Series convergence without sigma notation











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Consider the following series:



$$frac{1}{1} + frac{10}{2} + frac{100}{3} - frac{37}{4} - frac{37}{5} - frac{37}{6} + frac{1}{7} + frac{10}{8} + frac{100}{9} - frac{37}{10} - frac{37}{11} - frac{37}{12} + dots$$



This series seems to converge but I am not able to prove it. It seems impossible to write thing whole thing into summation notation. I tried to group by modulo 6, but then for example sum of $frac{1}{6n+1}$ diverges already so that doesn't seem right. Also tried to change the 37s into powers of 10 to maybe use the comparison test but also failed.
Any suggestions?










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    Consider the following series:



    $$frac{1}{1} + frac{10}{2} + frac{100}{3} - frac{37}{4} - frac{37}{5} - frac{37}{6} + frac{1}{7} + frac{10}{8} + frac{100}{9} - frac{37}{10} - frac{37}{11} - frac{37}{12} + dots$$



    This series seems to converge but I am not able to prove it. It seems impossible to write thing whole thing into summation notation. I tried to group by modulo 6, but then for example sum of $frac{1}{6n+1}$ diverges already so that doesn't seem right. Also tried to change the 37s into powers of 10 to maybe use the comparison test but also failed.
    Any suggestions?










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      up vote
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      down vote

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      up vote
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      down vote

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      Consider the following series:



      $$frac{1}{1} + frac{10}{2} + frac{100}{3} - frac{37}{4} - frac{37}{5} - frac{37}{6} + frac{1}{7} + frac{10}{8} + frac{100}{9} - frac{37}{10} - frac{37}{11} - frac{37}{12} + dots$$



      This series seems to converge but I am not able to prove it. It seems impossible to write thing whole thing into summation notation. I tried to group by modulo 6, but then for example sum of $frac{1}{6n+1}$ diverges already so that doesn't seem right. Also tried to change the 37s into powers of 10 to maybe use the comparison test but also failed.
      Any suggestions?










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      Consider the following series:



      $$frac{1}{1} + frac{10}{2} + frac{100}{3} - frac{37}{4} - frac{37}{5} - frac{37}{6} + frac{1}{7} + frac{10}{8} + frac{100}{9} - frac{37}{10} - frac{37}{11} - frac{37}{12} + dots$$



      This series seems to converge but I am not able to prove it. It seems impossible to write thing whole thing into summation notation. I tried to group by modulo 6, but then for example sum of $frac{1}{6n+1}$ diverges already so that doesn't seem right. Also tried to change the 37s into powers of 10 to maybe use the comparison test but also failed.
      Any suggestions?







      sequences-and-series






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      asked 3 hours ago









      Markus Punnar

      1228




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          First, notice that $$F_n=frac{1}{6n+1}+frac{10}{6n+2}+frac{100}{6n+3}-frac{37}{6n+4}-frac{37}{6n+5}-frac{37}{6n+6} geq frac{111}{6n+3}-frac{111}{6n+4} > 0.$$
          Then, notice that $$F_n leq frac{111}{6n+1}-frac{111}{6n+6}=frac{555}{(6n+1)(6n+6)}.$$



          Thus the sum of $F_n$ converges. How can you infer the final result from this?






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          • Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
            – Robert Israel
            3 hours ago


















          up vote
          3
          down vote













          I presume the pattern $(1, 10, 100, -37, -37, -37)$ continues forever, so this is
          $$ sum_{n=1}^infty frac{a_n}{n}$$
          where $$ a_n = cases{1 & if $n equiv 1 mod 6$cr
          10 & if $n equiv 2 mod 6$cr
          100 & if $n equiv 3 mod 6$cr
          -37 & if $n equiv 4,5$ or $0 mod 6$cr}$$

          Note that $1+10+100 - 3times 37 = 0$. Then the $6m$
          -th partial sum
          $$ eqalign{S_m &= sum_{n=1}^{6m} frac{a_n}{n}cr
          & = sum_{j=0}^m frac{1}{6j+1} +
          10 sum_{j=0}^m frac{1}{6j+2} + 100 sum_{j=0}^m frac{1}{6j+3}
          - 37 sum_{k=4}^6 sum_{j=0}^m frac{1}{6j+k}cr
          &= frac{1}{6} left(Psi(m+1+1/6) - Psi(1/6) + 10 (Psi(m+1+2/6) - Psi(2/6)) + 100 (Psi(m+1+3/6) - Psi(3/6)) - 37 (Psi(m+1+4/6)+Psi(m+1+5/6)+Psi(m+1+6/6)-Psi(4/6)-Psi(5/6)-Psi(6/6))right)cr}$$



          and since $Psi(x) = ln(x) + O(1/x)$ as $x to infty$, the sum converges.
          In fact, the limit is
          $$ frac{64}{3} ln(2) - frac{63}{4} ln(3) +frac{161}{36} pi sqrt{3}$$






          share|cite|improve this answer





















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            2 Answers
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            up vote
            6
            down vote













            First, notice that $$F_n=frac{1}{6n+1}+frac{10}{6n+2}+frac{100}{6n+3}-frac{37}{6n+4}-frac{37}{6n+5}-frac{37}{6n+6} geq frac{111}{6n+3}-frac{111}{6n+4} > 0.$$
            Then, notice that $$F_n leq frac{111}{6n+1}-frac{111}{6n+6}=frac{555}{(6n+1)(6n+6)}.$$



            Thus the sum of $F_n$ converges. How can you infer the final result from this?






            share|cite|improve this answer








            New contributor




            Mindlack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.


















            • Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
              – Robert Israel
              3 hours ago















            up vote
            6
            down vote













            First, notice that $$F_n=frac{1}{6n+1}+frac{10}{6n+2}+frac{100}{6n+3}-frac{37}{6n+4}-frac{37}{6n+5}-frac{37}{6n+6} geq frac{111}{6n+3}-frac{111}{6n+4} > 0.$$
            Then, notice that $$F_n leq frac{111}{6n+1}-frac{111}{6n+6}=frac{555}{(6n+1)(6n+6)}.$$



            Thus the sum of $F_n$ converges. How can you infer the final result from this?






            share|cite|improve this answer








            New contributor




            Mindlack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.


















            • Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
              – Robert Israel
              3 hours ago













            up vote
            6
            down vote










            up vote
            6
            down vote









            First, notice that $$F_n=frac{1}{6n+1}+frac{10}{6n+2}+frac{100}{6n+3}-frac{37}{6n+4}-frac{37}{6n+5}-frac{37}{6n+6} geq frac{111}{6n+3}-frac{111}{6n+4} > 0.$$
            Then, notice that $$F_n leq frac{111}{6n+1}-frac{111}{6n+6}=frac{555}{(6n+1)(6n+6)}.$$



            Thus the sum of $F_n$ converges. How can you infer the final result from this?






            share|cite|improve this answer








            New contributor




            Mindlack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.









            First, notice that $$F_n=frac{1}{6n+1}+frac{10}{6n+2}+frac{100}{6n+3}-frac{37}{6n+4}-frac{37}{6n+5}-frac{37}{6n+6} geq frac{111}{6n+3}-frac{111}{6n+4} > 0.$$
            Then, notice that $$F_n leq frac{111}{6n+1}-frac{111}{6n+6}=frac{555}{(6n+1)(6n+6)}.$$



            Thus the sum of $F_n$ converges. How can you infer the final result from this?







            share|cite|improve this answer








            New contributor




            Mindlack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.









            share|cite|improve this answer



            share|cite|improve this answer






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            answered 3 hours ago









            Mindlack

            3794




            3794




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            New contributor





            Mindlack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.






            Mindlack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
            Check out our Code of Conduct.












            • Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
              – Robert Israel
              3 hours ago


















            • Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
              – Robert Israel
              3 hours ago
















            Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
            – Robert Israel
            3 hours ago




            Alternatively, you can write $$F_n = {frac {303264,{n}^{4}+649296,{n}^{3}+471096,{n}^{2}+132516,n+ 11892}{ left( 6,n+1 right) left( 6,n+2 right) left( 6,n+3 right) left( 6,n+4 right) left( 6,n+5 right) left( 6,n+6 right) }} $$ and note that the numerator has degree $4$ and denominator degree $6$, so $F_n = O(1/n^2)$.
            – Robert Israel
            3 hours ago










            up vote
            3
            down vote













            I presume the pattern $(1, 10, 100, -37, -37, -37)$ continues forever, so this is
            $$ sum_{n=1}^infty frac{a_n}{n}$$
            where $$ a_n = cases{1 & if $n equiv 1 mod 6$cr
            10 & if $n equiv 2 mod 6$cr
            100 & if $n equiv 3 mod 6$cr
            -37 & if $n equiv 4,5$ or $0 mod 6$cr}$$

            Note that $1+10+100 - 3times 37 = 0$. Then the $6m$
            -th partial sum
            $$ eqalign{S_m &= sum_{n=1}^{6m} frac{a_n}{n}cr
            & = sum_{j=0}^m frac{1}{6j+1} +
            10 sum_{j=0}^m frac{1}{6j+2} + 100 sum_{j=0}^m frac{1}{6j+3}
            - 37 sum_{k=4}^6 sum_{j=0}^m frac{1}{6j+k}cr
            &= frac{1}{6} left(Psi(m+1+1/6) - Psi(1/6) + 10 (Psi(m+1+2/6) - Psi(2/6)) + 100 (Psi(m+1+3/6) - Psi(3/6)) - 37 (Psi(m+1+4/6)+Psi(m+1+5/6)+Psi(m+1+6/6)-Psi(4/6)-Psi(5/6)-Psi(6/6))right)cr}$$



            and since $Psi(x) = ln(x) + O(1/x)$ as $x to infty$, the sum converges.
            In fact, the limit is
            $$ frac{64}{3} ln(2) - frac{63}{4} ln(3) +frac{161}{36} pi sqrt{3}$$






            share|cite|improve this answer

























              up vote
              3
              down vote













              I presume the pattern $(1, 10, 100, -37, -37, -37)$ continues forever, so this is
              $$ sum_{n=1}^infty frac{a_n}{n}$$
              where $$ a_n = cases{1 & if $n equiv 1 mod 6$cr
              10 & if $n equiv 2 mod 6$cr
              100 & if $n equiv 3 mod 6$cr
              -37 & if $n equiv 4,5$ or $0 mod 6$cr}$$

