How to prove that $g(x)equiv f(x)$?

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Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.



I've solved it if we add the condition that $f(x)$ is continuous.



But the continuity condition is un-needed.



==============
additional remarks =================



I know the fact that



$limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,



which is called Heine Theorem in my textbook.



But I don't think you can put two different sequence here even both of them turn to infinity.







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    up vote
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    down vote

    favorite
    1












    Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.



    I've solved it if we add the condition that $f(x)$ is continuous.



    But the continuity condition is un-needed.



    ==============
    additional remarks =================



    I know the fact that



    $limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,



    which is called Heine Theorem in my textbook.



    But I don't think you can put two different sequence here even both of them turn to infinity.







    share|cite|improve this question























      up vote
      2
      down vote

      favorite
      1









      up vote
      2
      down vote

      favorite
      1






      1





      Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.



      I've solved it if we add the condition that $f(x)$ is continuous.



      But the continuity condition is un-needed.



      ==============
      additional remarks =================



      I know the fact that



      $limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,



      which is called Heine Theorem in my textbook.



      But I don't think you can put two different sequence here even both of them turn to infinity.







      share|cite|improve this question













      Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.



      I've solved it if we add the condition that $f(x)$ is continuous.



      But the continuity condition is un-needed.



      ==============
      additional remarks =================



      I know the fact that



      $limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,



      which is called Heine Theorem in my textbook.



      But I don't think you can put two different sequence here even both of them turn to infinity.









      share|cite|improve this question












      share|cite|improve this question




      share|cite|improve this question








      edited Jul 17 at 16:44
























      asked Jul 17 at 4:13









      闫嘉琦

      41219




      41219




















          1 Answer
          1






          active

          oldest

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          up vote
          6
          down vote













          The idea is:



          Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
          $$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
          Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.






          share|cite|improve this answer























          • The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
            – é—«å˜‰ç¦
            Jul 17 at 5:07











          • @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
            – Hanul Jeon
            Jul 17 at 6:00











          • @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
            – é—«å˜‰ç¦
            Jul 17 at 6:22






          • 1




            @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
            – Holo
            Jul 17 at 10:21







          • 2




            Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
            – Peter Szilas
            Jul 17 at 18:25











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          active

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          up vote
          6
          down vote













          The idea is:



          Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
          $$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
          Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.






          share|cite|improve this answer























          • The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
            – é—«å˜‰ç¦
            Jul 17 at 5:07











          • @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
            – Hanul Jeon
            Jul 17 at 6:00











          • @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
            – é—«å˜‰ç¦
            Jul 17 at 6:22






          • 1




            @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
            – Holo
            Jul 17 at 10:21







          • 2




            Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
            – Peter Szilas
            Jul 17 at 18:25















          up vote
          6
          down vote













          The idea is:



          Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
          $$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
          Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.






          share|cite|improve this answer























          • The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
            – é—«å˜‰ç¦
            Jul 17 at 5:07











          • @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
            – Hanul Jeon
            Jul 17 at 6:00











          • @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
            – é—«å˜‰ç¦
            Jul 17 at 6:22






          • 1




            @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
            – Holo
            Jul 17 at 10:21







          • 2




            Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
            – Peter Szilas
            Jul 17 at 18:25













          up vote
          6
          down vote










          up vote
          6
          down vote









          The idea is:



          Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
          $$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
          Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.






          share|cite|improve this answer















          The idea is:



          Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
          $$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
          Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.







          share|cite|improve this answer















          share|cite|improve this answer



          share|cite|improve this answer








          edited Jul 17 at 4:36


























          answered Jul 17 at 4:28









          Inzit

          935412




          935412











          • The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
            – é—«å˜‰ç¦
            Jul 17 at 5:07











          • @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
            – Hanul Jeon
            Jul 17 at 6:00











          • @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
            – é—«å˜‰ç¦
            Jul 17 at 6:22






          • 1




            @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
            – Holo
            Jul 17 at 10:21







          • 2




            Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
            – Peter Szilas
            Jul 17 at 18:25

















          • The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
            – é—«å˜‰ç¦
            Jul 17 at 5:07











          • @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
            – Hanul Jeon
            Jul 17 at 6:00











          • @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
            – é—«å˜‰ç¦
            Jul 17 at 6:22






          • 1




            @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
            – Holo
            Jul 17 at 10:21







          • 2




            Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
            – Peter Szilas
            Jul 17 at 18:25
















          The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
          – é—«å˜‰ç¦
          Jul 17 at 5:07





          The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ¥、$y+mp_1=y+np_2$.
          – é—«å˜‰ç¦
          Jul 17 at 5:07













          @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
          – Hanul Jeon
          Jul 17 at 6:00





          @闫嘉琦 The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
          – Hanul Jeon
          Jul 17 at 6:00













          @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
          – é—«å˜‰ç¦
          Jul 17 at 6:22




          @HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
          – é—«å˜‰ç¦
          Jul 17 at 6:22




          1




          1




          @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
          – Holo
          Jul 17 at 10:21





          @闫嘉琦 Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
          – Holo
          Jul 17 at 10:21





          2




          2




          Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
          – Peter Szilas
          Jul 17 at 18:25





          Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
          – Peter Szilas
          Jul 17 at 18:25













           

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