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How to evaluate the integral $int_0^infty fraclog xx^p (x-1), dx$?
How many ways to calculate: $sum_n=-infty^+inftyfrac1(u+n)^2$ where $u not in BbbZ$Finding integral $int_0^infty fracx^alphalogx1-x^2dx$ using complex analysis - residuesHow much makes $sumlimits_i=-infty^infty frac1i2pi+x$?integral with $logleft(fracx+1x-1right)$Evaluating $frac1pi int_0^pi e^2costheta dtheta$How to evaluate improper integral $int_0^inftyfractan^-1xe^ax-1dx$?Evaluate $int_-infty^infty fraclog(1+x^2) dx1+x^2$ Using Complex AnalysisEvaluate $int_ - infty^infty fracdx1+x^2$ using complex integrationAn explanation or hints to evaluate $textPVint_0^infty frace^-x(x+log (x))1-e^-xdx$Evaluate log improper integral $int_a^infty -frac1c (1-x) x$.Integral $int_frac12^1 fracln xsqrtx^2+1dx$How to evaluate $int_-infty^inftyfracxarctanfrac1x log(1+x^2)1+x^2dx$A line integral involving $log zeta(s)$
$begingroup$
How to evaluate the integral
$$
I_p=int_0^infty fraclog xx^p (x-1), dx,?$$
Attempt:
My first idea was to find the area of convergence, which gives me $p in (0,1)$. Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
But I'm stuck here. Because there is no term to describe $I_1$. Any ideas?
integration complex-analysis
$endgroup$
add a comment |
$begingroup$
How to evaluate the integral
$$
I_p=int_0^infty fraclog xx^p (x-1), dx,?$$
Attempt:
My first idea was to find the area of convergence, which gives me $p in (0,1)$. Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
But I'm stuck here. Because there is no term to describe $I_1$. Any ideas?
integration complex-analysis
$endgroup$
$begingroup$
math.stackexchange.com/a/2843081/155436
$endgroup$
– Count Iblis
Mar 18 at 0:18
add a comment |
$begingroup$
How to evaluate the integral
$$
I_p=int_0^infty fraclog xx^p (x-1), dx,?$$
Attempt:
My first idea was to find the area of convergence, which gives me $p in (0,1)$. Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
But I'm stuck here. Because there is no term to describe $I_1$. Any ideas?
integration complex-analysis
$endgroup$
How to evaluate the integral
$$
I_p=int_0^infty fraclog xx^p (x-1), dx,?$$
Attempt:
My first idea was to find the area of convergence, which gives me $p in (0,1)$. Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
But I'm stuck here. Because there is no term to describe $I_1$. Any ideas?
integration complex-analysis
integration complex-analysis
edited Mar 17 at 19:28
Jack
27.6k1782203
27.6k1782203
asked Mar 17 at 15:54
openspaceopenspace
3,3512822
3,3512822
$begingroup$
math.stackexchange.com/a/2843081/155436
$endgroup$
– Count Iblis
Mar 18 at 0:18
add a comment |
$begingroup$
math.stackexchange.com/a/2843081/155436
$endgroup$
– Count Iblis
Mar 18 at 0:18
$begingroup$
math.stackexchange.com/a/2843081/155436
$endgroup$
– Count Iblis
Mar 18 at 0:18
$begingroup$
math.stackexchange.com/a/2843081/155436
$endgroup$
– Count Iblis
Mar 18 at 0:18
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
We can take from here the following representation of the digamma function:
$$psi(s+1)=-gamma+int_0^1 frac1-x^s1-xdxRightarrow psi_1(s)=int_0^1 fracx^s-1ln xx-1dx$$
Now we are just one step away to solve it by splitting the integral into two pieces because we have:
$$int_1^infty fracln xx^p(x-1)dxoversetlarge x=frac1t=int_0^1 fract^p-1ln tt-1dt$$
$$int_0^infty fracln xx^p(x-1)dx=int_0^1 fracx^-pln xx-1dx+int_0^1fracx^p-1ln xx-1dx=psi_1(1-p)+psi_1(p)=boxedfracpi^2sin^2 (pi p)$$
Above follows by the the trigamma's reflection formula.
