log integral
Formula involving integrals of logarithm function
We prove the following formula involving the integral of powers of the function
ln(1−x).
∫10lnn(1−x)ln(x)xdx=(−1)n+1n![(n+12)ζ(n+2)−12n−1∑k=1ζ(k+1)ζ(n+1−k)]
Some examples for the first naturals:
n=1∫10ln(1−x)ln(x)xdx=ζ(3)n=2∫10ln2(1−x)ln(x)xdx=−3ζ(4)+ζ(2)ζ(2)=−π4180n=3∫10ln3(1−x)ln(x)xdx=12ζ(5)−6ζ(2)ζ(3)=12ζ(5)−π2ζ(3)n=4∫10ln4(1−x)ln(x)xdx=−60ζ(6)+24ζ(2)ζ(4)+12ζ(3)ζ(3)=12ζ2(3)−2π6105n=5∫10ln5(1−x)ln(x)xdx=360ζ(7)−120ζ(2)ζ(5)−120ζ(3)ζ(4)=−4π4ζ(3)3−20π2ζ(5)+360ζ(7)
Proof
∫10lnn(1−x)ln(x)xdx=∫10[dndtn|t=0+(1−x)t][dds|s=0+xs]xdx=dndtn|t=0+dds|s=0+∫10(1−x)txsxdx=dndtn|t=0+dds|s=0+∫10(1−x)txs−1dx=dndtn|t=0+dds|s=0+B(t+1,s)
Now, recall the series expansion of the beta function (proof in Appendix 1):
B(x,y)=∞∑j=0(1−y)jj!(x+j)y>0
Therefore
dndtn|t=0+dds|s=0+B(t+1,s)=dndtn|t=0+dds|s=0+∞∑j=0(1−s)jj!(t+1+j)=dndtn|t=0+[lims→0+dds∞∑j=0(1−s)jj!(t+1+j)]=dndtn|t=0+[lims→0+∞∑j=0(1−s)j(ψ(0)(1−s)−ψ(0)(1−s+j))j!(t+1+j)]=dndtn|t=0+[∞∑j=0(1)j(ψ(0)(1)−ψ(0)(1+j))j!(t+1+j)]=dndtn|t=0+[∞∑j=0Γ(j+1)(−γ−ψ(0)(1+j))j!(t+1+j)]=dndtn|t=0+[∞∑j=0−Γ(j+1)Hjj!(t+1+j)]=dndtn|t=0+[∞∑j=0−Hj(t+1+j)]=limt→0+∞∑j=0dndtn−Hj(t+1+j)=limt→0+∞∑j=0(−1)n+1n!Hj(t+1+j)n+1=∞∑j=0(−1)n+1n!Hj(1+j)n+1
Now we know that
Hj+1=1j+1+Hj therefore
∞∑j=0(−1)n+1n!Hj(1+j)n+1=∞∑j=0(−1)n+1n!Hj+1(1+j)n+1−∞∑j=0(−1)n+1n!(1+j)n+2=(−1)n+1n![∞∑j=0Hj+1(1+j)n+1−∞∑j=01(1+j)n+2]
If
m=j+1 then
(−1)n+1n![∞∑j=0Hj+1(1+j)n+1−∞∑j=01(1+j)n+2]=(−1)n+1n![∞∑m=1Hmmn+1−∞∑m=11mn+2]=(−1)n+1n![∞∑m=1Hmmn+1−ζ(n+2)]
Now, recall this beutiful result proven by Euler (Proof in Appendix 2):
∞∑n=1Hnnr=(1+r2)ζ(r+1)−12r−2∑k=1ζ(k+1)ζ(r−k)
Then
(−1)n+1n![∞∑m=1Hmmn+1−ζ(n+2)]=(−1)n+1n![(1+n+12)ζ(n+2)−12n−1∑k=1ζ(k+1)ζ(n+1−k)−ζ(n+2)]=(−1)n+1n![(n+12)ζ(n+2)−12n−1∑k=1ζ(k+1)ζ(n+1−k)]
Therefore
∫10lnn(1−x)ln(x)xdx=(−1)n+1n![(n+12)ζ(n+2)−12n−1∑k=1ζ(k+1)ζ(n+1−k)]
Apendix 1: Series expansion for the beta function
From the definition of the beta function:
B(x,y)=∫10tx−1(1−t)y−1dt
Recall the series expansion of
(1−t)y−1
(1-t)^{y-1} = \sum_{j=0}^{\infty} (-1)^{j} \binom{y-1}{j}t^{j}
\begin{align*}
\int_{0}^{1} t^{x-1}(1-x)^{y-1}dt =& \int_{0}^{1} t^{x-1}\sum_{j=0}^{\infty} (-1)^{j} \binom{y-1}{j}t^{j}dt\\
