Let $sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}$, does it converge or does it diverge?












4















Let $displaystyle sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}$, does it converge or does it diverge?




I'm not completely sure that my calculation is correct, check it please.



$$begin{align}sum_{nge 0} frac{(-1)^{n+1}}{3 n+6 (-1)^n}&=-frac13sum_{nge 0} frac{(-1)^{n}}{n+2 (-1)^n}cdotfrac{(-1)^n}{(-1)^n}\&=-frac13sum_{nge 0} frac1{n(-1)^n+2}\&=-frac13lim_{mtoinfty}left(sum_{n=0\ 2mid n}^mfrac1{n+2}+sum_{n=0\ 2nmid n}^mfrac1{-n+2}right)\&=-frac13lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)end{align}$$



If $m$ is even then



$$begin{align}lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)&=lim_{mtoinfty}left(left(sum_{n=1}^{m-2}frac{(-1)^{n}}nright)+frac1m+frac1{m+2}+1right)\&=-log (2)+1end{align}$$



If $m$ is odd then



$$lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)=lim_{mtoinfty}left(left(sum_{n=1}^{m-1}frac{(-1)^{n}}nright)+frac1{m+1}+1right)=-log (2)+1$$



Then finally



$$bbox[2pt,border:yellow solid 2px]{sum_{n=0}^inftyfrac{(-1)^{n+1}}{3 n+6 (-1)^n}=frac{log(2)-1}{3}}$$



Are my calculations correct?










share|cite|improve this question





























    4















    Let $displaystyle sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}$, does it converge or does it diverge?




    I'm not completely sure that my calculation is correct, check it please.



    $$begin{align}sum_{nge 0} frac{(-1)^{n+1}}{3 n+6 (-1)^n}&=-frac13sum_{nge 0} frac{(-1)^{n}}{n+2 (-1)^n}cdotfrac{(-1)^n}{(-1)^n}\&=-frac13sum_{nge 0} frac1{n(-1)^n+2}\&=-frac13lim_{mtoinfty}left(sum_{n=0\ 2mid n}^mfrac1{n+2}+sum_{n=0\ 2nmid n}^mfrac1{-n+2}right)\&=-frac13lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)end{align}$$



    If $m$ is even then



    $$begin{align}lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)&=lim_{mtoinfty}left(left(sum_{n=1}^{m-2}frac{(-1)^{n}}nright)+frac1m+frac1{m+2}+1right)\&=-log (2)+1end{align}$$



    If $m$ is odd then



    $$lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)=lim_{mtoinfty}left(left(sum_{n=1}^{m-1}frac{(-1)^{n}}nright)+frac1{m+1}+1right)=-log (2)+1$$



    Then finally



    $$bbox[2pt,border:yellow solid 2px]{sum_{n=0}^inftyfrac{(-1)^{n+1}}{3 n+6 (-1)^n}=frac{log(2)-1}{3}}$$



    Are my calculations correct?










    share|cite|improve this question



























      4












      4








      4








      Let $displaystyle sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}$, does it converge or does it diverge?




      I'm not completely sure that my calculation is correct, check it please.



      $$begin{align}sum_{nge 0} frac{(-1)^{n+1}}{3 n+6 (-1)^n}&=-frac13sum_{nge 0} frac{(-1)^{n}}{n+2 (-1)^n}cdotfrac{(-1)^n}{(-1)^n}\&=-frac13sum_{nge 0} frac1{n(-1)^n+2}\&=-frac13lim_{mtoinfty}left(sum_{n=0\ 2mid n}^mfrac1{n+2}+sum_{n=0\ 2nmid n}^mfrac1{-n+2}right)\&=-frac13lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)end{align}$$



      If $m$ is even then



      $$begin{align}lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)&=lim_{mtoinfty}left(left(sum_{n=1}^{m-2}frac{(-1)^{n}}nright)+frac1m+frac1{m+2}+1right)\&=-log (2)+1end{align}$$



      If $m$ is odd then



      $$lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)=lim_{mtoinfty}left(left(sum_{n=1}^{m-1}frac{(-1)^{n}}nright)+frac1{m+1}+1right)=-log (2)+1$$



      Then finally



      $$bbox[2pt,border:yellow solid 2px]{sum_{n=0}^inftyfrac{(-1)^{n+1}}{3 n+6 (-1)^n}=frac{log(2)-1}{3}}$$



      Are my calculations correct?










      share|cite|improve this question
















      Let $displaystyle sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}$, does it converge or does it diverge?




