Proof for this identity: $sumlimits_k=0^infty(zeta(2k+3)-1) = frac14$

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$$
sum_k=0^infty (zeta(2k+3)
-1) = frac14
$$
I found this nice identity and want to prove it, but I don't know how.
My first thought was that I can create a geometric series with the same limit like:
$$sum_k=0^infty frac18 cdot frac12^k = frac14$$
But I have no idea, how to write the first one in a way, that these are the same. They are obviously only equal, if we sum infinitely many terms together.
Do you have ideas, how to proof this identity or do you know, where the proof can be found?
Thank you for your answers!










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  • 2




    Where did you found it? All terms are greater than 1!
    – gammatester
    Mar 30 at 13:41







  • 2




    You have a typo, should be $sum_k=0^infty (zeta(2k+3) - colorred1) = frac14$.
    – achille hui
    Mar 30 at 13:50










  • Actually , this sum seems to be $k+frac54$ with a very small error already for $k=10$
    – Peter
    Mar 30 at 13:51














up vote
2
down vote

favorite
1












$$
sum_k=0^infty (zeta(2k+3)
-1) = frac14
$$
I found this nice identity and want to prove it, but I don't know how.
My first thought was that I can create a geometric series with the same limit like:
$$sum_k=0^infty frac18 cdot frac12^k = frac14$$
But I have no idea, how to write the first one in a way, that these are the same. They are obviously only equal, if we sum infinitely many terms together.
Do you have ideas, how to proof this identity or do you know, where the proof can be found?
Thank you for your answers!










share|cite|improve this question



















  • 2




    Where did you found it? All terms are greater than 1!
    – gammatester
    Mar 30 at 13:41







  • 2




    You have a typo, should be $sum_k=0^infty (zeta(2k+3) - colorred1) = frac14$.
    – achille hui
    Mar 30 at 13:50










  • Actually , this sum seems to be $k+frac54$ with a very small error already for $k=10$
    – Peter
    Mar 30 at 13:51












up vote
2
down vote

favorite
1









up vote
2
down vote

favorite
1






1





$$
sum_k=0^infty (zeta(2k+3)
-1) = frac14
$$
I found this nice identity and want to prove it, but I don't know how.
My first thought was that I can create a geometric series with the same limit like:
$$sum_k=0^infty frac18 cdot frac12^k = frac14$$
But I have no idea, how to write the first one in a way, that these are the same. They are obviously only equal, if we sum infinitely many terms together.
Do you have ideas, how to proof this identity or do you know, where the proof can be found?
Thank you for your answers!










share|cite|improve this question















$$
sum_k=0^infty (zeta(2k+3)
-1) = frac14
$$
I found this nice identity and want to prove it, but I don't know how.
My first thought was that I can create a geometric series with the same limit like:
$$sum_k=0^infty frac18 cdot frac12^k = frac14$$
But I have no idea, how to write the first one in a way, that these are the same. They are obviously only equal, if we sum infinitely many terms together.
Do you have ideas, how to proof this identity or do you know, where the proof can be found?
Thank you for your answers!







sequences-and-series riemann-zeta geometric-series






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edited Mar 30 at 16:32









Marco Cantarini

28.8k23272




28.8k23272










asked Mar 30 at 13:37









Mister Set

617310




617310







  • 2




    Where did you found it? All terms are greater than 1!
    – gammatester
    Mar 30 at 13:41







  • 2




    You have a typo, should be $sum_k=0^infty (zeta(2k+3) - colorred1) = frac14$.
    – achille hui
    Mar 30 at 13:50










  • Actually , this sum seems to be $k+frac54$ with a very small error already for $k=10$
    – Peter
    Mar 30 at 13:51












  • 2




    Where did you found it? All terms are greater than 1!
    – gammatester
    Mar 30 at 13:41







  • 2




    You have a typo, should be $sum_k=0^infty (zeta(2k+3) - colorred1) = frac14$.
    – achille hui
    Mar 30 at 13:50










  • Actually , this sum seems to be $k+frac54$ with a very small error already for $k=10$
    – Peter
    Mar 30 at 13:51







2




2




Where did you found it? All terms are greater than 1!
– gammatester
Mar 30 at 13:41





Where did you found it? All terms are greater than 1!
– gammatester
Mar 30 at 13:41





2




2




You have a typo, should be $sum_k=0^infty (zeta(2k+3) - colorred1) = frac14$.
– achille hui
Mar 30 at 13:50




You have a typo, should be $sum_k=0^infty (zeta(2k+3) - colorred1) = frac14$.
– achille hui
Mar 30 at 13:50












Actually , this sum seems to be $k+frac54$ with a very small error already for $k=10$
– Peter
Mar 30 at 13:51




Actually , this sum seems to be $k+frac54$ with a very small error already for $k=10$
– Peter
Mar 30 at 13:51










3 Answers
3






active

oldest

votes

















up vote
5
down vote













There is a typo in the question.
The sum $sumlimits_k=0^infty zeta(2k+3)$ diverges because $limlimits_ktoinfty zeta(2k+3) = 1$.

