Analytic continuation of harmonic series












1














Is there an accepted analytic continuation of $sum_{n=1}^m frac{1}{n}$? Even a continuation to positive reals would be of interested, though negative and complex arguments would also be interesting.



I don't have a specific application in mind, but I'd very much like to understand how / if such a continuation could be accomplished. I've Googled but haven't come up with anything meaningful - perhaps because it's not possible?










share|cite|improve this question


















  • 1




    As is the case with the Gamma function, it would help to specify what properties you want the analytic continuation to retain. For instance, Euler tells us that $H_n=int_0^1 frac {1-x^n}{1-x}dx$. Replacing $n$ by a continuous parameter gives you an analytic continuation (at least for $n>0$). Is it useful?
    – lulu
    1 hour ago


















1














Is there an accepted analytic continuation of $sum_{n=1}^m frac{1}{n}$? Even a continuation to positive reals would be of interested, though negative and complex arguments would also be interesting.



I don't have a specific application in mind, but I'd very much like to understand how / if such a continuation could be accomplished. I've Googled but haven't come up with anything meaningful - perhaps because it's not possible?










share|cite|improve this question


















  • 1




    As is the case with the Gamma function, it would help to specify what properties you want the analytic continuation to retain. For instance, Euler tells us that $H_n=int_0^1 frac {1-x^n}{1-x}dx$. Replacing $n$ by a continuous parameter gives you an analytic continuation (at least for $n>0$). Is it useful?
    – lulu
    1 hour ago
















1












1








1







Is there an accepted analytic continuation of $sum_{n=1}^m frac{1}{n}$? Even a continuation to positive reals would be of interested, though negative and complex arguments would also be interesting.



I don't have a specific application in mind, but I'd very much like to understand how / if such a continuation could be accomplished. I've Googled but haven't come up with anything meaningful - perhaps because it's not possible?










share|cite|improve this question













Is there an accepted analytic continuation of $sum_{n=1}^m frac{1}{n}$? Even a continuation to positive reals would be of interested, though negative and complex arguments would also be interesting.



I don't have a specific application in mind, but I'd very much like to understand how / if such a continuation could be accomplished. I've Googled but haven't come up with anything meaningful - perhaps because it's not possible?







sequences-and-series harmonic-functions harmonic-numbers analytic-continuation






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked 1 hour ago









Richard Burke-Ward

3068




3068








  • 1




    As is the case with the Gamma function, it would help to specify what properties you want the analytic continuation to retain. For instance, Euler tells us that $H_n=int_0^1 frac {1-x^n}{1-x}dx$. Replacing $n$ by a continuous parameter gives you an analytic continuation (at least for $n>0$). Is it useful?
    – lulu
    1 hour ago
















  • 1




    As is the case with the Gamma function, it would help to specify what properties you want the analytic continuation to retain. For instance, Euler tells us that $H_n=int_0^1 frac {1-x^n}{1-x}dx$. Replacing $n$ by a continuous parameter gives you an analytic continuation (at least for $n>0$). Is it useful?
    – lulu
    1 hour ago










1




1




As is the case with the Gamma function, it would help to specify what properties you want the analytic continuation to retain. For instance, Euler tells us that $H_n=int_0^1 frac {1-x^n}{1-x}dx$. Replacing $n$ by a continuous parameter gives you an analytic continuation (at least for $n>0$). Is it useful?
– lulu
1 hour ago






As is the case with the Gamma function, it would help to specify what properties you want the analytic continuation to retain. For instance, Euler tells us that $H_n=int_0^1 frac {1-x^n}{1-x}dx$. Replacing $n$ by a continuous parameter gives you an analytic continuation (at least for $n>0$). Is it useful?
– lulu
1 hour ago












2 Answers
2






active

oldest

votes


















3














I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:



enter image description here



Here, the digamma function is $psi_0(x)=frac{Gamma'(x)}{Gamma(x)}$, which I believe is defined for all numbers in the complex plane except for negative real integers.






share|cite|improve this answer





























    0














    Let's try it in an elementary manner




    1. You can use the defining recursion valid for $nin Z^{+}$


    $$H_{n} = H_{n-1} + frac{1}{n}, H_{1}=1tag{1a}$$



    also for any complex $z$



    $$H_{z} = H_{z-1} + frac{1}{z}, H_{1}=1tag{1b}$$



    For instance for $z=1$ we obtain $$H_{1} = H_{0} + frac{1}{1}$$



    from which we conclude that $H_{0}=0$.



