if a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that $exists M >0$ , $|f(x)| leq M$...












0












$begingroup$


The question is given below:



Let $f$ be a continuous function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that there exists $M >0$ such that $|f(x)| leq M$ for all $x in [a, +infty).$



My Thoughts



1-But I wonder how can I prove this, will I use a proof similar to the comparison test?



2- Will the proof include the use this problem:



Let $f$ be an increasing function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that $f(x) leq L$ for all $x in [a, +infty).$and that $f$ is bounded on $[a, +infty).$



Any help will be appreciated.










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    The question is given below:



    Let $f$ be a continuous function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that there exists $M >0$ such that $|f(x)| leq M$ for all $x in [a, +infty).$



    My Thoughts



    1-But I wonder how can I prove this, will I use a proof similar to the comparison test?



    2- Will the proof include the use this problem:



    Let $f$ be an increasing function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that $f(x) leq L$ for all $x in [a, +infty).$and that $f$ is bounded on $[a, +infty).$



    Any help will be appreciated.










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      The question is given below:



      Let $f$ be a continuous function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that there exists $M >0$ such that $|f(x)| leq M$ for all $x in [a, +infty).$



      My Thoughts



      1-But I wonder how can I prove this, will I use a proof similar to the comparison test?



      2- Will the proof include the use this problem:



      Let $f$ be an increasing function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that $f(x) leq L$ for all $x in [a, +infty).$and that $f$ is bounded on $[a, +infty).$



      Any help will be appreciated.










      share|cite|improve this question











      $endgroup$




      The question is given below:



      Let $f$ be a continuous function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that there exists $M >0$ such that $|f(x)| leq M$ for all $x in [a, +infty).$



      My Thoughts



      1-But I wonder how can I prove this, will I use a proof similar to the comparison test?



      2- Will the proof include the use this problem:



      Let $f$ be an increasing function for $x geq a,$ and suppose that a finite limit $L = lim_{x rightarrow infty} f(x) $ exists. Prove that $f(x) leq L$ for all $x in [a, +infty).$and that $f$ is bounded on $[a, +infty).$



      Any help will be appreciated.







      real-analysis calculus improper-integrals






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 7 '18 at 5:24







      hopefully

















      asked Dec 7 '18 at 5:17









      hopefullyhopefully

      290113




      290113






















          2 Answers
          2






          active

          oldest

          votes


















          1












          $begingroup$

          By definition of limit, we know there exist a $Xin [a,+infty)$, such that
          $$|f(x)-L|<1,,,,x>X$$
          which implies that
          $$|f(x)|<1+|L|,,,,x>X$$
          If $X=a$, let $M=max{(1+|L|,|f(a)|)}$, then we've done.



          If $X>a$, we know there exist $N>0$ such that
          $$|f(x)|<N,,,xin[a,X]$$
          by the continuity of $f$ on $[a,X]$. Let $M=max{(1+|L|,N)}$, we've done.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Could you please clarify from where your forth line come ?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:55






          • 1




            $begingroup$
            $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
            $endgroup$
            – Lau
            Dec 7 '18 at 7:58












          • $begingroup$
            is this a well known fact?
            $endgroup$
            – hopefully
            Dec 7 '18 at 8:22










          • $begingroup$
            I think so .....
            $endgroup$
            – Lau
            Dec 7 '18 at 8:24






          • 1




            $begingroup$
            @hopefully edited
            $endgroup$
            – Lau
            Dec 7 '18 at 9:05



















          2












          $begingroup$

          We know that for all $epsilon > 0$, there is an $N>0$ such that $x>N$, then $|f(x)-L| < epsilon$.



          In particular, there is an $N_1>0$, such that $x > N$, then $|f(x)-L| < 1$. then we have $|f(x) | < |L|+1$.



          Let $W = max_{x in [a, N]} |f(x)|.$



          I'm leaving the last step for you to use these to construct an upper bound for $|f(x)|$.