      I'm not completely sure that my calculation is correct, check it please.



      $$begin{align}sum_{nge 0} frac{(-1)^{n+1}}{3 n+6 (-1)^n}&=-frac13sum_{nge 0} frac{(-1)^{n}}{n+2 (-1)^n}cdotfrac{(-1)^n}{(-1)^n}\&=-frac13sum_{nge 0} frac1{n(-1)^n+2}\&=-frac13lim_{mtoinfty}left(sum_{n=0\ 2mid n}^mfrac1{n+2}+sum_{n=0\ 2nmid n}^mfrac1{-n+2}right)\&=-frac13lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)end{align}$$



      If $m$ is even then



      $$begin{align}lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)&=lim_{mtoinfty}left(left(sum_{n=1}^{m-2}frac{(-1)^{n}}nright)+frac1m+frac1{m+2}+1right)\&=-log (2)+1end{align}$$



      If $m$ is odd then



      $$lim_{mtoinfty}left(sum_{n=2\ 2mid n}^{m+2}frac1{n}-sum_{n=-2\ 2nmid n}^{m-2}frac1{n}right)=lim_{mtoinfty}left(left(sum_{n=1}^{m-1}frac{(-1)^{n}}nright)+frac1{m+1}+1right)=-log (2)+1$$



      Then finally



      $$bbox[2pt,border:yellow solid 2px]{sum_{n=0}^inftyfrac{(-1)^{n+1}}{3 n+6 (-1)^n}=frac{log(2)-1}{3}}$$



      Are my calculations correct?







      calculus sequences-and-series limits proof-verification closed-form






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      edited Nov 30 at 16:17

























      asked Oct 30 '16 at 8:58









      Masacroso

      12.8k41746




      12.8k41746






















          2 Answers
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          2














          One may set
          $$
          S_N=sum_{n=0}^N frac{(-1)^{n+1}}{3 n+6 (-1)^n},quad Nge0, tag1
          $$ giving
          $$
          begin{align}
          S_{2N}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=1}^{N} frac{1}{6p-9}
          \\S_{2N}&=-frac16+frac13log 2+psiBig(N-frac12 Big)-psileft(N+2 right) tag2
          end{align}
          $$ and, as $N to infty$, giving
          $$
          lim_{N to infty}S_{2N}=-frac13+frac13log 2, tag3
          $$
          where $psi$ denotes the standard digamma function.



          Similarly,$$
          begin{align}
          S_{2N+1}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=0}^{N} frac{1}{6p-3}
          \\&S_{2N+1}=-frac13+frac13log 2+psiBig(N+frac12 Big)-psileft(N+2 right)
          end{align}
          $$ and, as $N to infty$, it gives
          $$
          lim_{N to infty}S_{2N+1}=-frac13+frac13log 2.
          $$



          We thus have




          $$
          sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}=-frac13+frac13log 2
          $$




          in agreement with your calculations.






          share|cite|improve this answer































            1














            Lemma: Suppose $a_nto 0$ and $sum_{n=1}^{infty}(a_{2n-1}+a_{2n})$ converges. Then $sum_{n=1}^{infty} a_n$ converges.



            Proof: Let $S_n$ denote the $n$th partial sum of $sum a_n.$ The hypotheses imply that $S_{2n}$ converges. Since $S_{2n+1} = S_{2n} + a_{2n+1}$ and $a_{2n+1}to 0,$ $S_{2n+1}$ also converges. This implies $S_n$ converges.



            In our problem, we have $a_nto 0.$ We also have



            $$a_{2n-1}+a_{2n} =frac{1}{3(2n-1) -6}-frac{1}{3(2n)+1) -6} = frac{15}{(6n-9)(6n+6)}.$$



            Thus for large $n,$ $a_{2n-1}+a_{2n}$ is positive and on the order of $1/n^2.$ It follows that $sum (a_{2n-1}+a_{2n})$ converges. By the lemma, $sum a_n$ converges.






            share|cite|improve this answer





















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              2 Answers
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              active

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              2 Answers
              2






              active

              oldest

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              active

              oldest

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              active

              oldest

              votes









              2














              One may set
              $$
              S_N=sum_{n=0}^N frac{(-1)^{n+1}}{3 n+6 (-1)^n},quad Nge0, tag1
              $$ giving
              $$
              begin{align}
              S_{2N}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=1}^{N} frac{1}{6p-9}
              \\S_{2N}&=-frac16+frac13log 2+psiBig(N-frac12 Big)-psileft(N+2 right) tag2
              end{align}
              $$ and, as $N to infty$, giving
              $$
              lim_{N to infty}S_{2N}=-frac13+frac13log 2, tag3
              $$
              where $psi$ denotes the standard digamma function.



