Calculating sum of infinite series

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I am really confused by sum of infinite series and was hoping to get some guidance. If I have a series $$sum_i=1^infty fracii+1$$



If I am asked to calculate the sum of the series, do I just take the limit?



If I take the limit it is 1, which I think means that series diverges so there is no sum.



Or is it the case that the sum is 1? Does a series only diverge if the limit is infinity or does it diverge if the limit doesn't equal 0?










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    up vote
    3
    down vote

    favorite












    I am really confused by sum of infinite series and was hoping to get some guidance. If I have a series $$sum_i=1^infty fracii+1$$



    If I am asked to calculate the sum of the series, do I just take the limit?



    If I take the limit it is 1, which I think means that series diverges so there is no sum.



    Or is it the case that the sum is 1? Does a series only diverge if the limit is infinity or does it diverge if the limit doesn't equal 0?










    share|cite|improve this question

























      up vote
      3
      down vote

      favorite









      up vote
      3
      down vote

      favorite











      I am really confused by sum of infinite series and was hoping to get some guidance. If I have a series $$sum_i=1^infty fracii+1$$



      If I am asked to calculate the sum of the series, do I just take the limit?



      If I take the limit it is 1, which I think means that series diverges so there is no sum.



      Or is it the case that the sum is 1? Does a series only diverge if the limit is infinity or does it diverge if the limit doesn't equal 0?










      share|cite|improve this question















      I am really confused by sum of infinite series and was hoping to get some guidance. If I have a series $$sum_i=1^infty fracii+1$$



      If I am asked to calculate the sum of the series, do I just take the limit?



      If I take the limit it is 1, which I think means that series diverges so there is no sum.



      Or is it the case that the sum is 1? Does a series only diverge if the limit is infinity or does it diverge if the limit doesn't equal 0?







      sequences-and-series






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      edited 59 mins ago

























      asked 1 hour ago









      Sam

      1136




      1136




















          6 Answers
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          down vote



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          Ok, let's start from the very beginning, to try to clarify your doubts.
          The writing $sum_i=1^inftyfracii+1$ doesn't mean that you are actually performing a sum of an infinite number of elements. It's a notation which stands for
          $$lim_nrightarrowinftysum_i=1^nfracii+1.$$
          This means that you are creating a sequence $sum_i=1^nfracii+1_n=1,2,...$
          called sequence of partial sums and you are studying its limit for $n$ approaching $infty$. The series $sum_i=1^inftyfracii+1$ is said convergent if the sequence of partial sums converges.

          Now when you say 'if I take the limit it is $1$' you are not referring to the limit of the parial sums. You are talking about $lim_irightarrowinftyfracii+1$. But why?
          Well, this is because you have a result which tells you that if you have a series of the form
          $sum_i=1^inftya_i$ which converges, then $lim_irightarrowinftya_i=0$.
          In your case $a_i=fracii+1$ and you have $lim_irightarrowinftyfracii+1=1$. This means that your series is not convergent. As explained in the other answers the sequence of parial sums is going to $infty$ and the point is exactly that $fracii+1$ is approaching $1$.

          Now the second part. Having $lim_irightarrowinftya_i=0$ is not enough to guaratee that the series $sum_i=1^infty a_i$ is convergent. And the standard example is the one you considered in one of the comments, namely $sum_i=1^inftyfrac1i$. For this series you have $lim_irightarrowinftyfrac1i=0$ but nonetheless the series is divergent.






          share|cite|improve this answer



























            up vote
            2
            down vote













            A series is a sequence of partial sums. In your case,



            $$frac12,frac12+frac23,frac12+frac23+frac34,frac12+frac23+frac34+frac45,cdots$$



            or



            $$frac12,frac76,frac2312,frac16360,cdots$$



            Does this sequence converge ?