          Remark: We can't assume that $f$ is monotonic.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Could you provide some details about this last step please?
            $endgroup$
            – hopefully
            Dec 7 '18 at 6:03






          • 1




            $begingroup$
            I think $L+1$ is not necessary a positive number.
            $endgroup$
            – Lau
            Dec 7 '18 at 6:08










          • $begingroup$
            true, thanks for the feedback.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:24










          • $begingroup$
            @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:28










          • $begingroup$
            Could you please clarify from where your third line come?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:57











          Your Answer





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          2 Answers
          2






          active

          oldest

          votes








          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1












          $begingroup$

          By definition of limit, we know there exist a $Xin [a,+infty)$, such that
          $$|f(x)-L|<1,,,,x>X$$
          which implies that
          $$|f(x)|<1+|L|,,,,x>X$$
          If $X=a$, let $M=max{(1+|L|,|f(a)|)}$, then we've done.



          If $X>a$, we know there exist $N>0$ such that
          $$|f(x)|<N,,,xin[a,X]$$
          by the continuity of $f$ on $[a,X]$. Let $M=max{(1+|L|,N)}$, we've done.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Could you please clarify from where your forth line come ?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:55






          • 1




            $begingroup$
            $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
            $endgroup$
            – Lau
            Dec 7 '18 at 7:58












          • $begingroup$
            is this a well known fact?
            $endgroup$
            – hopefully
            Dec 7 '18 at 8:22










          • $begingroup$
            I think so .....
            $endgroup$
            – Lau
            Dec 7 '18 at 8:24






          • 1




            $begingroup$
            @hopefully edited
            $endgroup$
            – Lau
            Dec 7 '18 at 9:05
















          1












          $begingroup$

          By definition of limit, we know there exist a $Xin [a,+infty)$, such that
          $$|f(x)-L|<1,,,,x>X$$
          which implies that
          $$|f(x)|<1+|L|,,,,x>X$$
          If $X=a$, let $M=max{(1+|L|,|f(a)|)}$, then we've done.



          If $X>a$, we know there exist $N>0$ such that
          $$|f(x)|<N,,,xin[a,X]$$
          by the continuity of $f$ on $[a,X]$. Let $M=max{(1+|L|,N)}$, we've done.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Could you please clarify from where your forth line come ?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:55






          • 1




            $begingroup$
            $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
            $endgroup$
            – Lau
            Dec 7 '18 at 7:58












          • $begingroup$
            is this a well known fact?
            $endgroup$
            – hopefully
            Dec 7 '18 at 8:22










          • $begingroup$
            I think so .....
            $endgroup$
            – Lau
            Dec 7 '18 at 8:24






          • 1




            $begingroup$
            @hopefully edited
            $endgroup$
            – Lau
            Dec 7 '18 at 9:05














          1












          1








          1





          $begingroup$

          By definition of limit, we know there exist a $Xin [a,+infty)$, such that
          $$|f(x)-L|<1,,,,x>X$$
          which implies that
          $$|f(x)|<1+|L|,,,,x>X$$
          If $X=a$, let $M=max{(1+|L|,|f(a)|)}$, then we've done.



          If $X>a$, we know there exist $N>0$ such that
          $$|f(x)|<N,,,xin[a,X]$$
          by the continuity of $f$ on $[a,X]$. Let $M=max{(1+|L|,N)}$, we've done.






          share|cite|improve this answer











          $endgroup$



          By definition of limit, we know there exist a $Xin [a,+infty)$, such that
          $$|f(x)-L|<1,,,,x>X$$
          which implies that
          $$|f(x)|<1+|L|,,,,x>X$$
          If $X=a$, let $M=max{(1+|L|,|f(a)|)}$, then we've done.



          If $X>a$, we know there exist $N>0$ such that
          $$|f(x)|<N,,,xin[a,X]$$
          by the continuity of $f$ on $[a,X]$. Let $M=max{(1+|L|,N)}$, we've done.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 7 '18 at 9:05

























          answered Dec 7 '18 at 5:28









          LauLau

          527315




          527315












          • $begingroup$
            Could you please clarify from where your forth line come ?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:55






          • 1




            $begingroup$
            $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
            $endgroup$
            – Lau
            Dec 7 '18 at 7:58












          • $begingroup$
            is this a well known fact?
            $endgroup$
            – hopefully
            Dec 7 '18 at 8:22