              Note that $1+10+100 - 3times 37 = 0$. Then the $6m$
              -th partial sum
              $$ eqalign{S_m &= sum_{n=1}^{6m} frac{a_n}{n}cr
              & = sum_{j=0}^m frac{1}{6j+1} +
              10 sum_{j=0}^m frac{1}{6j+2} + 100 sum_{j=0}^m frac{1}{6j+3}
              - 37 sum_{k=4}^6 sum_{j=0}^m frac{1}{6j+k}cr
              &= frac{1}{6} left(Psi(m+1+1/6) - Psi(1/6) + 10 (Psi(m+1+2/6) - Psi(2/6)) + 100 (Psi(m+1+3/6) - Psi(3/6)) - 37 (Psi(m+1+4/6)+Psi(m+1+5/6)+Psi(m+1+6/6)-Psi(4/6)-Psi(5/6)-Psi(6/6))right)cr}$$



              and since $Psi(x) = ln(x) + O(1/x)$ as $x to infty$, the sum converges.
              In fact, the limit is
              $$ frac{64}{3} ln(2) - frac{63}{4} ln(3) +frac{161}{36} pi sqrt{3}$$






              share|cite|improve this answer























                up vote
                3
                down vote










                up vote
                3
                down vote









                I presume the pattern $(1, 10, 100, -37, -37, -37)$ continues forever, so this is
                $$ sum_{n=1}^infty frac{a_n}{n}$$
                where $$ a_n = cases{1 & if $n equiv 1 mod 6$cr
                10 & if $n equiv 2 mod 6$cr
                100 & if $n equiv 3 mod 6$cr
                -37 & if $n equiv 4,5$ or $0 mod 6$cr}$$

                Note that $1+10+100 - 3times 37 = 0$. Then the $6m$
                -th partial sum
                $$ eqalign{S_m &= sum_{n=1}^{6m} frac{a_n}{n}cr
                & = sum_{j=0}^m frac{1}{6j+1} +
                10 sum_{j=0}^m frac{1}{6j+2} + 100 sum_{j=0}^m frac{1}{6j+3}
                - 37 sum_{k=4}^6 sum_{j=0}^m frac{1}{6j+k}cr
                &= frac{1}{6} left(Psi(m+1+1/6) - Psi(1/6) + 10 (Psi(m+1+2/6) - Psi(2/6)) + 100 (Psi(m+1+3/6) - Psi(3/6)) - 37 (Psi(m+1+4/6)+Psi(m+1+5/6)+Psi(m+1+6/6)-Psi(4/6)-Psi(5/6)-Psi(6/6))right)cr}$$



                and since $Psi(x) = ln(x) + O(1/x)$ as $x to infty$, the sum converges.
                In fact, the limit is
                $$ frac{64}{3} ln(2) - frac{63}{4} ln(3) +frac{161}{36} pi sqrt{3}$$






                share|cite|improve this answer












                I presume the pattern $(1, 10, 100, -37, -37, -37)$ continues forever, so this is
                $$ sum_{n=1}^infty frac{a_n}{n}$$
                where $$ a_n = cases{1 & if $n equiv 1 mod 6$cr
                10 & if $n equiv 2 mod 6$cr
                100 & if $n equiv 3 mod 6$cr
                -37 & if $n equiv 4,5$ or $0 mod 6$cr}$$

                Note that $1+10+100 - 3times 37 = 0$. Then the $6m$
                -th partial sum
                $$ eqalign{S_m &= sum_{n=1}^{6m} frac{a_n}{n}cr
                & = sum_{j=0}^m frac{1}{6j+1} +
                10 sum_{j=0}^m frac{1}{6j+2} + 100 sum_{j=0}^m frac{1}{6j+3}
                - 37 sum_{k=4}^6 sum_{j=0}^m frac{1}{6j+k}cr
                &= frac{1}{6} left(Psi(m+1+1/6) - Psi(1/6) + 10 (Psi(m+1+2/6) - Psi(2/6)) + 100 (Psi(m+1+3/6) - Psi(3/6)) - 37 (Psi(m+1+4/6)+Psi(m+1+5/6)+Psi(m+1+6/6)-Psi(4/6)-Psi(5/6)-Psi(6/6))right)cr}$$



                and since $Psi(x) = ln(x) + O(1/x)$ as $x to infty$, the sum converges.
                In fact, the limit is
                $$ frac{64}{3} ln(2) - frac{63}{4} ln(3) +frac{161}{36} pi sqrt{3}$$







                share|cite|improve this answer












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                answered 3 hours ago









                Robert Israel

                316k23206457




                316k23206457






























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