$endgroup$
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
1
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
add a comment |
$begingroup$
Well use $$Re s>0implies int_0^inftyfracue^-su1-e^-udu=sum_nge 0frac1(n+s)^2.$$With the substitution $u=ln x$ followed by the identity $int_Bbb Rg(u)du=int_0^infty [g(u)+g(-u)]du$, $$I_1=int_0^inftyfracu (e^-pu+e^-(1-p)u)1-e^-udu=sum_nge 0left(frac1(n+1-p)^2+frac1(n+p)^2right).$$
This is evaluated here as $pi^2csc^2pi p.$
$endgroup$
1
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
1
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
|
show 2 more comments
$begingroup$
The OP stated at the end of the posted question
Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
Rather than analyze a contour integral of $f(z)=fraclog^2 zz^p (z-1)$, we analyze the integral
$$I(p)=oint_C fraclog(z)z^p(z-1),dz$$
where $C$ is the classical keyhole contour where the branch cut for $log(z)$ is take along the positive real axis and $arg(z)in [0,2pi)$. Inasmuch as $fraclog(z)z^p(z-1)$ is analytic in and on $C$, Cauchy's Integral Theorem guarantees that
$$beginalign
0&=oint_C fraclog(z)z^p(z-1),dz\\
&=int_epsilon^R fraclog(x)x^p(x-1),dx+underbraceint_0^2pi fraclog(Re^iphi)R^pe^ipphi(Re^iphi-1),iRe^iphi,dphi_to 0,,textas,,Rto infty+int_R^1+nufraclog(x)+i2pix^pe^i2pi p(x-1),dx\\
&+underbraceint_2pi^pi fraclog(1+nu e^iphi)(1+nu e^iphi)^pnu e^iphi,inu e^iphi,dphi_to -ipi (i2pi )e^-i2pi p,,textas,,nuto0+int_1-nu^epsilon fraclog(x)+i2pix^pe^i2pi p(x-1),dx+underbraceint_2pi^0 fraclog(epsilon e^iphi)(epsilon e^iphi)^p(epsilon e^iphi-1),iepsilon e^iphi,dphi_to 0,,textas,,epsilonto 0tag1
endalign$$
Letting $epsilonto 0^+$, $Rto infty$, and $nu to 0^+$ in $(1)$, we obtain
$$(1-e^-i2pi p)int_0^infty fraclog(x)x^p(x-1),dx=i2pi e^-i2pi p,,textPVleft(int_0^infty frac1x^p(x-1),dxright)+(ipi)left(i2pi e^-i2pi p right)tag2$$
where $textPVleft(int_0^infty frac1x^p(x-1),dx+ipiright)$ denotes the Cauchy Principal Value (CPV) integral.
We can evaluate the CVP integral as follows.
$$beginalign
textPVint_0^infty frac1x^p(x-1),dx&=lim_nuto 0^+left(int_0^1-nufrac1x^p(x-1),dx+int_1+nu^inftyfrac1x^p(x-1),dxright)\\
&=lim_nuto 0^+left(-sum_n=0^inftyint_0^1-nux^n-p,dx+sum_n=0^inftyint_1+nu^infty x^-n-p-1,dxright)\\
&=sum_n=0^infty left(frac1n+p-frac1n-p+1right)tag3
endalign$$
The series on the right-hand side of $(3)$ is the partial fraction representation of $pi cot(pi p)$ for $pin (0,1)$ (SEE HERE).
Substituting $(3)$ into $(2)$, dividing both sides by $(1-e^-i2pi p)$, we find
$$int_0^infty fraclog(x)x^p(x-1),dx=fracpi^2sin^2(pi p)$$
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$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
add a comment |
$begingroup$
Too Long for comment
If $p≥2$ were some natural number, we could write the indefinite integral as,
$$
I = log x left[ log left( 1-frac 1x right) + sum_r=2^p frac 1(r-1)x^r-1 right] - text Li _2 left( frac 1x right) + sum_r=2^p frac 1(r-1)^2x^r-1 + C
$$
However, I don't know how this can be related to your problem, as you say $0<p<1$ which seems absurd.
$endgroup$
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
add a comment |
Your Answer
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
We can take from here the following representation of the digamma function:
$$psi(s+1)=-gamma+int_0^1 frac1-x^s1-xdxRightarrow psi_1(s)=int_0^1 fracx^s-1ln xx-1dx$$
Now we are just one step away to solve it by splitting the integral into two pieces because we have:
$$int_1^infty fracln xx^p(x-1)dxoversetlarge x=frac1t=int_0^1 fract^p-1ln tt-1dt$$
$$int_0^infty fracln xx^p(x-1)dx=int_0^1 fracx^-pln xx-1dx+int_0^1fracx^p-1ln xx-1dx=psi_1(1-p)+psi_1(p)=boxedfracpi^2sin^2 (pi p)$$
Above follows by the the trigamma's reflection formula.