            Hint:




            $$frac ii+1gefrac12implies S_n>frac n2$$







            share|cite|improve this answer





























              up vote
              1
              down vote













              It means that the sum diverges. You are summing numbers that are larger than $frac12$, meaning that the sum of the first $n$ terms will be at least $fracn2$. That is, the partial sums grow without bound. As the sum of an infinite series is defined to be the limit of the partial sums, the series diverges.






              share|cite|improve this answer




















              • If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                – Sam
                1 hour ago










              • I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                – Theo Bendit
                56 mins ago

















              up vote
              1
              down vote













              You seem to be confusing the sequence $$a_i = frac ii+1$$
              And the sequence of partial sums
              $$s_n=sum_i=1^na_i$$
              Now $$sum_i=1^infty fracii+1$$
              converges iff $s_n$ converges. Assuming that $s_n$ converges then $lim_ntoinfty s_n=lim_ntoinfty s_n-1$ so $lim_ntoinfty (s_n-s_n-1)=0$ however $$s_n-s_n-1=a_1+a_2+cdots +a_n-(a_1+a_2+cdots+a_n-1)=a_n$$
              So it must be $lim_ntoinfty a_n=0$ otherwise the sum doesn't converge. However if $lim_ntoinftya_n = 0$ that doesn't mean that the series converge but if $lim_ntoinftya_n = L$ and $Lneq 0$ then it diverges.






              share|cite|improve this answer



























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                down vote













                You may consider partial sums and split them up:
                $$sum_i=1^n fracii+1 = sum_i=1^n left(1 - frac1i+1 right) = n - sum_i=1^n frac1i+1$$ $$stackrelsum_i=1^n frac1i+1 leq ln ngeq n - ln n = n(1- fracln nn) stackreln to infty longrightarrow infty$$






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                  note:
                  $$sum_r=1^inftyfracrr+1=sum_r=1^infty1-sum_r=1^inftyfrac1r+1$$
                  neither of these series converge






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                    6 Answers
                    6






                    active

                    oldest

                    votes








                    6 Answers
                    6






                    active

                    oldest

                    votes









                    active

                    oldest

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                    active

                    oldest

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                    up vote
                    2
                    down vote



                    accepted










                    Ok, let's start from the very beginning, to try to clarify your doubts.
                    The writing $sum_i=1^inftyfracii+1$ doesn't mean that you are actually performing a sum of an infinite number of elements. It's a notation which stands for
                    $$lim_nrightarrowinftysum_i=1^nfracii+1.$$
                    This means that you are creating a sequence $sum_i=1^nfracii+1_n=1,2,...$
                    called sequence of partial sums and you are studying its limit for $n$ approaching $infty$. The series $sum_i=1^inftyfracii+1$ is said convergent if the sequence of partial sums converges.

                    Now when you say 'if I take the limit it is $1$' you are not referring to the limit of the parial sums. You are talking about $lim_irightarrowinftyfracii+1$. But why?
                    Well, this is because you have a result which tells you that if you have a series of the form
                    $sum_i=1^inftya_i$ which converges, then $lim_irightarrowinftya_i=0$.
                    In your case $a_i=fracii+1$ and you have $lim_irightarrowinftyfracii+1=1$. This means that your series is not convergent. As explained in the other answers the sequence of parial sums is going to $infty$ and the point is exactly that $fracii+1$ is approaching $1$.

                    Now the second part. Having $lim_irightarrowinftya_i=0$ is not enough to guaratee that the series $sum_i=1^infty a_i$ is convergent. And the standard example is the one you considered in one of the comments, namely $sum_i=1^inftyfrac1i$. For this series you have $lim_irightarrowinftyfrac1i=0$ but nonetheless the series is divergent.






                    share|cite|improve this answer
























                      up vote
                      2
                      down vote



                      accepted










                      Ok, let's start from the very beginning, to try to clarify your doubts.
                      The writing $sum_i=1^inftyfracii+1$ doesn't mean that you are actually performing a sum of an infinite number of elements. It's a notation which stands for
                      $$lim_nrightarrowinftysum_i=1^nfracii+1.$$
                      This means that you are creating a sequence $sum_i=1^nfracii+1_n=1,2,...$
                      called sequence of partial sums and you are studying its limit for $n$ approaching $infty$. The series $sum_i=1^inftyfracii+1$ is said convergent if the sequence of partial sums converges.

