Limit of product of infinite sequences and infinite series












2












$begingroup$


If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:



$$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
therefore$$
$$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
$$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.










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$endgroup$

















    2












    $begingroup$


    If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:



    $$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
    therefore$$
    $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
    From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
    $$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
    but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.










    share|cite|improve this question











    $endgroup$















      2












      2








      2





      $begingroup$


      If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:



      $$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
      therefore$$
      $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
      From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
      $$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
      but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.










      share|cite|improve this question











      $endgroup$




      If the limit $$lim_{nto infty} n^p u_n =A tag{1}\ pgt 1;A lt infty$$ then I have to show that the limit of the series $sum_{n=0} ^{infty}u_n $ converges. I know that since $n^p$ blows up at infinity $u_n$ must somehow go to zero. I have tried the following:



      $$lim_{nto infty} n^p u_n =A tag{2} = lim_{nto infty} n^p cdot lim_{nto infty} u_n \
      therefore$$
      $$lim_{nto infty}u_n = frac{lim_{nto infty}n^p}{A} tag{3}$$
      From there I concluded that $u_n = frac{n^p}{A}$. Then applying the ratio test:
      $$lim_{nto infty}frac{u_{n+1}}{u_n} = lim_{nto infty} {}frac{(n+1)^p}{n^p} lt 1 tag{4}$$
      but obviously this doesn't work and is wrong. I am not convinced that $(2)$ is correct because I am not sure if the limits ($lim_{n to infty }n^p $ is $infty$ ???) exist. Also $(4)$ doesn't really work I guess because the limit is probably $=1$.







      real-analysis sequences-and-series limits






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      share|cite|improve this question








      edited Jan 25 at 12:20







      daljit97

















      asked Jan 25 at 12:08









      daljit97daljit97

      178111




      178111






















          3 Answers
          3






          active

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          2












          $begingroup$

          First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.



          Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
          Can you figure it out now?






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
            $endgroup$
            – daljit97
            Jan 25 at 15:32












          • $begingroup$
            No, $(n^p u_n)$ is just bounded, but that's sufficient.
            $endgroup$
            – Klaus
            Jan 25 at 15:38










          • $begingroup$
            Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
            $endgroup$
            – daljit97
            Jan 25 at 15:43












          • $begingroup$
            Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
            $endgroup$
            – Klaus
            Jan 25 at 15:45










          • $begingroup$
            Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
            $endgroup$
            – daljit97
            Jan 26 at 2:11



















          1












          $begingroup$

          As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say



          $vert n^p u_n vert le B$ with $B >0$.



          then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.






          share|cite|improve this answer









          $endgroup$





















            0












            $begingroup$

            $(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.



            So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?






            share|cite|improve this answer









            $endgroup$













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              3 Answers
              3






              active

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              3 Answers
              3






              active

              oldest

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              active

              oldest

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              active

              oldest

              votes









              2












              $begingroup$

              First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.



              Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
              Can you figure it out now?






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
                $endgroup$
                – daljit97
                Jan 25 at 15:32












              • $begingroup$
                No, $(n^p u_n)$ is just bounded, but that's sufficient.
                $endgroup$
                – Klaus
                Jan 25 at 15:38










              • $begingroup$
                Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
                $endgroup$
                – daljit97
                Jan 25 at 15:43












              • $begingroup$
                Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
                $endgroup$
                – Klaus
                Jan 25 at 15:45










              • $begingroup$
                Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
                $endgroup$
                – daljit97
                Jan 26 at 2:11
















              2












              $begingroup$

              First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.



              Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
              Can you figure it out now?






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
                $endgroup$
                – daljit97
                Jan 25 at 15:32












              • $begingroup$
                No, $(n^p u_n)$ is just bounded, but that's sufficient.
                $endgroup$
                – Klaus
                Jan 25 at 15:38










              • $begingroup$
                Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
                $endgroup$
                – daljit97
                Jan 25 at 15:43












              • $begingroup$
                Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
                $endgroup$
                – Klaus
                Jan 25 at 15:45










              • $begingroup$
                Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
                $endgroup$
                – daljit97
                Jan 26 at 2:11














              2












              2








              2





              $begingroup$

              First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.



              Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
              Can you figure it out now?






              share|cite|improve this answer











              $endgroup$



              First of all, you are doing a lot of illegal stuff there. (2), as you already pointed out, does not work as $n^p$ is not convergent. Then you are essentially dividing by $0$ from (2) to (3) (among another elementary mistake) and after that, you conclude $u_n = frac{n^p}{A}$ from $limlimits_{nto infty}u_n = frac{limlimits_{nto infty}n^p}{A}$, which is obviously not correct, either.