$endgroup$
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
1
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
add a comment |
$begingroup$
We can take from here the following representation of the digamma function:
$$psi(s+1)=-gamma+int_0^1 frac1-x^s1-xdxRightarrow psi_1(s)=int_0^1 fracx^s-1ln xx-1dx$$
Now we are just one step away to solve it by splitting the integral into two pieces because we have:
$$int_1^infty fracln xx^p(x-1)dxoversetlarge x=frac1t=int_0^1 fract^p-1ln tt-1dt$$
$$int_0^infty fracln xx^p(x-1)dx=int_0^1 fracx^-pln xx-1dx+int_0^1fracx^p-1ln xx-1dx=psi_1(1-p)+psi_1(p)=boxedfracpi^2sin^2 (pi p)$$
Above follows by the the trigamma's reflection formula.
$endgroup$
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
1
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
add a comment |
$begingroup$
We can take from here the following representation of the digamma function:
$$psi(s+1)=-gamma+int_0^1 frac1-x^s1-xdxRightarrow psi_1(s)=int_0^1 fracx^s-1ln xx-1dx$$
Now we are just one step away to solve it by splitting the integral into two pieces because we have:
$$int_1^infty fracln xx^p(x-1)dxoversetlarge x=frac1t=int_0^1 fract^p-1ln tt-1dt$$
$$int_0^infty fracln xx^p(x-1)dx=int_0^1 fracx^-pln xx-1dx+int_0^1fracx^p-1ln xx-1dx=psi_1(1-p)+psi_1(p)=boxedfracpi^2sin^2 (pi p)$$
Above follows by the the trigamma's reflection formula.
$endgroup$
We can take from here the following representation of the digamma function:
$$psi(s+1)=-gamma+int_0^1 frac1-x^s1-xdxRightarrow psi_1(s)=int_0^1 fracx^s-1ln xx-1dx$$
Now we are just one step away to solve it by splitting the integral into two pieces because we have:
$$int_1^infty fracln xx^p(x-1)dxoversetlarge x=frac1t=int_0^1 fract^p-1ln tt-1dt$$
$$int_0^infty fracln xx^p(x-1)dx=int_0^1 fracx^-pln xx-1dx+int_0^1fracx^p-1ln xx-1dx=psi_1(1-p)+psi_1(p)=boxedfracpi^2sin^2 (pi p)$$
Above follows by the the trigamma's reflection formula.
edited Mar 17 at 18:28
answered Mar 17 at 18:20
ZackyZacky
7,75511062
7,75511062
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
1
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
add a comment |
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
1
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:26
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
$begingroup$
@MarkViola that is cute and probably what OP intended to find (seeing the complex analysis tag)! I wish I knew to do that myself from scratch (I don't have a good knowledge of complex analysis).
$endgroup$
– Zacky
Mar 18 at 9:47
1
1
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
$begingroup$
Thank you. And I thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function, which was key to my solution herein.
