Analytic continuation of harmonic series
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
add a comment |
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
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
add a comment |
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
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
sequences-and-series harmonic-functions harmonic-numbers analytic-continuation
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
add a comment |
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
add a comment |
2 Answers
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I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:
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.
add a comment |
Let's try it in an elementary manner
- 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.
- 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
$$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$
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2 Answers
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I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:
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.
add a comment |
I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:
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.
add a comment |
I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:
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.
I am not sure if this is what you meant, but Wolfram Alpha has an analytic formula for the $n^{text{th}}$ harmonic number:
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.
answered 1 hour ago
Noble Mushtak
14.7k1734
14.7k1734
add a comment |
add a comment |
Let's try it in an elementary manner
- 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.
- 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
$$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$
add a comment |
Let's try it in an elementary manner
- 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.
- 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
$$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$
add a comment |
Let's try it in an elementary manner
- 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.
- 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
$$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$
Let's try it in an elementary manner
- 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.
- 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
$$H_{z} = int_0^1 frac{1-x^{z}}{1-x},dx tag{5}$$
answered 24 mins ago
Dr. Wolfgang Hintze
3,150617
3,150617
add a comment |
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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