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=∞∑j=0(−1)j(y−1j)tj
∫10tx−1(1−x)y−1dt=∫10tx−1∞∑j=0(−1)j(y−1j)tjdt=∞∑j=0(−1)j(y−1j)∫∞0tx+j−1dt=∞∑j=0(−1)j(y−1j)1(x+j)
Then we have
∞∑j=0(−1)j(y−1j)1(x+j)=∞∑j=0(j−yj)1(x+j)=∞∑j=0(j−y)!j!(−y)!1(x+j)=∞∑j=0Γ(j−y+1)j!Γ(1−y)1(x+j)=∞∑j=0(1−y)jj!(x+j)
where the last equality is the definnition of the Pochhammer polynomials.
Therefore
B(x,y)=∞∑j=0(1−y)jj!(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
ξ be a kernel function and let
r(s) be a rational function with
O(s−2) at infinity. Then
∑α{Res(r(s)ξ(s),α)|α is a pole of ξ that is not a pole of r(s)}⏟infinite series=−∑β{Res(r(s)ξ(s),β)|β is a pole of r(s)}⏟finite series
We will apply the lemma to the kernel
ξ(s)=(ψ(−s)+γ)2
Propositon. If
ξ(s)=(ψ(−s)+γ)2 then
∑α{Res(r(s)ξ(s),α)|α is a pole of ξ that is not a pole of r(s)}=2∞∑n=1Hnr(n)+∞∑n=1r′(n)
Proof
The kernel
ξ(s) has a countable number of pole at the positive integers.
Let
n∈N, then the function
(ψ(−s)+γ) has the following expansion at
s→n.
ψ(−s)+γ=1(s−n)+Hn+∞∑k=1[(−1)KHk+1n−ζ(k+1)](s−n)k
Therefore
(ψ(−s)+γ)2=[1(s−n)+Hn+∞∑k=1[(−1)KHk+1n−ζ(k+1)]⏟Ak(s−n)k]2=(1(s−n)+Hn)2+2(1(s−n)+Hn)∞∑k=1Ak(s−n)k+(∞∑k=1Ak(s−n)k)2=1(s−n)2+2Hn(s−n)+H2n+2(1(s−n)+Hn)∞∑k=1Ak(s−n)k+(∞∑k=1Ak(s−n)k)2=1(s−n)2+2Hn(s−n)+H2n+2A1+O(s−n)
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
O(s−2)). Then for
s→n:
r(s)=r(n)+r′(n)(s−n)+r″(n)2!(s−n)2+r‴(n)3!(s−n)3+...
Therefore
r(s)(ψ(−s)+γ)2=r(n)(ψ(−s)+γ)2+r′(n)(s−n)(ψ(−s)+γ)2+r″(n)2!(s−n)2(ψ(−s)+γ)2+...=r(n)(s−n)2+2Hnr(n)(s−n)+H2nr(n)+2A1r(n)+O(s−n)r(n)+r′(n)(s−n)+2Hnr′(n)+H2nr′(n)(s−n)+2A1r′(n)(s−n)+O(s−n)r′(n)(s−n)+...=H2nr(n)+2A1r(n)+2Hnr′(n)+r″(n)2!+1s−n[2Hnr(n)+r′(n)]⏟residue at n+r(n)(s−n)2+O(s−n)
∴Res(r(s)(ψ(−s)+γ)2,n)=2Hnr(n)+r′(n)
Summing each
n∈N∖{0}
∞∑n=1Res(r(s)(ψ(−s)+γ)2,n)=2∞∑n=1Hnr(n)+∞∑n=1r′(n)
or, equivalently
∑α{Res(r(s)ξ(s),α)|α is a pole of ξ that is not a pole of r(s)}=2∞∑n=1Hnr(n)+∞∑n=1r′(n)
We are ready to prove Euler's theorem
Theorem (Euler). For integer
≥2,
∞∑n=1Hnnq=(1+q2)ζ(q+1)−12q−2∑k=1ζ(k+1)ζ(q−k)
Proof
If we apply the last result to the function
r(s)=1sq we have
∞∑n=1Res((ψ(−s)+γ)2sq,n)=2∞∑n=1Hnnq−qζ(q+1)
By the first lemma,
−∑β{Res((ψ(−s)+γ)2sq,β)|β is a pole of 1sq}=2∞∑n=1Hnnq−qζ(q+1)
Given that
r(s)=1sq has a single pole at
s=0 of order
q we have:
−Res((ψ(−s)+γ)2sq,0)=2∞∑n=1Hnnq−qζ(q+1)
To find the residue we can expand
(ψ(−s)+γ)2sq:
First, recall the series expansion of
ψ(−s)+γ at
s=0:
ψ(−s)+γ=1s−∞∑k=1ζ(k+1)sk
Therefore
(ψ(−s)+γ)2=[1s−∞∑k=1ζ(k+1)sk]2=1s2−2∞∑k=1ζ(k+1)sk−1+(∞∑k=1ζ(k+1)sk)2=1s2−2∞∑k=1ζ(k+1)sk−1+∞∑k=1k∑j=1ζ(j+1)ζ(k−j+2)sk+1
Then
(ψ(−s)+γ)2sq=1s2+q−2∞∑k=1ζ(k+1)sk−1−q⏟A+∞∑k=1k∑j=1ζ(j+1)ζ(k−j+2)sk+1−q⏟B
From here is easy to show that the residue in A is the coefficient of
sk−1−q=s−1
⟹k−1−q=−1⟹k=q
and the residue in B is coefficient of
sk+1−q=s−1 where
⟹k+1−q=−1⟹k=q−2
Therefore
Res((ψ(−s)+γ)2sq,0)=−2ζ(q+1)+q−2∑j=1ζ(j+1)ζ(q−j)
Then
2ζ(q+1)−q−2∑j=1ζ(j+1)ζ(q−j)=2∞∑n=1Hnnq−qζ(q+1)
∴∞∑n=1Hnnq=(1+q2)ζ(q+1)−12q−2∑k=1ζ(k+1)ζ(q−k)