              Now for your actual question. Write $$sum_{n=0}^{infty}u_n = sum_{n=0}^{infty} frac{1}{n^p}(n^pu_n).$$
              Can you figure it out now?







              share|cite|improve this answer














              share|cite|improve this answer



              share|cite|improve this answer








              edited Jan 25 at 12:23

























              answered Jan 25 at 12:17









              KlausKlaus

              2,56713




              2,56713












              • $begingroup$
                I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
                $endgroup$
                – daljit97
                Jan 25 at 15:32












              • $begingroup$
                No, $(n^p u_n)$ is just bounded, but that's sufficient.
                $endgroup$
                – Klaus
                Jan 25 at 15:38










              • $begingroup$
                Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
                $endgroup$
                – daljit97
                Jan 25 at 15:43












              • $begingroup$
                Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
                $endgroup$
                – Klaus
                Jan 25 at 15:45










              • $begingroup$
                Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
                $endgroup$
                – daljit97
                Jan 26 at 2:11


















              • $begingroup$
                I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
                $endgroup$
                – daljit97
                Jan 25 at 15:32












              • $begingroup$
                No, $(n^p u_n)$ is just bounded, but that's sufficient.
                $endgroup$
                – Klaus
                Jan 25 at 15:38










              • $begingroup$
                Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
                $endgroup$
                – daljit97
                Jan 25 at 15:43












              • $begingroup$
                Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
                $endgroup$
                – Klaus
                Jan 25 at 15:45










              • $begingroup$
                Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
                $endgroup$
                – daljit97
                Jan 26 at 2:11
















              $begingroup$
              I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
              $endgroup$
              – daljit97
              Jan 25 at 15:32






              $begingroup$
              I know that $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ is convergent and I suppose that $sum_{n=0} ^ {infty} {n^p u_n}$ is also convergent?
              $endgroup$
              – daljit97
              Jan 25 at 15:32














              $begingroup$
              No, $(n^p u_n)$ is just bounded, but that's sufficient.
              $endgroup$
              – Klaus
              Jan 25 at 15:38




              $begingroup$
              No, $(n^p u_n)$ is just bounded, but that's sufficient.
              $endgroup$
              – Klaus
              Jan 25 at 15:38












              $begingroup$
              Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
              $endgroup$
              – daljit97
              Jan 25 at 15:43






              $begingroup$
              Ok so I can say $sum_{n=0}^{infty}u_n lt sum_{n=0}^{infty} frac{1}{n^p}(M).$ for some $M in mathbb{R}$?
              $endgroup$
              – daljit97
              Jan 25 at 15:43














              $begingroup$
              Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
              $endgroup$
              – Klaus
              Jan 25 at 15:45




              $begingroup$
              Better use an absolute value (i.e. $|u_n|$), but otherwise yes.
              $endgroup$
              – Klaus
              Jan 25 at 15:45












              $begingroup$
              Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
              $endgroup$
              – daljit97
              Jan 26 at 2:11




              $begingroup$
              Ok since $sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges obviously $M sum_{n=0} ^ {infty} {frac{1}{n^p}}$ converges right?
              $endgroup$
              – daljit97
              Jan 26 at 2:11











              1












              $begingroup$

              As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say



              $vert n^p u_n vert le B$ with $B >0$.



              then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.






              share|cite|improve this answer









              $endgroup$


















                1












                $begingroup$

                As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say



                $vert n^p u_n vert le B$ with $B >0$.



                then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.






                share|cite|improve this answer









                $endgroup$
















                  1












                  1








                  1





                  $begingroup$

                  As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say



                  $vert n^p u_n vert le B$ with $B >0$.



                  then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.






                  share|cite|improve this answer









                  $endgroup$



                  As $lim_{nto infty} n^p u_n =A $ is finite, $(n^p u_n)$ is a bounded sequence (a sequence that converges is bounded). Let's say



                  $vert n^p u_n vert le B$ with $B >0$.



                  then you have $$vert u_n vert le frac{B}{n^p}$$ proving that $sum u_n$ is absolutely converging as for $p > 1$, $sum 1/n^p$ converges.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Jan 25 at 12:17









                  mathcounterexamples.netmathcounterexamples.net

                  27k22158




                  27k22158























                      0












                      $begingroup$

                      $(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.



                      So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?






                      share|cite|improve this answer









                      $endgroup$


















                        0












                        $begingroup$

                        $(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.



                        So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?






                        share|cite|improve this answer









                        $endgroup$
















                          0












                          0








                          0





                          $begingroup$

                          $(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.



                          So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?






                          share|cite|improve this answer









                          $endgroup$



                          $(2)$ and $(3)$ are meaningless. $(4)$ is actually right (except that the limit is equal to $1$), but your derivation isn’t.



                          So, here, the ratio test is useless. The actual useful assumption is that $n^pu_n$ is convergent. Therefore it is bounded: so $|u_n| leq Mn^{-p}$ for each $n$. Can you take it from there?







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Jan 25 at 12:21









                          MindlackMindlack

                          4,920211




                          4,920211






























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