How does this infinite series simplify to an integral?












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How does the infinite series in the link above simplify to that integral?



$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$



I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.










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  • 1




    $begingroup$
    en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
    $endgroup$
    – lab bhattacharjee
    3 hours ago










  • $begingroup$
    It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
    $endgroup$
    – StubbornAtom
    3 hours ago
















2












$begingroup$


How does the infinite series in the link above simplify to that integral?



$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$



I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
    $endgroup$
    – lab bhattacharjee
    3 hours ago










  • $begingroup$
    It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
    $endgroup$
    – StubbornAtom
    3 hours ago














2












2








2


0



$begingroup$


How does the infinite series in the link above simplify to that integral?



$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$



I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.










share|cite|improve this question











$endgroup$




How does the infinite series in the link above simplify to that integral?



$$1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots=int_0^1frac{dx}{1+x^3}$$



I thought of simplifying the series to the sum to infinity of $frac{1}{6n-5} - frac{1}{6n-2}$, but this did not help.







integration sequences-and-series power-series improper-integrals






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edited 3 hours ago









Zacky

7,87511062




7,87511062










asked 3 hours ago









ShreeShree

114




114








  • 1




    $begingroup$
    en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
    $endgroup$
    – lab bhattacharjee
    3 hours ago










  • $begingroup$
    It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
    $endgroup$
    – StubbornAtom
    3 hours ago














  • 1




    $begingroup$
    en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
    $endgroup$
    – lab bhattacharjee
    3 hours ago










  • $begingroup$
    It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
    $endgroup$
    – StubbornAtom
    3 hours ago








1




1




$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago




$begingroup$
en.wikipedia.org/wiki/Binomial_series $$(1+x^3)^{-1}$$
$endgroup$
– lab bhattacharjee
3 hours ago












$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago




$begingroup$
It can be obtained by integrating term by term the infinite series expansion of $(1+x^3)^{-1}$.
$endgroup$
– StubbornAtom
3 hours ago










3 Answers
3






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5












$begingroup$

$$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
$$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$






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$endgroup$













  • $begingroup$
    For those who aren't clear on the first equality, it's a geometric series formula.
    $endgroup$
    – Alexis Olson
    18 mins ago



















2












$begingroup$

for $x$ real, $ngeq 0$ integer
begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
end{align}



For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}



Therefore,
begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
&=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
end{align}



For $xin[0;1],ngeq 0$, integer,
begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
and,
begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
Therefore,
begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
Therefore,
begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}






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$endgroup$





















    1












    $begingroup$

    If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}






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      3 Answers
      3






      active

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      3 Answers
      3






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      active

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      5












      $begingroup$

      $$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
      $$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$






      share|cite|improve this answer









      $endgroup$













      • $begingroup$
        For those who aren't clear on the first equality, it's a geometric series formula.
        $endgroup$
        – Alexis Olson
        18 mins ago
















      5












      $begingroup$

      $$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
      $$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$






      share|cite|improve this answer









      $endgroup$













      • $begingroup$
        For those who aren't clear on the first equality, it's a geometric series formula.
        $endgroup$
        – Alexis Olson
        18 mins ago














      5












      5








      5





      $begingroup$

      $$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
      $$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$






      share|cite|improve this answer









      $endgroup$



      $$int_{0}^{1}{frac{1}{1-(-x)^3}=int_{0}^{1}{sum_{n=0}^{infty}{(-x)}^{3n}}}dx=sum_{n=0}^{infty}{(-1)^{3n}int_{0}^{1}{x^{3n}}dx}$$
      $$=sum_{n=0}^{infty}{frac{(-1)^{3n}}{3n+1}}= 1-frac{1}{4}+frac{1}{7}-frac{1}{10}+cdots $$







      share|cite|improve this answer












      share|cite|improve this answer



      share|cite|improve this answer










      answered 3 hours ago









      HAMIDINE SOUMAREHAMIDINE SOUMARE

      2,184212




      2,184212












      • $begingroup$
        For those who aren't clear on the first equality, it's a geometric series formula.
        $endgroup$
        – Alexis Olson
        18 mins ago


















      • $begingroup$
        For those who aren't clear on the first equality, it's a geometric series formula.
        $endgroup$
        – Alexis Olson
        18 mins ago
