The correct statement should be



$$sum_k=0^infty (zeta(2k+3) - 1) = frac14$$



The proof is relatively simple,



$$beginalign sum_k=0^infty (zeta(2k+3)-1)
= &sum_k=0^infty sum_n=2^infty frac1n^2k+3
stackrelcolorblue[1]= sum_n=2^infty sum_k=0^infty frac1n^2k+3
= sum_n=2^infty frac1n^3sum_k=0^infty (n^-2)^k\
= &sum_n=2^infty frac1n^3frac11 - n^-2
= sum_n=2^infty frac1(n-1)n(n+1)
= sum_n=2^infty frac12left(frac1(n-1)n - frac1n(n+1)right)\
stackrelcolorblue[2]= & frac12left(frac1(2-1)2right) = frac14endalign
$$



Notes



  • $colorblue[1]$ - all terms are non-negative, it is legal to exchange order of summation.


  • $colorblue[2]$ - the sum is a telescoping one.






share|cite|improve this answer




















  • Wow, so easy is that (if one has the "know-how") (+1)
    – Peter
    Mar 30 at 14:16










  • The concept of a telescoping sum is very fascinating. I was not aware of this type of series
    – Mister Set
    Mar 30 at 17:15

















up vote
3
down vote













By the integral representation of $zetaleft(sright)$ we have $$zetaleft(2k+3right)-1=frac1left(2k+2right)!int_0^inftyfracu^2k+2e^uleft(e^u-1right)du$$ then $$sum_kgeq0left(zetaleft(2k+3right)-1right)=int_0^inftyfrac1e^uleft(e^u-1right)sum_kgeq0fracu^2k+2left(2k+2right)!du$$ $$=int_0^inftyfraccoshleft(uright)-1e^uleft(e^u-1right)du=frac12int_0^inftye^-u-e^-2udu=color redfrac14.$$






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    $newcommandbbx[1],bbox[15px,border:1px groove navy]displaystyle#1,
    newcommandbraces[1]leftlbrace,#1,rightrbrace
    newcommandbracks[1]leftlbrack,#1,rightrbrack
    newcommandddmathrmd
    newcommandds[1]displaystyle#1
    newcommandexpo[1],mathrme^#1,
    newcommandicmathrmi
    newcommandmc[1]mathcal#1
    newcommandmrm[1]mathrm#1
    newcommandpars[1]left(,#1,right)
    newcommandpartiald[3]fracpartial^#1 #2partial #3^#1
    newcommandroot[2],sqrt[#1],#2,,
    newcommandtotald[3]fracmathrmd^#1 #2mathrmd #3^#1
    newcommandverts[1]leftvert,#1,rightvert$
    beginalign
    &bbox[10px,#ffd]dssum_k = 0^inftybrackszetapars2k + 3 - 1 =
    sum_k = 1^inftybrackszetapars2k + 1 - 1
    \[5mm] = &
    lim_z to 1bracks1 over 2z - 1 over 2,pi,cotparspi z -
    1 over 1 - z^2 + 1 - gamma - Psipars1 + z = bbx1 over 4
    endalign




    where $dsPsi$ is the Digamma Function. See $dsmathbfcolorblack6.3.15$ in A & S Table.


    Note that
    $dsPsipars2 = Psipars1 + 1 = -gamma + 1$.







    share|cite|improve this answer






















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      3 Answers
      3






      active

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      3 Answers
      3






      active

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      active

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      active

      oldest

      votes








      up vote
      5
      down vote













      There is a typo in the question.
      The sum $sumlimits_k=0^infty zeta(2k+3)$ diverges because $limlimits_ktoinfty zeta(2k+3) = 1$.

      The correct statement should be



      $$sum_k=0^infty (zeta(2k+3) - 1) = frac14$$



      The proof is relatively simple,



      $$beginalign sum_k=0^infty (zeta(2k+3)-1)
      = &sum_k=0^infty sum_n=2^infty frac1n^2k+3
      stackrelcolorblue[1]= sum_n=2^infty sum_k=0^infty frac1n^2k+3
      = sum_n=2^infty frac1n^3sum_k=0^infty (n^-2)^k\
      = &sum_n=2^infty frac1n^3frac11 - n^-2
      = sum_n=2^infty frac1(n-1)n(n+1)
      = sum_n=2^infty frac12left(frac1(n-1)n - frac1n(n+1)right)\
      stackrelcolorblue[2]= & frac12left(frac1(2-1)2right) = frac14endalign
      $$



      Notes



      • $colorblue[1]$ - all terms are non-negative, it is legal to exchange order of summation.