$endgroup$
– Mark Viola
Mar 18 at 16:45
add a comment |
$begingroup$
Well use $$Re s>0implies int_0^inftyfracue^-su1-e^-udu=sum_nge 0frac1(n+s)^2.$$With the substitution $u=ln x$ followed by the identity $int_Bbb Rg(u)du=int_0^infty [g(u)+g(-u)]du$, $$I_1=int_0^inftyfracu (e^-pu+e^-(1-p)u)1-e^-udu=sum_nge 0left(frac1(n+1-p)^2+frac1(n+p)^2right).$$
This is evaluated here as $pi^2csc^2pi p.$
$endgroup$
1
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
1
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
|
show 2 more comments
$begingroup$
Well use $$Re s>0implies int_0^inftyfracue^-su1-e^-udu=sum_nge 0frac1(n+s)^2.$$With the substitution $u=ln x$ followed by the identity $int_Bbb Rg(u)du=int_0^infty [g(u)+g(-u)]du$, $$I_1=int_0^inftyfracu (e^-pu+e^-(1-p)u)1-e^-udu=sum_nge 0left(frac1(n+1-p)^2+frac1(n+p)^2right).$$
This is evaluated here as $pi^2csc^2pi p.$
$endgroup$
1
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
1
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
|
show 2 more comments
$begingroup$
Well use $$Re s>0implies int_0^inftyfracue^-su1-e^-udu=sum_nge 0frac1(n+s)^2.$$With the substitution $u=ln x$ followed by the identity $int_Bbb Rg(u)du=int_0^infty [g(u)+g(-u)]du$, $$I_1=int_0^inftyfracu (e^-pu+e^-(1-p)u)1-e^-udu=sum_nge 0left(frac1(n+1-p)^2+frac1(n+p)^2right).$$
This is evaluated here as $pi^2csc^2pi p.$
$endgroup$
Well use $$Re s>0implies int_0^inftyfracue^-su1-e^-udu=sum_nge 0frac1(n+s)^2.$$With the substitution $u=ln x$ followed by the identity $int_Bbb Rg(u)du=int_0^infty [g(u)+g(-u)]du$, $$I_1=int_0^inftyfracu (e^-pu+e^-(1-p)u)1-e^-udu=sum_nge 0left(frac1(n+1-p)^2+frac1(n+p)^2right).$$
This is evaluated here as $pi^2csc^2pi p.$
edited Mar 18 at 17:44
answered Mar 17 at 16:57
J.G.J.G.
32.1k23250
32.1k23250
1
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
1
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
|
show 2 more comments
1
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
1
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
1
1
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
That's beautiful! Thanks for your answer, I was quite interested in this problem.+1
$endgroup$
– Awe Kumar Jha
Mar 17 at 17:02
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
$begingroup$
Using the relation: $zeta(2, x)=psi_1(x)$ further simplification is avaible using the reflection formula for the trigamma function. I didn't find that formula for the Hurwitz Zeta function (on google), altough there must exist one.
$endgroup$
– Zacky
Mar 17 at 18:22
1
1
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@Zacky Thanks; fixed.
$endgroup$
– J.G.
Mar 17 at 18:26
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
@J.G. Nice solution. I posted one in which I avoid any reference to special functions and instead uses contour integration and makes use of the partial fraction expansion of the cotangent function. Let me know what you think.
$endgroup$
– Mark Viola
Mar 18 at 3:27
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
$begingroup$
Your solution is fine; you might consider providing readers with a reference to your first equation since this is the thrust of the development. And I just thought it might be of interest to you to see THESE derivations of the partial fraction representation of the cotangent function.
$endgroup$
– Mark Viola
Mar 18 at 16:42
|
show 2 more comments
$begingroup$
The OP stated at the end of the posted question
Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
Rather than analyze a contour integral of $f(z)=fraclog^2 zz^p (z-1)$, we analyze the integral
$$I(p)=oint_C fraclog(z)z^p(z-1),dz$$
where $C$ is the classical keyhole contour where the branch cut for $log(z)$ is take along the positive real axis and $arg(z)in [0,2pi)$. Inasmuch as $fraclog(z)z^p(z-1)$ is analytic in and on $C$, Cauchy's Integral Theorem guarantees that
$$beginalign
0&=oint_C fraclog(z)z^p(z-1),dz\\
&=int_epsilon^R fraclog(x)x^p(x-1),dx+underbraceint_0^2pi fraclog(Re^iphi)R^pe^ipphi(Re^iphi-1),iRe^iphi,dphi_to 0,,textas,,Rto infty+int_R^1+nufraclog(x)+i2pix^pe^i2pi p(x-1),dx\\
&+underbraceint_2pi^pi fraclog(1+nu e^iphi)(1+nu e^iphi)^pnu e^iphi,inu e^iphi,dphi_to -ipi (i2pi )e^-i2pi p,,textas,,nuto0+int_1-nu^epsilon fraclog(x)+i2pix^pe^i2pi p(x-1),dx+underbraceint_2pi^0 fraclog(epsilon e^iphi)(epsilon e^iphi)^p(epsilon e^iphi-1),iepsilon e^iphi,dphi_to 0,,textas,,epsilonto 0tag1
endalign$$
Letting $epsilonto 0^+$, $Rto infty$, and $nu to 0^+$ in $(1)$, we obtain
$$(1-e^-i2pi p)int_0^infty fraclog(x)x^p(x-1),dx=i2pi e^-i2pi p,,textPVleft(int_0^infty frac1x^p(x-1),dxright)+(ipi)left(i2pi e^-i2pi p right)tag2$$
where $textPVleft(int_0^infty frac1x^p(x-1),dx+ipiright)$ denotes the Cauchy Principal Value (CPV) integral.