    If we try to find $H_{-1}$ we encounter the problem that from $H_0 = 0 = lim_{zto0}(H_{-1+z} + frac{1}{z})$ we find that $H_{z} simeq frac{1}{z}$ for $zsimeq 0$. In other words, $H_{z}$ has a simple pole at $z=-1$.



    We could continue in this manner to go to further into the region of negative $z$ but we prefer the following approach.




    1. Starting with this formula for the harmonic number which is valid for $nin Z^{+}$


    $$H_{n} = frac{1}{2}+ ... + frac{1}{n}\\=frac{1}{1}+ frac{1}{2}+ ... + frac{1}{n} +frac{1}{1+n}+frac{1}{n+2} + ... \;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;-frac{1}{1+n}- frac{1}{n+2} + ...\=sum_{k=0}^infty left(frac{1}{k}-frac{1}{ (k+n)}right)tag{2}$$



    The sum can be written as



    $$H_{n}= sum_{k=0}^infty frac{n}{k (k+n)}tag{3}$$



    and this can be extended immediately to complex values $z$ in place of $n$



    $$H_{z}= sum_{k=0}^infty frac{z}{k (k+z)}=sum_{k=0}^infty left(frac{1}{k} -frac{1}{k+z}right)tag{4}$$



    This sum is convergent (the proof is left to the reader) for any $z$ except for $z=-1, -2, ...$ where $H_{z}$ has simple poles with residue $-1$.



    Hence $(4)$ gives the analytic continuation.



    For instance close to $z=0$ we have as in 1. that



    $$H_{z} simeq z sum_{k=0}^infty frac{1}{k^2} = z;zeta(2) =z;frac{pi^2}{6}to 0 $$



    We can also derive an integral representation from the second form of $(4)$ writing



    $$frac{1}{k} -frac{1}{k+z} =int_0^1 (x^{k-1}-x^{z+k-1}),dx $$



    Performing the sum under the integral is just doing a geometric sum and gives



    $$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$






    share|cite|improve this answer





















      Your Answer





      StackExchange.ifUsing("editor", function () {
      return StackExchange.using("mathjaxEditing", function () {
      StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
      StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
      });
      });
      }, "mathjax-editing");

      StackExchange.ready(function() {
      var channelOptions = {
      tags: "".split(" "),
      id: "69"
      };
      initTagRenderer("".split(" "), "".split(" "), channelOptions);

      StackExchange.using("externalEditor", function() {
      // Have to fire editor after snippets, if snippets enabled
      if (StackExchange.settings.snippets.snippetsEnabled) {
      StackExchange.using("snippets", function() {
      createEditor();
      });
      }
      else {
      createEditor();
      }
      });

      function createEditor() {
      StackExchange.prepareEditor({
      heartbeatType: 'answer',
      autoActivateHeartbeat: false,
      convertImagesToLinks: true,
      noModals: true,
      showLowRepImageUploadWarning: true,
      reputationToPostImages: 10,
      bindNavPrevention: true,
      postfix: "",
      imageUploader: {
      brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
      contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
      allowUrls: true
      },
      noCode: true, onDemand: true,
      discardSelector: ".discard-answer"
      ,immediatelyShowMarkdownHelp:true
      });


      }
      });














      draft saved

      draft discarded


















      StackExchange.ready(
      function () {
      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3058500%2fanalytic-continuation-of-harmonic-series%23new-answer', 'question_page');
      }
      );

      Post as a guest















      Required, but never shown

























      2 Answers
      2






      active

      oldest

      votes








      2 Answers
      2






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      3














      I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:



      enter image description here



      Here, the digamma function is $psi_0(x)=frac{Gamma'(x)}{Gamma(x)}$, which I believe is defined for all numbers in the complex plane except for negative real integers.