                      Now when you say 'if I take the limit it is $1$' you are not referring to the limit of the parial sums. You are talking about $lim_irightarrowinftyfracii+1$. But why?
                      Well, this is because you have a result which tells you that if you have a series of the form
                      $sum_i=1^inftya_i$ which converges, then $lim_irightarrowinftya_i=0$.
                      In your case $a_i=fracii+1$ and you have $lim_irightarrowinftyfracii+1=1$. This means that your series is not convergent. As explained in the other answers the sequence of parial sums is going to $infty$ and the point is exactly that $fracii+1$ is approaching $1$.

                      Now the second part. Having $lim_irightarrowinftya_i=0$ is not enough to guaratee that the series $sum_i=1^infty a_i$ is convergent. And the standard example is the one you considered in one of the comments, namely $sum_i=1^inftyfrac1i$. For this series you have $lim_irightarrowinftyfrac1i=0$ but nonetheless the series is divergent.






                      share|cite|improve this answer






















                        up vote
                        2
                        down vote



                        accepted







                        up vote
                        2
                        down vote



                        accepted






                        Ok, let's start from the very beginning, to try to clarify your doubts.
                        The writing $sum_i=1^inftyfracii+1$ doesn't mean that you are actually performing a sum of an infinite number of elements. It's a notation which stands for
                        $$lim_nrightarrowinftysum_i=1^nfracii+1.$$
                        This means that you are creating a sequence $sum_i=1^nfracii+1_n=1,2,...$
                        called sequence of partial sums and you are studying its limit for $n$ approaching $infty$. The series $sum_i=1^inftyfracii+1$ is said convergent if the sequence of partial sums converges.

                        Now when you say 'if I take the limit it is $1$' you are not referring to the limit of the parial sums. You are talking about $lim_irightarrowinftyfracii+1$. But why?
                        Well, this is because you have a result which tells you that if you have a series of the form
                        $sum_i=1^inftya_i$ which converges, then $lim_irightarrowinftya_i=0$.
                        In your case $a_i=fracii+1$ and you have $lim_irightarrowinftyfracii+1=1$. This means that your series is not convergent. As explained in the other answers the sequence of parial sums is going to $infty$ and the point is exactly that $fracii+1$ is approaching $1$.

                        Now the second part. Having $lim_irightarrowinftya_i=0$ is not enough to guaratee that the series $sum_i=1^infty a_i$ is convergent. And the standard example is the one you considered in one of the comments, namely $sum_i=1^inftyfrac1i$. For this series you have $lim_irightarrowinftyfrac1i=0$ but nonetheless the series is divergent.






                        share|cite|improve this answer












                        Ok, let's start from the very beginning, to try to clarify your doubts.
                        The writing $sum_i=1^inftyfracii+1$ doesn't mean that you are actually performing a sum of an infinite number of elements. It's a notation which stands for
                        $$lim_nrightarrowinftysum_i=1^nfracii+1.$$
                        This means that you are creating a sequence $sum_i=1^nfracii+1_n=1,2,...$
                        called sequence of partial sums and you are studying its limit for $n$ approaching $infty$. The series $sum_i=1^inftyfracii+1$ is said convergent if the sequence of partial sums converges.

                        Now when you say 'if I take the limit it is $1$' you are not referring to the limit of the parial sums. You are talking about $lim_irightarrowinftyfracii+1$. But why?
                        Well, this is because you have a result which tells you that if you have a series of the form
                        $sum_i=1^inftya_i$ which converges, then $lim_irightarrowinftya_i=0$.
                        In your case $a_i=fracii+1$ and you have $lim_irightarrowinftyfracii+1=1$. This means that your series is not convergent. As explained in the other answers the sequence of parial sums is going to $infty$ and the point is exactly that $fracii+1$ is approaching $1$.

