Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find...












1














Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.



Useful hints will work. Please give some nice hint to solve the problem.










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  • Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
    – xavierm02
    Feb 2 '17 at 14:36










  • @xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
    – Piyush Raut
    Feb 2 '17 at 14:39












  • But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
    – xavierm02
    Feb 2 '17 at 14:42












  • I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
    – Piyush Raut
    Feb 2 '17 at 14:45










  • No I meant with $g$, under the assumption that the limit exists with $f$.
    – xavierm02
    Feb 2 '17 at 14:45
















1














Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.



Useful hints will work. Please give some nice hint to solve the problem.










share|cite|improve this question
























  • Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
    – xavierm02
    Feb 2 '17 at 14:36










  • @xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
    – Piyush Raut
    Feb 2 '17 at 14:39












  • But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
    – xavierm02
    Feb 2 '17 at 14:42












  • I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
    – Piyush Raut
    Feb 2 '17 at 14:45










  • No I meant with $g$, under the assumption that the limit exists with $f$.
    – xavierm02
    Feb 2 '17 at 14:45














1












1








1


1





Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.



Useful hints will work. Please give some nice hint to solve the problem.










share|cite|improve this question















Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.



Useful hints will work. Please give some nice hint to solve the problem.







integration limits functions continuity






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share|cite|improve this question













share|cite|improve this question




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edited Nov 29 '18 at 21:00









Shaun

8,820113681




8,820113681










asked Feb 2 '17 at 14:32









Piyush RautPiyush Raut

577




577












  • Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
    – xavierm02
    Feb 2 '17 at 14:36










  • @xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
    – Piyush Raut
    Feb 2 '17 at 14:39












  • But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
    – xavierm02
    Feb 2 '17 at 14:42












  • I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
    – Piyush Raut
    Feb 2 '17 at 14:45










  • No I meant with $g$, under the assumption that the limit exists with $f$.
    – xavierm02
    Feb 2 '17 at 14:45


















  • Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
    – xavierm02
    Feb 2 '17 at 14:36










  • @xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
    – Piyush Raut
    Feb 2 '17 at 14:39












  • But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
    – xavierm02
    Feb 2 '17 at 14:42












  • I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
    – Piyush Raut
    Feb 2 '17 at 14:45










  • No I meant with $g$, under the assumption that the limit exists with $f$.
    – xavierm02
    Feb 2 '17 at 14:45
















Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36




Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36












@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39






@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39














But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42






But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42














I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45




I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45












No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45




No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45










2 Answers
2






active

oldest

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4














Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
$$
(c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
$$

Now, integrating from $N$ to $t$ gives:
$$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
$$
and hence
$$
(c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
$$
Taking $ttoinfty$ yields
$$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
$$
Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.






share|cite|improve this answer





























    3





    +50









    One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.



    We have
    begin{align}
    lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
    &=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
    &=lim_{xtoinfty} F(x) +F'(x)notag\
    &=Lnotag
    end{align}

    and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$






    share|cite|improve this answer

















    • 1




      Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
      – zhw.
      Nov 30 '18 at 17:51












    • @zhw : I should have added more details. Thanks for putting these details in your comment.
      – Paramanand Singh
      Nov 30 '18 at 17:55











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    2 Answers
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    2 Answers
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    4














    Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
    $$
    (c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
    $$

    Now, integrating from $N$ to $t$ gives:
    $$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
    $$
    and hence
    $$
    (c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
    $$
    Taking $ttoinfty$ yields
    $$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
    $$
    Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.






    share|cite|improve this answer


























      4














      Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
      $$
      (c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
      $$

      Now, integrating from $N$ to $t$ gives:
      $$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
      $$
      and hence
      $$
      (c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
      $$
      Taking $ttoinfty$ yields
      $$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
      $$
      Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.






      share|cite|improve this answer
























        4












        4








        4






        Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
        $$
        (c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
        $$

        Now, integrating from $N$ to $t$ gives:
        $$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
        $$
        and hence
        $$
        (c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
        $$
        Taking $ttoinfty$ yields
        $$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
        $$
        Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.






        share|cite|improve this answer












        Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
        $$
        (c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
        $$

        Now, integrating from $N$ to $t$ gives:
        $$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
        $$
        and hence
        $$
        (c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
        $$
        Taking $ttoinfty$ yields
        $$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
        $$
        Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Nov 27 '18 at 7:45









        SongSong

        6,948421




        6,948421























            3





            +50









            One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.



            We have
            begin{align}
            lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
            &=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
            &=lim_{xtoinfty} F(x) +F'(x)notag\
            &=Lnotag
            end{align}

            and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$






            share|cite|improve this answer

















            • 1




              Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
              – zhw.
              Nov 30 '18 at 17:51












            • @zhw : I should have added more details. Thanks for putting these details in your comment.
              – Paramanand Singh
              Nov 30 '18 at 17:55
















            3





            +50









            One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.



            We have
            begin{align}
            lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
            &=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
            &=lim_{xtoinfty} F(x) +F'(x)notag\
            &=Lnotag
            end{align}

            and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$






            share|cite|improve this answer

















            • 1




              Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
              – zhw.
              Nov 30 '18 at 17:51












            • @zhw : I should have added more details. Thanks for putting these details in your comment.
              – Paramanand Singh
              Nov 30 '18 at 17:55














            3





            +50







            3





            +50



            3




            +50




            One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.



            We have
            begin{align}
            lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
            &=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
            &=lim_{xtoinfty} F(x) +F'(x)notag\
            &=Lnotag
            end{align}

            and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$






            share|cite|improve this answer












            One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.



            We have
            begin{align}
            lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
            &=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
            &=lim_{xtoinfty} F(x) +F'(x)notag\
            &=Lnotag
            end{align}

            and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Nov 30 '18 at 2:37









            Paramanand SinghParamanand Singh

            49.2k555161




            49.2k555161








            • 1




              Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
              – zhw.
              Nov 30 '18 at 17:51












            • @zhw : I should have added more details. Thanks for putting these details in your comment.
              – Paramanand Singh
              Nov 30 '18 at 17:55














            • 1




              Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
              – zhw.
              Nov 30 '18 at 17:51












            • @zhw : I should have added more details. Thanks for putting these details in your comment.
              – Paramanand Singh
              Nov 30 '18 at 17:55








            1




            1




            Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
            – zhw.
            Nov 30 '18 at 17:51






            Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
            – zhw.
            Nov 30 '18 at 17:51














            @zhw : I should have added more details. Thanks for putting these details in your comment.
            – Paramanand Singh
            Nov 30 '18 at 17:55




            @zhw : I should have added more details. Thanks for putting these details in your comment.
            – Paramanand Singh
            Nov 30 '18 at 17:55


















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