We can evaluate the CVP integral as follows.
$$beginalign
textPVint_0^infty frac1x^p(x-1),dx&=lim_nuto 0^+left(int_0^1-nufrac1x^p(x-1),dx+int_1+nu^inftyfrac1x^p(x-1),dxright)\\
&=lim_nuto 0^+left(-sum_n=0^inftyint_0^1-nux^n-p,dx+sum_n=0^inftyint_1+nu^infty x^-n-p-1,dxright)\\
&=sum_n=0^infty left(frac1n+p-frac1n-p+1right)tag3
endalign$$
The series on the right-hand side of $(3)$ is the partial fraction representation of $pi cot(pi p)$ for $pin (0,1)$ (SEE HERE).
Substituting $(3)$ into $(2)$, dividing both sides by $(1-e^-i2pi p)$, we find
$$int_0^infty fraclog(x)x^p(x-1),dx=fracpi^2sin^2(pi p)$$
$endgroup$
$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
add a comment |
$begingroup$
The OP stated at the end of the posted question
Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
Rather than analyze a contour integral of $f(z)=fraclog^2 zz^p (z-1)$, we analyze the integral
$$I(p)=oint_C fraclog(z)z^p(z-1),dz$$
where $C$ is the classical keyhole contour where the branch cut for $log(z)$ is take along the positive real axis and $arg(z)in [0,2pi)$. Inasmuch as $fraclog(z)z^p(z-1)$ is analytic in and on $C$, Cauchy's Integral Theorem guarantees that
$$beginalign
0&=oint_C fraclog(z)z^p(z-1),dz\\
&=int_epsilon^R fraclog(x)x^p(x-1),dx+underbraceint_0^2pi fraclog(Re^iphi)R^pe^ipphi(Re^iphi-1),iRe^iphi,dphi_to 0,,textas,,Rto infty+int_R^1+nufraclog(x)+i2pix^pe^i2pi p(x-1),dx\\
&+underbraceint_2pi^pi fraclog(1+nu e^iphi)(1+nu e^iphi)^pnu e^iphi,inu e^iphi,dphi_to -ipi (i2pi )e^-i2pi p,,textas,,nuto0+int_1-nu^epsilon fraclog(x)+i2pix^pe^i2pi p(x-1),dx+underbraceint_2pi^0 fraclog(epsilon e^iphi)(epsilon e^iphi)^p(epsilon e^iphi-1),iepsilon e^iphi,dphi_to 0,,textas,,epsilonto 0tag1
endalign$$
Letting $epsilonto 0^+$, $Rto infty$, and $nu to 0^+$ in $(1)$, we obtain
$$(1-e^-i2pi p)int_0^infty fraclog(x)x^p(x-1),dx=i2pi e^-i2pi p,,textPVleft(int_0^infty frac1x^p(x-1),dxright)+(ipi)left(i2pi e^-i2pi p right)tag2$$
where $textPVleft(int_0^infty frac1x^p(x-1),dx+ipiright)$ denotes the Cauchy Principal Value (CPV) integral.
We can evaluate the CVP integral as follows.
$$beginalign
textPVint_0^infty frac1x^p(x-1),dx&=lim_nuto 0^+left(int_0^1-nufrac1x^p(x-1),dx+int_1+nu^inftyfrac1x^p(x-1),dxright)\\
&=lim_nuto 0^+left(-sum_n=0^inftyint_0^1-nux^n-p,dx+sum_n=0^inftyint_1+nu^infty x^-n-p-1,dxright)\\
&=sum_n=0^infty left(frac1n+p-frac1n-p+1right)tag3
endalign$$
The series on the right-hand side of $(3)$ is the partial fraction representation of $pi cot(pi p)$ for $pin (0,1)$ (SEE HERE).