      • $colorblue[2]$ - the sum is a telescoping one.






      share|cite|improve this answer




















      • Wow, so easy is that (if one has the "know-how") (+1)
        – Peter
        Mar 30 at 14:16










      • The concept of a telescoping sum is very fascinating. I was not aware of this type of series
        – Mister Set
        Mar 30 at 17:15














      up vote
      5
      down vote













      There is a typo in the question.
      The sum $sumlimits_k=0^infty zeta(2k+3)$ diverges because $limlimits_ktoinfty zeta(2k+3) = 1$.

      The correct statement should be



      $$sum_k=0^infty (zeta(2k+3) - 1) = frac14$$



      The proof is relatively simple,



      $$beginalign sum_k=0^infty (zeta(2k+3)-1)
      = &sum_k=0^infty sum_n=2^infty frac1n^2k+3
      stackrelcolorblue[1]= sum_n=2^infty sum_k=0^infty frac1n^2k+3
      = sum_n=2^infty frac1n^3sum_k=0^infty (n^-2)^k\
      = &sum_n=2^infty frac1n^3frac11 - n^-2
      = sum_n=2^infty frac1(n-1)n(n+1)
      = sum_n=2^infty frac12left(frac1(n-1)n - frac1n(n+1)right)\
      stackrelcolorblue[2]= & frac12left(frac1(2-1)2right) = frac14endalign
      $$



      Notes



      • $colorblue[1]$ - all terms are non-negative, it is legal to exchange order of summation.


      • $colorblue[2]$ - the sum is a telescoping one.






      share|cite|improve this answer




















      • Wow, so easy is that (if one has the "know-how") (+1)
        – Peter
        Mar 30 at 14:16










      • The concept of a telescoping sum is very fascinating. I was not aware of this type of series
        – Mister Set
        Mar 30 at 17:15












      up vote
      5
      down vote










      up vote
      5
      down vote









      There is a typo in the question.
      The sum $sumlimits_k=0^infty zeta(2k+3)$ diverges because $limlimits_ktoinfty zeta(2k+3) = 1$.

      The correct statement should be



      $$sum_k=0^infty (zeta(2k+3) - 1) = frac14$$



      The proof is relatively simple,



      $$beginalign sum_k=0^infty (zeta(2k+3)-1)
      = &sum_k=0^infty sum_n=2^infty frac1n^2k+3
      stackrelcolorblue[1]= sum_n=2^infty sum_k=0^infty frac1n^2k+3
      = sum_n=2^infty frac1n^3sum_k=0^infty (n^-2)^k\
      = &sum_n=2^infty frac1n^3frac11 - n^-2
      = sum_n=2^infty frac1(n-1)n(n+1)
      = sum_n=2^infty frac12left(frac1(n-1)n - frac1n(n+1)right)\
      stackrelcolorblue[2]= & frac12left(frac1(2-1)2right) = frac14endalign
      $$



      Notes



      • $colorblue[1]$ - all terms are non-negative, it is legal to exchange order of summation.


      • $colorblue[2]$ - the sum is a telescoping one.






      share|cite|improve this answer












      There is a typo in the question.
      The sum $sumlimits_k=0^infty zeta(2k+3)$ diverges because $limlimits_ktoinfty zeta(2k+3) = 1$.

      The correct statement should be



      $$sum_k=0^infty (zeta(2k+3) - 1) = frac14$$



      The proof is relatively simple,



      $$beginalign sum_k=0^infty (zeta(2k+3)-1)
      = &sum_k=0^infty sum_n=2^infty frac1n^2k+3
      stackrelcolorblue[1]= sum_n=2^infty sum_k=0^infty frac1n^2k+3
      = sum_n=2^infty frac1n^3sum_k=0^infty (n^-2)^k\
      = &sum_n=2^infty frac1n^3frac11 - n^-2
      = sum_n=2^infty frac1(n-1)n(n+1)
      = sum_n=2^infty frac12left(frac1(n-1)n - frac1n(n+1)right)\
      stackrelcolorblue[2]= & frac12left(frac1(2-1)2right) = frac14endalign
      $$



      Notes



      • $colorblue[1]$ - all terms are non-negative, it is legal to exchange order of summation.