          • $begingroup$
            I think so .....
            $endgroup$
            – Lau
            Dec 7 '18 at 8:24






          • 1




            $begingroup$
            @hopefully edited
            $endgroup$
            – Lau
            Dec 7 '18 at 9:05


















          • $begingroup$
            Could you please clarify from where your forth line come ?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:55






          • 1




            $begingroup$
            $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
            $endgroup$
            – Lau
            Dec 7 '18 at 7:58












          • $begingroup$
            is this a well known fact?
            $endgroup$
            – hopefully
            Dec 7 '18 at 8:22










          • $begingroup$
            I think so .....
            $endgroup$
            – Lau
            Dec 7 '18 at 8:24






          • 1




            $begingroup$
            @hopefully edited
            $endgroup$
            – Lau
            Dec 7 '18 at 9:05
















          $begingroup$
          Could you please clarify from where your forth line come ?
          $endgroup$
          – hopefully
          Dec 7 '18 at 7:55




          $begingroup$
          Could you please clarify from where your forth line come ?
          $endgroup$
          – hopefully
          Dec 7 '18 at 7:55




          1




          1




          $begingroup$
          $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
          $endgroup$
          – Lau
          Dec 7 '18 at 7:58






          $begingroup$
          $|f(x)-L|<1$ is also $-1<f(x)-L<1$ and $-1+L<f(x)<1+L$. Then $$|f(x)|<max(|L-1|,|L+1|) leq 1+|L|$$
          $endgroup$
          – Lau
          Dec 7 '18 at 7:58














          $begingroup$
          is this a well known fact?
          $endgroup$
          – hopefully
          Dec 7 '18 at 8:22




          $begingroup$
          is this a well known fact?
          $endgroup$
          – hopefully
          Dec 7 '18 at 8:22












          $begingroup$
          I think so .....
          $endgroup$
          – Lau
          Dec 7 '18 at 8:24




          $begingroup$
          I think so .....
          $endgroup$
          – Lau
          Dec 7 '18 at 8:24




          1




          1




          $begingroup$
          @hopefully edited
          $endgroup$
          – Lau
          Dec 7 '18 at 9:05




          $begingroup$
          @hopefully edited
          $endgroup$
          – Lau
          Dec 7 '18 at 9:05











          2












          $begingroup$

          We know that for all $epsilon > 0$, there is an $N>0$ such that $x>N$, then $|f(x)-L| < epsilon$.



          In particular, there is an $N_1>0$, such that $x > N$, then $|f(x)-L| < 1$. then we have $|f(x) | < |L|+1$.



          Let $W = max_{x in [a, N]} |f(x)|.$



          I'm leaving the last step for you to use these to construct an upper bound for $|f(x)|$.



          Remark: We can't assume that $f$ is monotonic.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Could you provide some details about this last step please?
            $endgroup$
            – hopefully
            Dec 7 '18 at 6:03






          • 1




            $begingroup$
            I think $L+1$ is not necessary a positive number.
            $endgroup$
            – Lau
            Dec 7 '18 at 6:08










          • $begingroup$
            true, thanks for the feedback.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:24










          • $begingroup$
            @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:28










          • $begingroup$
            Could you please clarify from where your third line come?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:57
















          2












          $begingroup$

          We know that for all $epsilon > 0$, there is an $N>0$ such that $x>N$, then $|f(x)-L| < epsilon$.



          In particular, there is an $N_1>0$, such that $x > N$, then $|f(x)-L| < 1$. then we have $|f(x) | < |L|+1$.



          Let $W = max_{x in [a, N]} |f(x)|.$



          I'm leaving the last step for you to use these to construct an upper bound for $|f(x)|$.



          Remark: We can't assume that $f$ is monotonic.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Could you provide some details about this last step please?
            $endgroup$
            – hopefully
            Dec 7 '18 at 6:03






          • 1




            $begingroup$
            I think $L+1$ is not necessary a positive number.
            $endgroup$
            – Lau
            Dec 7 '18 at 6:08










          • $begingroup$
            true, thanks for the feedback.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:24










          • $begingroup$
            @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:28










          • $begingroup$
            Could you please clarify from where your third line come?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:57














          2












          2








          2





          $begingroup$

          We know that for all $epsilon > 0$, there is an $N>0$ such that $x>N$, then $|f(x)-L| < epsilon$.