                        Now the second part. Having $lim_irightarrowinftya_i=0$ is not enough to guaratee that the series $sum_i=1^infty a_i$ is convergent. And the standard example is the one you considered in one of the comments, namely $sum_i=1^inftyfrac1i$. For this series you have $lim_irightarrowinftyfrac1i=0$ but nonetheless the series is divergent.







                        share|cite|improve this answer












                        share|cite|improve this answer



                        share|cite|improve this answer










                        answered 39 mins ago









                        Uskebasi

                        9021417




                        9021417




















                            up vote
                            2
                            down vote













                            A series is a sequence of partial sums. In your case,



                            $$frac12,frac12+frac23,frac12+frac23+frac34,frac12+frac23+frac34+frac45,cdots$$



                            or



                            $$frac12,frac76,frac2312,frac16360,cdots$$



                            Does this sequence converge ?



                            Hint:




                            $$frac ii+1gefrac12implies S_n>frac n2$$







                            share|cite|improve this answer


























                              up vote
                              2
                              down vote













                              A series is a sequence of partial sums. In your case,



                              $$frac12,frac12+frac23,frac12+frac23+frac34,frac12+frac23+frac34+frac45,cdots$$



                              or



                              $$frac12,frac76,frac2312,frac16360,cdots$$



                              Does this sequence converge ?



                              Hint:




                              $$frac ii+1gefrac12implies S_n>frac n2$$







                              share|cite|improve this answer
























                                up vote
                                2
                                down vote










                                up vote
                                2
                                down vote









                                A series is a sequence of partial sums. In your case,



                                $$frac12,frac12+frac23,frac12+frac23+frac34,frac12+frac23+frac34+frac45,cdots$$



                                or



                                $$frac12,frac76,frac2312,frac16360,cdots$$



                                Does this sequence converge ?



                                Hint:




                                $$frac ii+1gefrac12implies S_n>frac n2$$







                                share|cite|improve this answer














                                A series is a sequence of partial sums. In your case,



                                $$frac12,frac12+frac23,frac12+frac23+frac34,frac12+frac23+frac34+frac45,cdots$$



                                or



                                $$frac12,frac76,frac2312,frac16360,cdots$$



                                Does this sequence converge ?



                                Hint:




                                $$frac ii+1gefrac12implies S_n>frac n2$$








                                share|cite|improve this answer














                                share|cite|improve this answer



                                share|cite|improve this answer








                                edited 50 mins ago

























                                answered 55 mins ago









                                Yves Daoust

                                115k666209




                                115k666209




















                                    up vote
                                    1
                                    down vote













                                    It means that the sum diverges. You are summing numbers that are larger than $frac12$, meaning that the sum of the first $n$ terms will be at least $fracn2$. That is, the partial sums grow without bound. As the sum of an infinite series is defined to be the limit of the partial sums, the series diverges.






                                    share|cite|improve this answer




















                                    • If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                                      – Sam
                                      1 hour ago










                                    • I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                                      – Theo Bendit
                                      56 mins ago














                                    up vote
                                    1
                                    down vote













                                    It means that the sum diverges. You are summing numbers that are larger than $frac12$, meaning that the sum of the first $n$ terms will be at least $fracn2$. That is, the partial sums grow without bound. As the sum of an infinite series is defined to be the limit of the partial sums, the series diverges.






                                    share|cite|improve this answer




















                                    • If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                                      – Sam
                                      1 hour ago










                                    • I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                                      – Theo Bendit
                                      56 mins ago












                                    up vote
                                    1
                                    down vote










                                    up vote
                                    1
                                    down vote









                                    It means that the sum diverges. You are summing numbers that are larger than $frac12$, meaning that the sum of the first $n$ terms will be at least $fracn2$. That is, the partial sums grow without bound. As the sum of an infinite series is defined to be the limit of the partial sums, the series diverges.






                                    share|cite|improve this answer












                                    It means that the sum diverges. You are summing numbers that are larger than $frac12$, meaning that the sum of the first $n$ terms will be at least $fracn2$. That is, the partial sums grow without bound. As the sum of an infinite series is defined to be the limit of the partial sums, the series diverges.