      share|cite|improve this answer


























        3














        I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:



        enter image description here



        Here, the digamma function is $psi_0(x)=frac{Gamma'(x)}{Gamma(x)}$, which I believe is defined for all numbers in the complex plane except for negative real integers.






        share|cite|improve this answer
























          3












          3








          3






          I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:



          enter image description here



          Here, the digamma function is $psi_0(x)=frac{Gamma'(x)}{Gamma(x)}$, which I believe is defined for all numbers in the complex plane except for negative real integers.






          share|cite|improve this answer












          I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:



          enter image description here



          Here, the digamma function is $psi_0(x)=frac{Gamma'(x)}{Gamma(x)}$, which I believe is defined for all numbers in the complex plane except for negative real integers.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered 1 hour ago









          Noble Mushtak

          14.7k1734




          14.7k1734























              0














              Let's try it in an elementary manner




              1. You can use the defining recursion valid for $nin Z^{+}$


              $$H_{n} = H_{n-1} + frac{1}{n}, H_{1}=1tag{1a}$$



              also for any complex $z$



              $$H_{z} = H_{z-1} + frac{1}{z}, H_{1}=1tag{1b}$$



              For instance for $z=1$ we obtain $$H_{1} = H_{0} + frac{1}{1}$$



              from which we conclude that $H_{0}=0$.



              If we try to find $H_{-1}$ we encounter the problem that from $H_0 = 0 = lim_{zto0}(H_{-1+z} + frac{1}{z})$ we find that $H_{z} simeq frac{1}{z}$ for $zsimeq 0$. In other words, $H_{z}$ has a simple pole at $z=-1$.



              We could continue in this manner to go to further into the region of negative $z$ but we prefer the following approach.




              1. Starting with this formula for the harmonic number which is valid for $nin Z^{+}$


              $$H_{n} = frac{1}{2}+ ... + frac{1}{n}\\=frac{1}{1}+ frac{1}{2}+ ... + frac{1}{n} +frac{1}{1+n}+frac{1}{n+2} + ... \;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;-frac{1}{1+n}- frac{1}{n+2} + ...\=sum_{k=0}^infty left(frac{1}{k}-frac{1}{ (k+n)}right)tag{2}$$



              The sum can be written as



              $$H_{n}= sum_{k=0}^infty frac{n}{k (k+n)}tag{3}$$



              and this can be extended immediately to complex values $z$ in place of $n$



              $$H_{z}= sum_{k=0}^infty frac{z}{k (k+z)}=sum_{k=0}^infty left(frac{1}{k} -frac{1}{k+z}right)tag{4}$$



              This sum is convergent (the proof is left to the reader) for any $z$ except for $z=-1, -2, ...$ where $H_{z}$ has simple poles with residue $-1$.



              Hence $(4)$ gives the analytic continuation.



              For instance close to $z=0$ we have as in 1. that



              $$H_{z} simeq z sum_{k=0}^infty frac{1}{k^2} = z;zeta(2) =z;frac{pi^2}{6}to 0 $$



              We can also derive an integral representation from the second form of $(4)$ writing



              $$frac{1}{k} -frac{1}{k+z} =int_0^1 (x^{k-1}-x^{z+k-1}),dx $$



              Performing the sum under the integral is just doing a geometric sum and gives



              $$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$






              share|cite|improve this answer


























                0














                Let's try it in an elementary manner




                1. You can use the defining recursion valid for $nin Z^{+}$


                $$H_{n} = H_{n-1} + frac{1}{n}, H_{1}=1tag{1a}$$



                also for any complex $z$



                $$H_{z} = H_{z-1} + frac{1}{z}, H_{1}=1tag{1b}$$



                For instance for $z=1$ we obtain $$H_{1} = H_{0} + frac{1}{1}$$



                from which we conclude that $H_{0}=0$.



                If we try to find $H_{-1}$ we encounter the problem that from $H_0 = 0 = lim_{zto0}(H_{-1+z} + frac{1}{z})$ we find that $H_{z} simeq frac{1}{z}$ for $zsimeq 0$. In other words, $H_{z}$ has a simple pole at $z=-1$.