          In particular, there is an $N_1>0$, such that $x > N$, then $|f(x)-L| < 1$. then we have $|f(x) | < |L|+1$.



          Let $W = max_{x in [a, N]} |f(x)|.$



          I'm leaving the last step for you to use these to construct an upper bound for $|f(x)|$.



          Remark: We can't assume that $f$ is monotonic.






          share|cite|improve this answer











          $endgroup$



          We know that for all $epsilon > 0$, there is an $N>0$ such that $x>N$, then $|f(x)-L| < epsilon$.



          In particular, there is an $N_1>0$, such that $x > N$, then $|f(x)-L| < 1$. then we have $|f(x) | < |L|+1$.



          Let $W = max_{x in [a, N]} |f(x)|.$



          I'm leaving the last step for you to use these to construct an upper bound for $|f(x)|$.



          Remark: We can't assume that $f$ is monotonic.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 7 '18 at 6:24

























          answered Dec 7 '18 at 5:23









          Siong Thye GohSiong Thye Goh

          100k1466117




          100k1466117












          • $begingroup$
            Could you provide some details about this last step please?
            $endgroup$
            – hopefully
            Dec 7 '18 at 6:03






          • 1




            $begingroup$
            I think $L+1$ is not necessary a positive number.
            $endgroup$
            – Lau
            Dec 7 '18 at 6:08










          • $begingroup$
            true, thanks for the feedback.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:24










          • $begingroup$
            @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:28










          • $begingroup$
            Could you please clarify from where your third line come?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:57


















          • $begingroup$
            Could you provide some details about this last step please?
            $endgroup$
            – hopefully
            Dec 7 '18 at 6:03






          • 1




            $begingroup$
            I think $L+1$ is not necessary a positive number.
            $endgroup$
            – Lau
            Dec 7 '18 at 6:08










          • $begingroup$
            true, thanks for the feedback.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:24










          • $begingroup$
            @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
            $endgroup$
            – Siong Thye Goh
            Dec 7 '18 at 6:28










          • $begingroup$
            Could you please clarify from where your third line come?
            $endgroup$
            – hopefully
            Dec 7 '18 at 7:57
















          $begingroup$
          Could you provide some details about this last step please?
          $endgroup$
          – hopefully
          Dec 7 '18 at 6:03




          $begingroup$
          Could you provide some details about this last step please?
          $endgroup$
          – hopefully
          Dec 7 '18 at 6:03




          1




          1




          $begingroup$
          I think $L+1$ is not necessary a positive number.
          $endgroup$
          – Lau
          Dec 7 '18 at 6:08




          $begingroup$
          I think $L+1$ is not necessary a positive number.
          $endgroup$
          – Lau
          Dec 7 '18 at 6:08












          $begingroup$
          true, thanks for the feedback.
          $endgroup$
          – Siong Thye Goh
          Dec 7 '18 at 6:24




          $begingroup$
          true, thanks for the feedback.
          $endgroup$
          – Siong Thye Goh
          Dec 7 '18 at 6:24












          $begingroup$
          @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
          $endgroup$
          – Siong Thye Goh
          Dec 7 '18 at 6:28




          $begingroup$
          @hopefully the answer has actually been provided in the other answer but the idea is that we know number that are big enough is bounded by a number $M_1$, number that are not too large is bounded by $M_2$. how do you select a common upper bound for both smaller number and big number at the same time? A simple analogy, you have two children, one spends more than the other, say one spend $5$ dollars per day and one spend $10$ dollars per day? how do you make sure they have enough to spend each day if you want to give them the same amount to spend.
          $endgroup$
          – Siong Thye Goh
          Dec 7 '18 at 6:28












          $begingroup$
          Could you please clarify from where your third line come?
          $endgroup$
          – hopefully
          Dec 7 '18 at 7:57




          $begingroup$
          Could you please clarify from where your third line come?
          $endgroup$
          – hopefully
          Dec 7 '18 at 7:57


















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