty$ count as existence of the limit?
    $endgroup$
    – Shashi
    Dec 25 '17 at 8:44














  • 1




    $begingroup$
    I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
    $endgroup$
    – Alejandro Nasif Salum
    Dec 25 '17 at 8:38












  • $begingroup$
    Does $pminfty$ count as existence of the limit?
    $endgroup$
    – Shashi
    Dec 25 '17 at 8:44








1




1




$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38






$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38














$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44




$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44










5 Answers
5






active

oldest

votes


















1












$begingroup$

$f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    What is $chi_{{bf{Q}}}$?
    $endgroup$
    – Holo
    Dec 25 '17 at 11:12










  • $begingroup$
    The function takes the value $1$ for all rational numbers and $0$ otherwise.
    $endgroup$
    – user284331
    Dec 25 '17 at 18:06



















1












$begingroup$

Take



$f(x)=ln(x)$



$f(2x)=ln(2x)=ln(2)+ln(x)$



$g(x)=f(x)-f(2x)=-ln(2)$



Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
    $endgroup$
    – Holo
    Dec 26 '17 at 7:47





















0












$begingroup$

Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      Here's an example that is rather continuous.



      Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
      $$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
      so
      $$lim_{xto 0}(f(x)-f(2x)) = 0.$$
      But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.






      share|cite|improve this answer









      $endgroup$













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        5 Answers
        5






        active

        oldest

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        5 Answers
        5






        active

        oldest

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        active

        oldest

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        active

        oldest

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        1












        $begingroup$

        $f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          What is $chi_{{bf{Q}}}$?
          $endgroup$
          – Holo
          Dec 25 '17 at 11:12










        • $begingroup$
          The function takes the value $1$ for all rational numbers and $0$ otherwise.
          $endgroup$
          – user284331
          Dec 25 '17 at 18:06
















        1












        $begingroup$

        $f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.






        share|cite|improve this answer









        $endgroup$













        • $begingroup$
          What is $chi_{{bf{Q}}}$?
          $endgroup$
          – Holo
          Dec 25 '17 at 11:12










        • $begingroup$
          The function takes the value $1$ for all rational numbers and $0$ otherwise.
          $endgroup$
          – user284331
          Dec 25 '17 at 18:06














        1












        1








        1





        $begingroup$

        $f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.






        share|cite|improve this answer









        $endgroup$



        $f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Dec 25 '17 at 8:39









        user284331user284331

        35.4k31646




        35.4k31646












        • $begingroup$
          What is $chi_{{bf{Q}}}$?
          $endgroup$
          – Holo
          Dec 25 '17 at 11:12










        • $begingroup$
          The function takes the value $1$ for all rational numbers and $0$ otherwise.
          $endgroup$
          – user284331
          Dec 25 '17 at 18:06


















        • $begingroup$
          What is $chi_{{bf{Q}}}$?
          $endgroup$
          – Holo
          Dec 25 '17 at 11:12










        • $begingroup$
          The function takes the value $1$ for all rational numbers and $0$ otherwise.
          $endgroup$
          – user284331
          Dec 25 '17 at 18:06
















        $begingroup$
        What is $chi_{{bf{Q}}}$?
        $endgroup$
        – Holo
        Dec 25 '17 at 11:12




        $begingroup$
        What is $chi_{{bf{Q}}}$?
        $endgroup$
        – Holo
        Dec 25 '17 at 11:12












        $begingroup$
        The function takes the value $1$ for all rational numbers and $0$ otherwise.
        $endgroup$
        – user284331
        Dec 25 '17 at 18:06




        $begingroup$
        The function takes the value $1$ for all rational numbers and $0$ otherwise.
        $endgroup$
        – user284331
        Dec 25 '17 at 18:06











        1












        $begingroup$

        Take



        $f(x)=ln(x)$



        $f(2x)=ln(2x)=ln(2)+ln(x)$



        $g(x)=f(x)-f(2x)=-ln(2)$



        Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
          $endgroup$
          – Holo
          Dec 26 '17 at 7:47


















        1












        $begingroup$

        Take



        $f(x)=ln(x)$



        $f(2x)=ln(2x)=ln(2)+ln(x)$



        $g(x)=f(x)-f(2x)=-ln(2)$



        Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
          $endgroup$
          – Holo
          Dec 26 '17 at 7:47
















        1












        1








        1





        $begingroup$

        Take



        $f(x)=ln(x)$



        $f(2x)=ln(2x)=ln(2)+ln(x)$



        $g(x)=f(x)-f(2x)=-ln(2)$



        Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant






        share|cite|improve this answer











        $endgroup$



        Take



        $f(x)=ln(x)$



        $f(2x)=ln(2x)=ln(2)+ln(x)$



        $g(x)=f(x)-f(2x)=-ln(2)$



        Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 25 '17 at 9:44

























        answered Dec 25 '17 at 9:36









        HoloHolo

        6,08421131




        6,08421131












        • $begingroup$
          Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
          $endgroup$
          – Holo
          Dec 26 '17 at 7:47




















        • $begingroup$
          Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
          $endgroup$
          – Holo
          Dec 26 '17 at 7:47


















        $begingroup$
        Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
        $endgroup$
        – Holo
        Dec 26 '17 at 7:47






        $begingroup$
        Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
        $endgroup$
        – Holo
        Dec 26 '17 at 7:47













        0












        $begingroup$

        Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.






        share|cite|improve this answer









        $endgroup$


















          0












          $begingroup$

          Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.






          share|cite|improve this answer









          $endgroup$
















            0












            0








            0





            $begingroup$

            Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.






            share|cite|improve this answer









            $endgroup$



            Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 25 '17 at 8:39









            ShashiShashi

            7,3151628




            7,3151628























                0












                $begingroup$

                Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).






                    share|cite|improve this answer









                    $endgroup$



                    Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 25 '17 at 8:42









                    Alejandro Nasif SalumAlejandro Nasif Salum

                    4,765118




                    4,765118























                        0












                        $begingroup$

                        Here's an example that is rather continuous.



                        Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
                        $$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
                        so
                        $$lim_{xto 0}(f(x)-f(2x)) = 0.$$
                        But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          Here's an example that is rather continuous.



                          Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
                          $$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
                          so
                          $$lim_{xto 0}(f(x)-f(2x)) = 0.$$
                          But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            Here's an example that is rather continuous.



                            Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
                            $$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
                            so
                            $$lim_{xto 0}(f(x)-f(2x)) = 0.$$
                            But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.






                            share|cite|improve this answer









                            $endgroup$



                            Here's an example that is rather continuous.



                            Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
                            $$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
                            so
                            $$lim_{xto 0}(f(x)-f(2x)) = 0.$$
                            But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 25 '17 at 9:25









                            md2perpemd2perpe

                            8,30911028




                            8,30911028






























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