                We could continue in this manner to go to further into the region of negative $z$ but we prefer the following approach.




                1. Starting with this formula for the harmonic number which is valid for $nin Z^{+}$


                $$H_{n} = frac{1}{2}+ ... + frac{1}{n}\\=frac{1}{1}+ frac{1}{2}+ ... + frac{1}{n} +frac{1}{1+n}+frac{1}{n+2} + ... \;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;-frac{1}{1+n}- frac{1}{n+2} + ...\=sum_{k=0}^infty left(frac{1}{k}-frac{1}{ (k+n)}right)tag{2}$$



                The sum can be written as



                $$H_{n}= sum_{k=0}^infty frac{n}{k (k+n)}tag{3}$$



                and this can be extended immediately to complex values $z$ in place of $n$



                $$H_{z}= sum_{k=0}^infty frac{z}{k (k+z)}=sum_{k=0}^infty left(frac{1}{k} -frac{1}{k+z}right)tag{4}$$



                This sum is convergent (the proof is left to the reader) for any $z$ except for $z=-1, -2, ...$ where $H_{z}$ has simple poles with residue $-1$.



                Hence $(4)$ gives the analytic continuation.



                For instance close to $z=0$ we have as in 1. that



                $$H_{z} simeq z sum_{k=0}^infty frac{1}{k^2} = z;zeta(2) =z;frac{pi^2}{6}to 0 $$



                We can also derive an integral representation from the second form of $(4)$ writing



                $$frac{1}{k} -frac{1}{k+z} =int_0^1 (x^{k-1}-x^{z+k-1}),dx $$



                Performing the sum under the integral is just doing a geometric sum and gives



                $$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$






                share|cite|improve this answer
























                  0












                  0








                  0






                  Let's try it in an elementary manner




                  1. You can use the defining recursion valid for $nin Z^{+}$


                  $$H_{n} = H_{n-1} + frac{1}{n}, H_{1}=1tag{1a}$$



                  also for any complex $z$



                  $$H_{z} = H_{z-1} + frac{1}{z}, H_{1}=1tag{1b}$$



                  For instance for $z=1$ we obtain $$H_{1} = H_{0} + frac{1}{1}$$



                  from which we conclude that $H_{0}=0$.



                  If we try to find $H_{-1}$ we encounter the problem that from $H_0 = 0 = lim_{zto0}(H_{-1+z} + frac{1}{z})$ we find that $H_{z} simeq frac{1}{z}$ for $zsimeq 0$. In other words, $H_{z}$ has a simple pole at $z=-1$.



                  We could continue in this manner to go to further into the region of negative $z$ but we prefer the following approach.




                  1. Starting with this formula for the harmonic number which is valid for $nin Z^{+}$


                  $$H_{n} = frac{1}{2}+ ... + frac{1}{n}\\=frac{1}{1}+ frac{1}{2}+ ... + frac{1}{n} +frac{1}{1+n}+frac{1}{n+2} + ... \;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;-frac{1}{1+n}- frac{1}{n+2} + ...\=sum_{k=0}^infty left(frac{1}{k}-frac{1}{ (k+n)}right)tag{2}$$



                  The sum can be written as



                  $$H_{n}= sum_{k=0}^infty frac{n}{k (k+n)}tag{3}$$



                  and this can be extended immediately to complex values $z$ in place of $n$



                  $$H_{z}= sum_{k=0}^infty frac{z}{k (k+z)}=sum_{k=0}^infty left(frac{1}{k} -frac{1}{k+z}right)tag{4}$$



                  This sum is convergent (the proof is left to the reader) for any $z$ except for $z=-1, -2, ...$ where $H_{z}$ has simple poles with residue $-1$.



                  Hence $(4)$ gives the analytic continuation.