      $begingroup$
      For those who aren't clear on the first equality, it's a geometric series formula.
      $endgroup$
      – Alexis Olson
      18 mins ago




      $begingroup$
      For those who aren't clear on the first equality, it's a geometric series formula.
      $endgroup$
      – Alexis Olson
      18 mins ago











      2












      $begingroup$

      for $x$ real, $ngeq 0$ integer
      begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
      &=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
      end{align}



      For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}



      Therefore,
      begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
      &=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
      &=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
      end{align}



      For $xin[0;1],ngeq 0$, integer,
      begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
      and,
      begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
      Therefore,
      begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
      begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
      Therefore,
      begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}






      share|cite|improve this answer









      $endgroup$


















        2












        $begingroup$

        for $x$ real, $ngeq 0$ integer
        begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
        &=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
        end{align}



        For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}



        Therefore,
        begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
        &=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
        &=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
        end{align}



        For $xin[0;1],ngeq 0$, integer,
        begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
        and,
        begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
        Therefore,
        begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
        begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
        Therefore,
        begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}






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        $endgroup$
















          2












          2








          2





          $begingroup$

          for $x$ real, $ngeq 0$ integer
          begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
          &=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
          end{align}



          For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}



          Therefore,
          begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
          &=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
          &=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
          end{align}



          For $xin[0;1],ngeq 0$, integer,
          begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
          and,
          begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
          Therefore,
          begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
          begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
          Therefore,
          begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}






          share|cite|improve this answer









          $endgroup$



          for $x$ real, $ngeq 0$ integer
          begin{align}frac{1}{1+x^3}&=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1-(-x^3)}\
          &=frac{1-(-x^3)^{n+1}}{1-(-x^3)}+frac{(-x^3)^{n+1}}{1+x^3}\
          end{align}



          For $xneq 1$, $ngeq 0$ integer, begin{align}sum_{k=0}^n x^k=frac{1-x^{n+1}}{1-x}end{align}



          Therefore,
          begin{align}int_0^1 frac{1}{1+x^3},dx&=int_0^1 left(sum_{k=0}^n (-x^3)^kright),dx+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
          &=sum_{k=0}^n left(int_0^1 (-x^3)^k,dxright)+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
          &=sum_{k=0}^n frac{(-1)^k}{3k+1}+int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dx\
          end{align}



          For $xin[0;1],ngeq 0$, integer,
          begin{align}frac{x^{3(n+1)}}{1+x^3}leq x^{3(n+1)}end{align}
          and,
          begin{align}int_0^1 x^{3(n+1)},dx=frac{1}{3n+4}end{align}
          Therefore,
          begin{align}left|int_0^1 frac{(-x^3)^{n+1}}{1+x^3},dxright|leq frac{1}{3n+4}end{align}
          begin{align}left|int_0^1 frac{1}{1+x^3},dx-sum_{k=0}^n frac{(-1)^k}{3k+1}right|leq frac{1}{3n+4}end{align}
          Therefore,
          begin{align}boxed{int_0^1 frac{1}{1+x^3},dx=sum_{k=0}^infty frac{(-1)^k}{3k+1}}end{align}







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          share|cite|improve this answer










          answered 3 hours ago









          FDPFDP

          6,12211929




          6,12211929























              1












              $begingroup$

              If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}






              share|cite|improve this answer









              $endgroup$


















                1












                $begingroup$

                If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}






                share|cite|improve this answer









                $endgroup$
















                  1












                  1








                  1





                  $begingroup$

                  If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}






                  share|cite|improve this answer









                  $endgroup$



                  If $lvert xrvert<1$, let$$f(x)=sum_{n=0}^inftyfrac{x^{3n+1}}{3n+1}.$$Then $$f'(x)=sum_{n=0}^infty x^{3n}=frac1{1-x^3}.$$Thereforebegin{align}1-frac14+frac17-frac1{10}+cdots&=lim_{xto1}f(x)\&=int_0^1f'(x),mathrm dx\&=int_0^1frac1{1-x^3},mathrm dx.end{align}







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered 3 hours ago









                  José Carlos SantosJosé Carlos Santos

                  174k23133243




                  174k23133243






























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