=& \sum_{j=0}^{\infty} (-1)^{j} \binom{y-1}{j} \int_{0}^{\infty} t^{x+j-1}dt\\
=& \sum_{j=0}^{\infty} (-1)^{j} \binom{y-1}{j} \frac{1}{(x+j)}
\end{align*}
Then we have
\begin{align*}
\sum_{j=0}^{\infty} (-1)^{j} \binom{y-1}{j}\frac{1}{(x+j)} =& \sum_{j=0}^{\infty}\binom{j-y}{j}\frac{1}{(x+j)}\\
=& \sum_{j=0}^{\infty}\frac{(j-y)!}{j!(-y)!}\frac{1}{(x+j)}\\
=& \sum_{j=0}^{\infty}\frac{\Gamma(j-y+1)}{j!\Gamma(1-y)} \frac{1}{(x+j)}\\
=& \sum_{j=0}^{\infty}\frac{(1-y)_{j}}{j!(x+j)}
\end{align*}
where the last equality is the definnition of the Pochhammer polynomials.
Therefore
\boxed{B(x,y) = \sum_{j=0}^{\infty}\frac{(1-y)_{j}}{j!(x+j)}}
Appendix 2: Proof of Euler's theorem for harmonic series
I found a beautiful proof of this result which relies on the residue theorem. I really liked this proof because it shows that the famous result due to Euler is just a residue of certain kernel function. I present here the original proof in the spirit of Flajolet et al.
[1] but I complete the omitted parts by the authors in the original paper.
We need the following Lemma due to Cauchy and Lindelöf.
Lemma. Let
\xi be a kernel function and let
r(s) be a rational function with
\mathscr{O}(s^{-2}) at infinity. Then
\underbrace{\sum_{\alpha } \left\{\operatorname{Res}\left(r(s)\xi(s), \alpha\right) \Big| \alpha \textrm{ is a pole of } \xi \textrm{ that is not a pole of } r(s) \right\}}_{\textrm{infinite series}} = -\underbrace{\sum_{\beta } \left\{\operatorname{Res}\left(r(s)\xi(s), \beta \right) \Big| \beta \textrm{ is a pole of } r(s) \right\}}_{\textrm{finite series}}
We will apply the lemma to the kernel
\displaystyle \xi(s) = (\psi(-s)+\gamma)^2
Propositon. If
\displaystyle \xi(s) = (\psi(-s)+\gamma)^2 then
\sum_{\alpha } \left\{\operatorname{Res}\left(r(s)\xi(s), \alpha\right) \Big| \alpha \textrm{ is a pole of } \xi \textrm{ that is not a pole of } r(s) \right\} = 2\sum_{n=1}^{\infty}H_{n}r(n)+ \sum_{n=1}^{\infty}r'(n)
Proof
The kernel
\xi(s) has a countable number of pole at the positive integers.
Let
\displaystyle n\in \mathbb{N}, then the function
\displaystyle (\psi(-s)+\gamma) has the following expansion at
s \to n.