                  For instance close to $z=0$ we have as in 1. that



                  $$H_{z} simeq z sum_{k=0}^infty frac{1}{k^2} = z;zeta(2) =z;frac{pi^2}{6}to 0 $$



                  We can also derive an integral representation from the second form of $(4)$ writing



                  $$frac{1}{k} -frac{1}{k+z} =int_0^1 (x^{k-1}-x^{z+k-1}),dx $$



                  Performing the sum under the integral is just doing a geometric sum and gives



                  $$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$






                  share|cite|improve this answer












                  Let's try it in an elementary manner




                  1. You can use the defining recursion valid for $nin Z^{+}$


                  $$H_{n} = H_{n-1} + frac{1}{n}, H_{1}=1tag{1a}$$



                  also for any complex $z$



                  $$H_{z} = H_{z-1} + frac{1}{z}, H_{1}=1tag{1b}$$



                  For instance for $z=1$ we obtain $$H_{1} = H_{0} + frac{1}{1}$$



                  from which we conclude that $H_{0}=0$.



                  If we try to find $H_{-1}$ we encounter the problem that from $H_0 = 0 = lim_{zto0}(H_{-1+z} + frac{1}{z})$ we find that $H_{z} simeq frac{1}{z}$ for $zsimeq 0$. In other words, $H_{z}$ has a simple pole at $z=-1$.



                  We could continue in this manner to go to further into the region of negative $z$ but we prefer the following approach.




                  1. Starting with this formula for the harmonic number which is valid for $nin Z^{+}$


                  $$H_{n} = frac{1}{2}+ ... + frac{1}{n}\\=frac{1}{1}+ frac{1}{2}+ ... + frac{1}{n} +frac{1}{1+n}+frac{1}{n+2} + ... \;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;-frac{1}{1+n}- frac{1}{n+2} + ...\=sum_{k=0}^infty left(frac{1}{k}-frac{1}{ (k+n)}right)tag{2}$$



                  The sum can be written as



                  $$H_{n}= sum_{k=0}^infty frac{n}{k (k+n)}tag{3}$$



                  and this can be extended immediately to complex values $z$ in place of $n$



                  $$H_{z}= sum_{k=0}^infty frac{z}{k (k+z)}=sum_{k=0}^infty left(frac{1}{k} -frac{1}{k+z}right)tag{4}$$



                  This sum is convergent (the proof is left to the reader) for any $z$ except for $z=-1, -2, ...$ where $H_{z}$ has simple poles with residue $-1$.



                  Hence $(4)$ gives the analytic continuation.



                  For instance close to $z=0$ we have as in 1. that



                  $$H_{z} simeq z sum_{k=0}^infty frac{1}{k^2} = z;zeta(2) =z;frac{pi^2}{6}to 0 $$



                  We can also derive an integral representation from the second form of $(4)$ writing



                  $$frac{1}{k} -frac{1}{k+z} =int_0^1 (x^{k-1}-x^{z+k-1}),dx $$



                  Performing the sum under the integral is just doing a geometric sum and gives



                  $$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered 24 mins ago









                  Dr. Wolfgang Hintze

                  3,150617




                  3,150617






























                      draft saved

                      draft discarded




















































                      Thanks for contributing an answer to Mathematics Stack Exchange!


                      • Please be sure to answer the question. Provide details and share your research!

                      But avoid



                      • Asking for help, clarification, or responding to other answers.

                      • Making statements based on opinion; back them up with references or personal experience.


                      Use MathJax to format equations. MathJax reference.


                      To learn more, see our tips on writing great answers.





                      Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


                      Please pay close attention to the following guidance:


                      • Please be sure to answer the question. Provide details and share your research!

                      But avoid



                      • Asking for help, clarification, or responding to other answers.

                      • Making statements based on opinion; back them up with references or personal experience.


                      To learn more, see our tips on writing great answers.




                      draft saved


                      draft discarded














                      StackExchange.ready(
                      function () {
                      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3058500%2fanalytic-continuation-of-harmonic-series%23new-answer', 'question_page');
                      }
                      );

                      Post as a guest















                      Required, but never shown





















































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown

































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown







                      Popular posts from this blog

                      Eastern Orthodox Church

                      Zagreb

                      Understanding the information contained in the Deep Space Network XML data?