              Similarly,$$
              begin{align}
              S_{2N+1}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=0}^{N} frac{1}{6p-3}
              \\&S_{2N+1}=-frac13+frac13log 2+psiBig(N+frac12 Big)-psileft(N+2 right)
              end{align}
              $$ and, as $N to infty$, it gives
              $$
              lim_{N to infty}S_{2N+1}=-frac13+frac13log 2.
              $$



              We thus have




              $$
              sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}=-frac13+frac13log 2
              $$




              in agreement with your calculations.






              share|cite|improve this answer




























                2














                One may set
                $$
                S_N=sum_{n=0}^N frac{(-1)^{n+1}}{3 n+6 (-1)^n},quad Nge0, tag1
                $$ giving
                $$
                begin{align}
                S_{2N}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=1}^{N} frac{1}{6p-9}
                \\S_{2N}&=-frac16+frac13log 2+psiBig(N-frac12 Big)-psileft(N+2 right) tag2
                end{align}
                $$ and, as $N to infty$, giving
                $$
                lim_{N to infty}S_{2N}=-frac13+frac13log 2, tag3
                $$
                where $psi$ denotes the standard digamma function.



                Similarly,$$
                begin{align}
                S_{2N+1}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=0}^{N} frac{1}{6p-3}
                \\&S_{2N+1}=-frac13+frac13log 2+psiBig(N+frac12 Big)-psileft(N+2 right)
                end{align}
                $$ and, as $N to infty$, it gives
                $$
                lim_{N to infty}S_{2N+1}=-frac13+frac13log 2.
                $$



                We thus have




                $$
                sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}=-frac13+frac13log 2
                $$




                in agreement with your calculations.






                share|cite|improve this answer


























                  2












                  2








                  2






                  One may set
                  $$
                  S_N=sum_{n=0}^N frac{(-1)^{n+1}}{3 n+6 (-1)^n},quad Nge0, tag1
                  $$ giving
                  $$
                  begin{align}
                  S_{2N}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=1}^{N} frac{1}{6p-9}
                  \\S_{2N}&=-frac16+frac13log 2+psiBig(N-frac12 Big)-psileft(N+2 right) tag2
                  end{align}
                  $$ and, as $N to infty$, giving
                  $$
                  lim_{N to infty}S_{2N}=-frac13+frac13log 2, tag3
                  $$
                  where $psi$ denotes the standard digamma function.



                  Similarly,$$
                  begin{align}
                  S_{2N+1}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=0}^{N} frac{1}{6p-3}
                  \\&S_{2N+1}=-frac13+frac13log 2+psiBig(N+frac12 Big)-psileft(N+2 right)
                  end{align}
                  $$ and, as $N to infty$, it gives
                  $$
                  lim_{N to infty}S_{2N+1}=-frac13+frac13log 2.
                  $$



                  We thus have




                  $$
                  sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}=-frac13+frac13log 2
                  $$




                  in agreement with your calculations.






                  share|cite|improve this answer














                  One may set
                  $$
                  S_N=sum_{n=0}^N frac{(-1)^{n+1}}{3 n+6 (-1)^n},quad Nge0, tag1
                  $$ giving
                  $$
                  begin{align}
                  S_{2N}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=1}^{N} frac{1}{6p-9}
                  \\S_{2N}&=-frac16+frac13log 2+psiBig(N-frac12 Big)-psileft(N+2 right) tag2
                  end{align}
                  $$ and, as $N to infty$, giving
                  $$
                  lim_{N to infty}S_{2N}=-frac13+frac13log 2, tag3
                  $$
                  where $psi$ denotes the standard digamma function.



                  Similarly,$$
                  begin{align}
                  S_{2N+1}&=sum_{p=0}^N frac{-1}{6p+6}+sum_{p=0}^{N} frac{1}{6p-3}
                  \\&S_{2N+1}=-frac13+frac13log 2+psiBig(N+frac12 Big)-psileft(N+2 right)
                  end{align}
                  $$ and, as $N to infty$, it gives
                  $$
                  lim_{N to infty}S_{2N+1}=-frac13+frac13log 2.
                  $$



                  We thus have




                  $$
                  sum_{n=0}^infty frac{(-1)^{n+1}}{3 n+6 (-1)^n}=-frac13+frac13log 2
                  $$




                  in agreement with your calculations.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Oct 30 '16 at 17:07

























                  answered Oct 30 '16 at 10:55









                  Olivier Oloa

                  107k17175293




                  107k17175293























                      1














                      Lemma: Suppose $a_nto 0$ and $sum_{n=1}^{infty}(a_{2n-1}+a_{2n})$ converges. Then $sum_{n=1}^{infty} a_n$ converges.