\psi(-s)+\gamma = \frac{1}{(s-n)}+ H_{n} + \sum_{k=1}^{\infty}\left[(-1)^K H_{n}^{k+1} - \zeta(k+1)\right](s-n)^{k}
Therefore
\begin{align*}
(\psi(-s)+\gamma)^2 =& \left[\frac{1}{(s-n)}+ H_{n} + \sum_{k=1}^{\infty}\underbrace{\left[(-1)^K H_{n}^{k+1} - \zeta(k+1)\right]}_{A_{k}}(s-n)^{k}\right]^2\\
=& \left(\frac{1}{(s-n)}+ H_{n}\right)^2 + 2\left(\frac{1}{(s-n)}+ H_{n}\right)\sum_{k=1}^{\infty}A_{k}(s-n)^{k} + \left(\sum_{k=1}^{\infty}A_{k}(s-n)^{k}\right)^2\\
=& \frac{1}{(s-n)^2}+\frac{2H_{n}}{(s-n)} + H^2_{n} + 2\left(\frac{1}{(s-n)}+ H_{n}\right)\sum_{k=1}^{\infty}A_{k}(s-n)^{k} + \left(\sum_{k=1}^{\infty}A_{k}(s-n)^{k}\right)^2\\
=& \frac{1}{(s-n)^2}+\frac{2H_{n}}{(s-n)} + H^2_{n} + 2A_{1} + \mathscr{O}(s-n)
\end{align*}
Now, suppose
r(s) is a rational function that can be expanded with its power series (in reality, we have assumed something weaker: that
r(s) is
\mathcal{O}(s^{-2})). Then for
s \to n:
r(s) = r(n)+ r'(n)(s-n)+\frac{r''(n)}{2!}(s-n)^2+\frac{r'''(n)}{3!}(s-n)^3+...
Therefore
\begin{align*}
r(s)(\psi(-s)+\gamma)^2 =& r(n)(\psi(-s)+\gamma)^2+ r'(n)(s-n)(\psi(-s)+\gamma)^2+\frac{r''(n)}{2!}(s-n)^2(\psi(-s)+\gamma)^2+...\\
=& \frac{r(n)}{(s-n)^2}+\frac{2H_{n}r(n)}{(s-n)} + H^2_{n}r(n) + 2A_{1}r(n) + \mathscr{O}(s-n)r(n) + \frac{r'(n)}{(s-n)}+2H_{n}r'(n) + H^2_{n}r'(n)(s-n) + 2A_{1}r'(n)(s-n) + \mathscr{O}(s-n)r'(n)(s-n) + ... \\
=& H^2_{n}r(n)+ 2A_{1}r(n)+2H_{n}r'(n)+\frac{r''(n)}{2!}+\frac{1}{s-n}\underbrace{\left[2H_{n}r(n)+r'(n)\right]}_{\textrm{residue at } n} +\frac{r(n)}{(s-n)^2} + \mathscr{O}(s-n)
\end{align*}
\therefore \operatorname{Res}\left(r(s)(\psi(-s)+\gamma)^2, n\right) = 2H_{n}r(n)+r'(n)
Summing each
n \in \mathbb{N}\setminus\left\{0\right\}
\boxed{\sum_{n=1}^{\infty} \operatorname{Res}\left(r(s)(\psi(-s)+\gamma)^2, n\right) = 2\sum_{n=1}^{\infty}H_{n}r(n)+ \sum_{n=1}^{\infty}r'(n)}
or, equivalently
\boxed{\sum_{\alpha } \left\{\operatorname{Res}\left(r(s)\xi(s), \alpha\right) \Big| \alpha \textrm{ is a pole of } \xi \textrm{ that is not a pole of } r(s) \right\} = 2\sum_{n=1}^{\infty}H_{n}r(n)+ \sum_{n=1}^{\infty}r'(n)}
We are ready to prove Euler's theorem
Theorem (Euler). For integer
\geq 2,