                                    share|cite|improve this answer












                                    share|cite|improve this answer



                                    share|cite|improve this answer










                                    answered 1 hour ago









                                    Theo Bendit

                                    13.7k12045




                                    13.7k12045











                                    • If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                                      – Sam
                                      1 hour ago










                                    • I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                                      – Theo Bendit
                                      56 mins ago
















                                    • If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                                      – Sam
                                      1 hour ago










                                    • I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                                      – Theo Bendit
                                      56 mins ago















                                    If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                                    – Sam
                                    1 hour ago




                                    If this series did not diverge, is there a way to find the sum of that series? For example if the series of $sum_i=0^infty frac1n$ then the sum is 0, but is there a way to have a sum other than 0?
                                    – Sam
                                    1 hour ago












                                    I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                                    – Theo Bendit
                                    56 mins ago




                                    I'm not sure what you mean. The series does diverge, and any method that begins by assuming the series converges will arrive at totally erroneous conclusions.
                                    – Theo Bendit
                                    56 mins ago










                                    up vote
                                    1
                                    down vote













                                    You seem to be confusing the sequence $$a_i = frac ii+1$$
                                    And the sequence of partial sums
                                    $$s_n=sum_i=1^na_i$$
                                    Now $$sum_i=1^infty fracii+1$$
                                    converges iff $s_n$ converges. Assuming that $s_n$ converges then $lim_ntoinfty s_n=lim_ntoinfty s_n-1$ so $lim_ntoinfty (s_n-s_n-1)=0$ however $$s_n-s_n-1=a_1+a_2+cdots +a_n-(a_1+a_2+cdots+a_n-1)=a_n$$
                                    So it must be $lim_ntoinfty a_n=0$ otherwise the sum doesn't converge. However if $lim_ntoinftya_n = 0$ that doesn't mean that the series converge but if $lim_ntoinftya_n = L$ and $Lneq 0$ then it diverges.






                                    share|cite|improve this answer
























                                      up vote
                                      1
                                      down vote













                                      You seem to be confusing the sequence $$a_i = frac ii+1$$
                                      And the sequence of partial sums
                                      $$s_n=sum_i=1^na_i$$
                                      Now $$sum_i=1^infty fracii+1$$
                                      converges iff $s_n$ converges. Assuming that $s_n$ converges then $lim_ntoinfty s_n=lim_ntoinfty s_n-1$ so $lim_ntoinfty (s_n-s_n-1)=0$ however $$s_n-s_n-1=a_1+a_2+cdots +a_n-(a_1+a_2+cdots+a_n-1)=a_n$$
                                      So it must be $lim_ntoinfty a_n=0$ otherwise the sum doesn't converge. However if $lim_ntoinftya_n = 0$ that doesn't mean that the series converge but if $lim_ntoinftya_n = L$ and $Lneq 0$ then it diverges.






                                      share|cite|improve this answer






















                                        up vote
                                        1
                                        down vote










                                        up vote
                                        1
                                        down vote









                                        You seem to be confusing the sequence $$a_i = frac ii+1$$
                                        And the sequence of partial sums
                                        $$s_n=sum_i=1^na_i$$
                                        Now $$sum_i=1^infty fracii+1$$
                                        converges iff $s_n$ converges. Assuming that $s_n$ converges then $lim_ntoinfty s_n=lim_ntoinfty s_n-1$ so $lim_ntoinfty (s_n-s_n-1)=0$ however $$s_n-s_n-1=a_1+a_2+cdots +a_n-(a_1+a_2+cdots+a_n-1)=a_n$$
                                        So it must be $lim_ntoinfty a_n=0$ otherwise the sum doesn't converge. However if $lim_ntoinftya_n = 0$ that doesn't mean that the series converge but if $lim_ntoinftya_n = L$ and $Lneq 0$ then it diverges.