                      Proof: Let $S_n$ denote the $n$th partial sum of $sum a_n.$ The hypotheses imply that $S_{2n}$ converges. Since $S_{2n+1} = S_{2n} + a_{2n+1}$ and $a_{2n+1}to 0,$ $S_{2n+1}$ also converges. This implies $S_n$ converges.



                      In our problem, we have $a_nto 0.$ We also have



                      $$a_{2n-1}+a_{2n} =frac{1}{3(2n-1) -6}-frac{1}{3(2n)+1) -6} = frac{15}{(6n-9)(6n+6)}.$$



                      Thus for large $n,$ $a_{2n-1}+a_{2n}$ is positive and on the order of $1/n^2.$ It follows that $sum (a_{2n-1}+a_{2n})$ converges. By the lemma, $sum a_n$ converges.






                      share|cite|improve this answer


























                        1














                        Lemma: Suppose $a_nto 0$ and $sum_{n=1}^{infty}(a_{2n-1}+a_{2n})$ converges. Then $sum_{n=1}^{infty} a_n$ converges.



                        Proof: Let $S_n$ denote the $n$th partial sum of $sum a_n.$ The hypotheses imply that $S_{2n}$ converges. Since $S_{2n+1} = S_{2n} + a_{2n+1}$ and $a_{2n+1}to 0,$ $S_{2n+1}$ also converges. This implies $S_n$ converges.



                        In our problem, we have $a_nto 0.$ We also have



                        $$a_{2n-1}+a_{2n} =frac{1}{3(2n-1) -6}-frac{1}{3(2n)+1) -6} = frac{15}{(6n-9)(6n+6)}.$$



                        Thus for large $n,$ $a_{2n-1}+a_{2n}$ is positive and on the order of $1/n^2.$ It follows that $sum (a_{2n-1}+a_{2n})$ converges. By the lemma, $sum a_n$ converges.






                        share|cite|improve this answer
























                          1












                          1








                          1






                          Lemma: Suppose $a_nto 0$ and $sum_{n=1}^{infty}(a_{2n-1}+a_{2n})$ converges. Then $sum_{n=1}^{infty} a_n$ converges.



                          Proof: Let $S_n$ denote the $n$th partial sum of $sum a_n.$ The hypotheses imply that $S_{2n}$ converges. Since $S_{2n+1} = S_{2n} + a_{2n+1}$ and $a_{2n+1}to 0,$ $S_{2n+1}$ also converges. This implies $S_n$ converges.



                          In our problem, we have $a_nto 0.$ We also have



                          $$a_{2n-1}+a_{2n} =frac{1}{3(2n-1) -6}-frac{1}{3(2n)+1) -6} = frac{15}{(6n-9)(6n+6)}.$$



                          Thus for large $n,$ $a_{2n-1}+a_{2n}$ is positive and on the order of $1/n^2.$ It follows that $sum (a_{2n-1}+a_{2n})$ converges. By the lemma, $sum a_n$ converges.






                          share|cite|improve this answer












                          Lemma: Suppose $a_nto 0$ and $sum_{n=1}^{infty}(a_{2n-1}+a_{2n})$ converges. Then $sum_{n=1}^{infty} a_n$ converges.



                          Proof: Let $S_n$ denote the $n$th partial sum of $sum a_n.$ The hypotheses imply that $S_{2n}$ converges. Since $S_{2n+1} = S_{2n} + a_{2n+1}$ and $a_{2n+1}to 0,$ $S_{2n+1}$ also converges. This implies $S_n$ converges.



                          In our problem, we have $a_nto 0.$ We also have



                          $$a_{2n-1}+a_{2n} =frac{1}{3(2n-1) -6}-frac{1}{3(2n)+1) -6} = frac{15}{(6n-9)(6n+6)}.$$



                          Thus for large $n,$ $a_{2n-1}+a_{2n}$ is positive and on the order of $1/n^2.$ It follows that $sum (a_{2n-1}+a_{2n})$ converges. By the lemma, $sum a_n$ converges.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Nov 30 at 16:51









                          zhw.

                          71.5k43075




                          71.5k43075






























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