\sum_{n=1}^{\infty} \frac{H_{n}}{n^q} = \left(1+\frac{q}{2}\right)\zeta(q+1) -\frac{1}{2}\sum_{k=1}^{q-2} \zeta(k+1)\zeta(q-k)
Proof
If we apply the last result to the function
\displaystyle r(s)= \frac{1}{s^q} we have
\sum_{n=1}^{\infty} \operatorname{Res}\left(\frac{(\psi(-s)+\gamma)^2}{s^q}, n\right) = 2\sum_{n=1}^{\infty}\frac{H_{n}}{n^q} -q\zeta(q+1)
By the first lemma,
\displaystyle -\sum_{\beta} \left\{\operatorname{Res}\left(\frac{(\psi(-s)+\gamma)^2}{s^q}, \beta \right) \Big| \beta \textrm{ is a pole of } \frac{1}{s^q} \right\} = 2\sum_{n=1}^{\infty}\frac{H_{n}}{n^q} -q\zeta(q+1)
Given that
\displaystyle r(s)= \frac{1}{s^q} has a single pole at
s=0 of order
q we have:
-\operatorname{Res}\left(\frac{(\psi(-s)+\gamma)^2}{s^q}, 0\right) =2\sum_{n=1}^{\infty}\frac{H_{n}}{n^q} -q\zeta(q+1)
To find the residue we can expand
\displaystyle \frac{(\psi(-s)+\gamma)^2}{s^q}:
First, recall the series expansion of
\psi(-s)+\gamma at
s=0:
\psi(-s)+\gamma = \frac{1}{s} - \sum_{k=1}^{\infty} \zeta(k+1)s^{k}
Therefore
\begin{align*}
(\psi(-s)+\gamma)^2 =& \left[\frac{1}{s} -\sum_{k=1}^{\infty} \zeta(k+1)s^{k}\right]^2 \\
=& \frac{1}{s^2} -2\sum_{k=1}^{\infty} \zeta(k+1)s^{k-1} + \left(\sum_{k=1}^{\infty} \zeta(k+1)s^{k}\right)^2\\
=& \frac{1}{s^2} -2\sum_{k=1}^{\infty} \zeta(k+1)s^{k-1} + \sum_{k=1}^{\infty}\sum_{j=1}^{k} \zeta(j+1)\zeta(k-j+2)s^{k+1}\\
\end{align*}
Then
\begin{align*}
\frac{(\psi(-s)+\gamma)^2}{s^q} = \frac{1}{s^{2+q}} -2\underbrace{\sum_{k=1}^{\infty} \zeta(k+1)s^{k-1-q}}_{A} + \underbrace{\sum_{k=1}^{\infty}\sum_{j=1}^{k} \zeta(j+1)\zeta(k-j+2)s^{k+1-q}}_{B}
\end{align*}
From here is easy to show that the residue in A is the coefficient of
\displaystyle s^{k-1-q}=s^{-1}
\Longrightarrow k-1-q=-1 \Longrightarrow k=q
and the residue in B is coefficient of
\displaystyle s^{k+1-q}=s^{-1} where
\Longrightarrow k+1-q=-1 \Longrightarrow k=q-2
Therefore
\operatorname{Res}\left(\frac{(\psi(-s)+\gamma)^2}{s^q},0\right) = -2\zeta(q+1) + \sum_{j=1}^{q-2} \zeta(j+1)\zeta(q-j)
Then
2\zeta(q+1) - \sum_{j=1}^{q-2} \zeta(j+1)\zeta(q-j) = 2\sum_{n=1}^{\infty}\frac{H_{n}}{n^q} -q\zeta(q+1)
\boxed{\therefore\sum_{n=1}^{\infty} \frac{H_{n}}{n^q} = \left(1+\frac{q}{2}\right)\zeta(q+1) -\frac{1}{2}\sum_{k=1}^{q-2} \zeta(k+1)\zeta(q-k)}