      • $colorblue[2]$ - the sum is a telescoping one.







      share|cite|improve this answer












      share|cite|improve this answer



      share|cite|improve this answer










      answered Mar 30 at 14:12









      achille hui

      92.8k5127249




      92.8k5127249











      • Wow, so easy is that (if one has the "know-how") (+1)
        – Peter
        Mar 30 at 14:16










      • The concept of a telescoping sum is very fascinating. I was not aware of this type of series
        – Mister Set
        Mar 30 at 17:15
















      • Wow, so easy is that (if one has the "know-how") (+1)
        – Peter
        Mar 30 at 14:16










      • The concept of a telescoping sum is very fascinating. I was not aware of this type of series
        – Mister Set
        Mar 30 at 17:15















      Wow, so easy is that (if one has the "know-how") (+1)
      – Peter
      Mar 30 at 14:16




      Wow, so easy is that (if one has the "know-how") (+1)
      – Peter
      Mar 30 at 14:16












      The concept of a telescoping sum is very fascinating. I was not aware of this type of series
      – Mister Set
      Mar 30 at 17:15




      The concept of a telescoping sum is very fascinating. I was not aware of this type of series
      – Mister Set
      Mar 30 at 17:15










      up vote
      3
      down vote













      By the integral representation of $zetaleft(sright)$ we have $$zetaleft(2k+3right)-1=frac1left(2k+2right)!int_0^inftyfracu^2k+2e^uleft(e^u-1right)du$$ then $$sum_kgeq0left(zetaleft(2k+3right)-1right)=int_0^inftyfrac1e^uleft(e^u-1right)sum_kgeq0fracu^2k+2left(2k+2right)!du$$ $$=int_0^inftyfraccoshleft(uright)-1e^uleft(e^u-1right)du=frac12int_0^inftye^-u-e^-2udu=color redfrac14.$$






      share|cite|improve this answer
























        up vote
        3
        down vote













        By the integral representation of $zetaleft(sright)$ we have $$zetaleft(2k+3right)-1=frac1left(2k+2right)!int_0^inftyfracu^2k+2e^uleft(e^u-1right)du$$ then $$sum_kgeq0left(zetaleft(2k+3right)-1right)=int_0^inftyfrac1e^uleft(e^u-1right)sum_kgeq0fracu^2k+2left(2k+2right)!du$$ $$=int_0^inftyfraccoshleft(uright)-1e^uleft(e^u-1right)du=frac12int_0^inftye^-u-e^-2udu=color redfrac14.$$






        share|cite|improve this answer






















          up vote
          3
          down vote










          up vote
          3
          down vote









          By the integral representation of $zetaleft(sright)$ we have $$zetaleft(2k+3right)-1=frac1left(2k+2right)!int_0^inftyfracu^2k+2e^uleft(e^u-1right)du$$ then $$sum_kgeq0left(zetaleft(2k+3right)-1right)=int_0^inftyfrac1e^uleft(e^u-1right)sum_kgeq0fracu^2k+2left(2k+2right)!du$$ $$=int_0^inftyfraccoshleft(uright)-1e^uleft(e^u-1right)du=frac12int_0^inftye^-u-e^-2udu=color redfrac14.$$






          share|cite|improve this answer












          By the integral representation of $zetaleft(sright)$ we have $$zetaleft(2k+3right)-1=frac1left(2k+2right)!int_0^inftyfracu^2k+2e^uleft(e^u-1right)du$$ then $$sum_kgeq0left(zetaleft(2k+3right)-1right)=int_0^inftyfrac1e^uleft(e^u-1right)sum_kgeq0fracu^2k+2left(2k+2right)!du$$ $$=int_0^inftyfraccoshleft(uright)-1e^uleft(e^u-1right)du=frac12int_0^inftye^-u-e^-2udu=color redfrac14.$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Mar 30 at 15:51









          Marco Cantarini

          28.8k23272




          28.8k23272




















              up vote
              0
              down vote













              $newcommandbbx[1],bbox[15px,border:1px groove navy]displaystyle#1,
              newcommandbraces[1]leftlbrace,#1,rightrbrace
              newcommandbracks[1]leftlbrack,#1,rightrbrack
              newcommandddmathrmd
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              newcommandexpo[1],mathrme^#1,
              newcommandicmathrmi
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              newcommandpartiald[3]fracpartial^#1 #2partial #3^#1
              newcommandroot[2],sqrt[#1],#2,,
              newcommandtotald[3]fracmathrmd^#1 #2mathrmd #3^#1
              newcommandverts[1]leftvert,#1,rightvert$
              beginalign
              &bbox[10px,#ffd]dssum_k = 0^inftybrackszetapars2k + 3 - 1 =
              sum_k = 1^inftybrackszetapars2k + 1 - 1
              \[5mm] = &
              lim_z to 1bracks1 over 2z - 1 over 2,pi,cotparspi z -
              1 over 1 - z^2 + 1 - gamma - Psipars1 + z = bbx1 over 4
              endalign




              where $dsPsi$ is the Digamma Function. See $dsmathbfcolorblack6.3.15$ in A & S Table.