Substituting $(3)$ into $(2)$, dividing both sides by $(1-e^-i2pi p)$, we find
$$int_0^infty fraclog(x)x^p(x-1),dx=fracpi^2sin^2(pi p)$$
$endgroup$
$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
add a comment |
$begingroup$
The OP stated at the end of the posted question
Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
Rather than analyze a contour integral of $f(z)=fraclog^2 zz^p (z-1)$, we analyze the integral
$$I(p)=oint_C fraclog(z)z^p(z-1),dz$$
where $C$ is the classical keyhole contour where the branch cut for $log(z)$ is take along the positive real axis and $arg(z)in [0,2pi)$. Inasmuch as $fraclog(z)z^p(z-1)$ is analytic in and on $C$, Cauchy's Integral Theorem guarantees that
$$beginalign
0&=oint_C fraclog(z)z^p(z-1),dz\\
&=int_epsilon^R fraclog(x)x^p(x-1),dx+underbraceint_0^2pi fraclog(Re^iphi)R^pe^ipphi(Re^iphi-1),iRe^iphi,dphi_to 0,,textas,,Rto infty+int_R^1+nufraclog(x)+i2pix^pe^i2pi p(x-1),dx\\
&+underbraceint_2pi^pi fraclog(1+nu e^iphi)(1+nu e^iphi)^pnu e^iphi,inu e^iphi,dphi_to -ipi (i2pi )e^-i2pi p,,textas,,nuto0+int_1-nu^epsilon fraclog(x)+i2pix^pe^i2pi p(x-1),dx+underbraceint_2pi^0 fraclog(epsilon e^iphi)(epsilon e^iphi)^p(epsilon e^iphi-1),iepsilon e^iphi,dphi_to 0,,textas,,epsilonto 0tag1
endalign$$
Letting $epsilonto 0^+$, $Rto infty$, and $nu to 0^+$ in $(1)$, we obtain
$$(1-e^-i2pi p)int_0^infty fraclog(x)x^p(x-1),dx=i2pi e^-i2pi p,,textPVleft(int_0^infty frac1x^p(x-1),dxright)+(ipi)left(i2pi e^-i2pi p right)tag2$$
where $textPVleft(int_0^infty frac1x^p(x-1),dx+ipiright)$ denotes the Cauchy Principal Value (CPV) integral.
We can evaluate the CVP integral as follows.
$$beginalign
textPVint_0^infty frac1x^p(x-1),dx&=lim_nuto 0^+left(int_0^1-nufrac1x^p(x-1),dx+int_1+nu^inftyfrac1x^p(x-1),dxright)\\
&=lim_nuto 0^+left(-sum_n=0^inftyint_0^1-nux^n-p,dx+sum_n=0^inftyint_1+nu^infty x^-n-p-1,dxright)\\
&=sum_n=0^infty left(frac1n+p-frac1n-p+1right)tag3
endalign$$
The series on the right-hand side of $(3)$ is the partial fraction representation of $pi cot(pi p)$ for $pin (0,1)$ (SEE HERE).
Substituting $(3)$ into $(2)$, dividing both sides by $(1-e^-i2pi p)$, we find
$$int_0^infty fraclog(x)x^p(x-1),dx=fracpi^2sin^2(pi p)$$
$endgroup$
The OP stated at the end of the posted question
Now let's consider the function $f(z) = fraclog^2 zz^p (z-1)$. Then we can use Cauchy theorem and residuals.
Rather than analyze a contour integral of $f(z)=fraclog^2 zz^p (z-1)$, we analyze the integral
$$I(p)=oint_C fraclog(z)z^p(z-1),dz$$
where $C$ is the classical keyhole contour where the branch cut for $log(z)$ is take along the positive real axis and $arg(z)in [0,2pi)$. Inasmuch as $fraclog(z)z^p(z-1)$ is analytic in and on $C$, Cauchy's Integral Theorem guarantees that
$$beginalign
0&=oint_C fraclog(z)z^p(z-1),dz\\
&=int_epsilon^R fraclog(x)x^p(x-1),dx+underbraceint_0^2pi fraclog(Re^iphi)R^pe^ipphi(Re^iphi-1),iRe^iphi,dphi_to 0,,textas,,Rto infty+int_R^1+nufraclog(x)+i2pix^pe^i2pi p(x-1),dx\\
&+underbraceint_2pi^pi fraclog(1+nu e^iphi)(1+nu e^iphi)^pnu e^iphi,inu e^iphi,dphi_to -ipi (i2pi )e^-i2pi p,,textas,,nuto0+int_1-nu^epsilon fraclog(x)+i2pix^pe^i2pi p(x-1),dx+underbraceint_2pi^0 fraclog(epsilon e^iphi)(epsilon e^iphi)^p(epsilon e^iphi-1),iepsilon e^iphi,dphi_to 0,,textas,,epsilonto 0tag1
endalign$$
Letting $epsilonto 0^+$, $Rto infty$, and $nu to 0^+$ in $(1)$, we obtain
$$(1-e^-i2pi p)int_0^infty fraclog(x)x^p(x-1),dx=i2pi e^-i2pi p,,textPVleft(int_0^infty frac1x^p(x-1),dxright)+(ipi)left(i2pi e^-i2pi p right)tag2$$
where $textPVleft(int_0^infty frac1x^p(x-1),dx+ipiright)$ denotes the Cauchy Principal Value (CPV) integral.