                                        share|cite|improve this answer












                                        You seem to be confusing the sequence $$a_i = frac ii+1$$
                                        And the sequence of partial sums
                                        $$s_n=sum_i=1^na_i$$
                                        Now $$sum_i=1^infty fracii+1$$
                                        converges iff $s_n$ converges. Assuming that $s_n$ converges then $lim_ntoinfty s_n=lim_ntoinfty s_n-1$ so $lim_ntoinfty (s_n-s_n-1)=0$ however $$s_n-s_n-1=a_1+a_2+cdots +a_n-(a_1+a_2+cdots+a_n-1)=a_n$$
                                        So it must be $lim_ntoinfty a_n=0$ otherwise the sum doesn't converge. However if $lim_ntoinftya_n = 0$ that doesn't mean that the series converge but if $lim_ntoinftya_n = L$ and $Lneq 0$ then it diverges.







                                        share|cite|improve this answer












                                        share|cite|improve this answer



                                        share|cite|improve this answer










                                        answered 35 mins ago









                                        kingW3

                                        10.8k72553




                                        10.8k72553




















                                            up vote
                                            0
                                            down vote













                                            You may consider partial sums and split them up:
                                            $$sum_i=1^n fracii+1 = sum_i=1^n left(1 - frac1i+1 right) = n - sum_i=1^n frac1i+1$$ $$stackrelsum_i=1^n frac1i+1 leq ln ngeq n - ln n = n(1- fracln nn) stackreln to infty longrightarrow infty$$






                                            share|cite|improve this answer
























                                              up vote
                                              0
                                              down vote













                                              You may consider partial sums and split them up:
                                              $$sum_i=1^n fracii+1 = sum_i=1^n left(1 - frac1i+1 right) = n - sum_i=1^n frac1i+1$$ $$stackrelsum_i=1^n frac1i+1 leq ln ngeq n - ln n = n(1- fracln nn) stackreln to infty longrightarrow infty$$






                                              share|cite|improve this answer






















                                                up vote
                                                0
                                                down vote










                                                up vote
                                                0
                                                down vote









                                                You may consider partial sums and split them up:
                                                $$sum_i=1^n fracii+1 = sum_i=1^n left(1 - frac1i+1 right) = n - sum_i=1^n frac1i+1$$ $$stackrelsum_i=1^n frac1i+1 leq ln ngeq n - ln n = n(1- fracln nn) stackreln to infty longrightarrow infty$$






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                                                You may consider partial sums and split them up:
                                                $$sum_i=1^n fracii+1 = sum_i=1^n left(1 - frac1i+1 right) = n - sum_i=1^n frac1i+1$$ $$stackrelsum_i=1^n frac1i+1 leq ln ngeq n - ln n = n(1- fracln nn) stackreln to infty longrightarrow infty$$







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                                                answered 41 mins ago









                                                trancelocation

                                                5,8801516




                                                5,8801516




















                                                    up vote
                                                    0
                                                    down vote













                                                    note:
                                                    $$sum_r=1^inftyfracrr+1=sum_r=1^infty1-sum_r=1^inftyfrac1r+1$$
                                                    neither of these series converge






                                                    share|cite|improve this answer
























                                                      up vote
                                                      0
                                                      down vote













                                                      note:
                                                      $$sum_r=1^inftyfracrr+1=sum_r=1^infty1-sum_r=1^inftyfrac1r+1$$
                                                      neither of these series converge






                                                      share|cite|improve this answer






















                                                        up vote
                                                        0
                                                        down vote










                                                        up vote
                                                        0
                                                        down vote









                                                        note:
                                                        $$sum_r=1^inftyfracrr+1=sum_r=1^infty1-sum_r=1^inftyfrac1r+1$$
                                                        neither of these series converge






                                                        share|cite|improve this answer












                                                        note:
                                                        $$sum_r=1^inftyfracrr+1=sum_r=1^infty1-sum_r=1^inftyfrac1r+1$$
                                                        neither of these series converge







                                                        share|cite|improve this answer












                                                        share|cite|improve this answer



                                                        share|cite|improve this answer










                                                        answered 17 mins ago









                                                        Henry Lee

                                                        87410




                                                        87410



























                                                             

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