              Note that
              $dsPsipars2 = Psipars1 + 1 = -gamma + 1$.







              share|cite|improve this answer


























                up vote
                0
                down vote













                $newcommandbbx[1],bbox[15px,border:1px groove navy]displaystyle#1,
                newcommandbraces[1]leftlbrace,#1,rightrbrace
                newcommandbracks[1]leftlbrack,#1,rightrbrack
                newcommandddmathrmd
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                newcommandexpo[1],mathrme^#1,
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                newcommandpartiald[3]fracpartial^#1 #2partial #3^#1
                newcommandroot[2],sqrt[#1],#2,,
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                beginalign
                &bbox[10px,#ffd]dssum_k = 0^inftybrackszetapars2k + 3 - 1 =
                sum_k = 1^inftybrackszetapars2k + 1 - 1
                \[5mm] = &
                lim_z to 1bracks1 over 2z - 1 over 2,pi,cotparspi z -
                1 over 1 - z^2 + 1 - gamma - Psipars1 + z = bbx1 over 4
                endalign




                where $dsPsi$ is the Digamma Function. See $dsmathbfcolorblack6.3.15$ in A & S Table.


                Note that
                $dsPsipars2 = Psipars1 + 1 = -gamma + 1$.







                share|cite|improve this answer
























                  up vote
                  0
                  down vote










                  up vote
                  0
                  down vote









                  $newcommandbbx[1],bbox[15px,border:1px groove navy]displaystyle#1,
                  newcommandbraces[1]leftlbrace,#1,rightrbrace
                  newcommandbracks[1]leftlbrack,#1,rightrbrack
                  newcommandddmathrmd
                  newcommandds[1]displaystyle#1
                  newcommandexpo[1],mathrme^#1,
                  newcommandicmathrmi
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                  newcommandmrm[1]mathrm#1
                  newcommandpars[1]left(,#1,right)
                  newcommandpartiald[3]fracpartial^#1 #2partial #3^#1
                  newcommandroot[2],sqrt[#1],#2,,
                  newcommandtotald[3]fracmathrmd^#1 #2mathrmd #3^#1
                  newcommandverts[1]leftvert,#1,rightvert$
                  beginalign
                  &bbox[10px,#ffd]dssum_k = 0^inftybrackszetapars2k + 3 - 1 =
                  sum_k = 1^inftybrackszetapars2k + 1 - 1
                  \[5mm] = &
                  lim_z to 1bracks1 over 2z - 1 over 2,pi,cotparspi z -
                  1 over 1 - z^2 + 1 - gamma - Psipars1 + z = bbx1 over 4
                  endalign




                  where $dsPsi$ is the Digamma Function. See $dsmathbfcolorblack6.3.15$ in A & S Table.


                  Note that
                  $dsPsipars2 = Psipars1 + 1 = -gamma + 1$.







                  share|cite|improve this answer














                  $newcommandbbx[1],bbox[15px,border:1px groove navy]displaystyle#1,
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                  newcommandmc[1]mathcal#1
                  newcommandmrm[1]mathrm#1
                  newcommandpars[1]left(,#1,right)
                  newcommandpartiald[3]fracpartial^#1 #2partial #3^#1
                  newcommandroot[2],sqrt[#1],#2,,
                  newcommandtotald[3]fracmathrmd^#1 #2mathrmd #3^#1
                  newcommandverts[1]leftvert,#1,rightvert$
                  beginalign
                  &bbox[10px,#ffd]dssum_k = 0^inftybrackszetapars2k + 3 - 1 =
                  sum_k = 1^inftybrackszetapars2k + 1 - 1
                  \[5mm] = &
                  lim_z to 1bracks1 over 2z - 1 over 2,pi,cotparspi z -
                  1 over 1 - z^2 + 1 - gamma - Psipars1 + z = bbx1 over 4
                  endalign




                  where $dsPsi$ is the Digamma Function. See $dsmathbfcolorblack6.3.15$ in A & S Table.


                  Note that
                  $dsPsipars2 = Psipars1 + 1 = -gamma + 1$.








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                  edited Sep 11 at 3:04

























                  answered Apr 2 at 3:41









                  Felix Marin

                  65.6k7107138




                  65.6k7107138



























                       

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