We can evaluate the CVP integral as follows.
$$beginalign
textPVint_0^infty frac1x^p(x-1),dx&=lim_nuto 0^+left(int_0^1-nufrac1x^p(x-1),dx+int_1+nu^inftyfrac1x^p(x-1),dxright)\\
&=lim_nuto 0^+left(-sum_n=0^inftyint_0^1-nux^n-p,dx+sum_n=0^inftyint_1+nu^infty x^-n-p-1,dxright)\\
&=sum_n=0^infty left(frac1n+p-frac1n-p+1right)tag3
endalign$$
The series on the right-hand side of $(3)$ is the partial fraction representation of $pi cot(pi p)$ for $pin (0,1)$ (SEE HERE).
Substituting $(3)$ into $(2)$, dividing both sides by $(1-e^-i2pi p)$, we find
$$int_0^infty fraclog(x)x^p(x-1),dx=fracpi^2sin^2(pi p)$$
edited Mar 18 at 16:10
answered Mar 18 at 3:24
Mark ViolaMark Viola
134k1278176
134k1278176
$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
add a comment |
$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
$begingroup$
@openspace Please let me know how I can improve my answer. I really want to give you the best answer I can. And feel free to up vote an answer as you see fit.
$endgroup$
– Mark Viola
2 days ago
add a comment |
$begingroup$
Too Long for comment
If $p≥2$ were some natural number, we could write the indefinite integral as,
$$
I = log x left[ log left( 1-frac 1x right) + sum_r=2^p frac 1(r-1)x^r-1 right] - text Li _2 left( frac 1x right) + sum_r=2^p frac 1(r-1)^2x^r-1 + C
$$
However, I don't know how this can be related to your problem, as you say $0<p<1$ which seems absurd.
$endgroup$
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
add a comment |
$begingroup$
Too Long for comment
If $p≥2$ were some natural number, we could write the indefinite integral as,
$$
I = log x left[ log left( 1-frac 1x right) + sum_r=2^p frac 1(r-1)x^r-1 right] - text Li _2 left( frac 1x right) + sum_r=2^p frac 1(r-1)^2x^r-1 + C
$$
However, I don't know how this can be related to your problem, as you say $0<p<1$ which seems absurd.
$endgroup$
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
add a comment |
$begingroup$
Too Long for comment
If $p≥2$ were some natural number, we could write the indefinite integral as,
$$
I = log x left[ log left( 1-frac 1x right) + sum_r=2^p frac 1(r-1)x^r-1 right] - text Li _2 left( frac 1x right) + sum_r=2^p frac 1(r-1)^2x^r-1 + C
$$
However, I don't know how this can be related to your problem, as you say $0<p<1$ which seems absurd.
$endgroup$
Too Long for comment
If $p≥2$ were some natural number, we could write the indefinite integral as,
$$
I = log x left[ log left( 1-frac 1x right) + sum_r=2^p frac 1(r-1)x^r-1 right] - text Li _2 left( frac 1x right) + sum_r=2^p frac 1(r-1)^2x^r-1 + C
$$
However, I don't know how this can be related to your problem, as you say $0<p<1$ which seems absurd.
answered Mar 17 at 16:39
Awe Kumar JhaAwe Kumar Jha
570113
570113
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
add a comment |
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
$begingroup$
web.archive.org/web/20110617053801/http://www.math.tu-berlin.de/…
$endgroup$
– Count Iblis
Mar 18 at 0:13
add a comment |
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– Count Iblis
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