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Originally this integral was interesting for me as it represents the specific harmonic number $H_{-\frac{1}{5}}$ and also because Mathematica returned the wrong value $0$ for the integral. I posted the probem here https://mathematica.stackexchange.com/questions/215089/possible-bug-integrate1-x-1-5-1-x-x-0-1-0

Later I found a way to get a reasonable result from Mathematica which, finally I could simplify to this form.

$$\int_0^1 \frac{1-\frac{1}{\sqrt[5]{x}}}{1-x} \, dx=\frac{\pi}{2} \sqrt{1+\frac{2 \sqrt{5}}{5}}-\frac{5}{4} \log (5)-\frac{\sqrt{5}}{2} \log \left(\frac{1}{2} \left(\sqrt{5}+1\right)\right)$$

So a presumed bug by has turned finally into a nice statement. Can you prove it?

Quanto
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Dr. Wolfgang Hintze
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4 Answers4

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\begin{align} I=&\int_0^1 \frac{1-\frac{1}{\sqrt[5]{x}}}{1-x} dx \overset{ x=t^5}=\int_0^1 \frac{-5t^3}{t^4+t^3+t^2+t+1}dt\\ =&\sqrt5\int_0^1\left( \frac{2\phi_-t+1}{t^2+2\phi_-t+1} - \frac{2\phi_+t+1}{t^2+2\phi_+t+1}\right)dt \\ =&\ \sqrt5 J(\phi_-) - \sqrt5 J(\phi_+) \end{align} where $\phi_{+}=\frac{1+\sqrt5}4= \cos \frac\pi5$, $\phi_{-}=\frac{1-\sqrt5}4= \cos \frac{3\pi}5$ and

\begin{align} J(\phi) = &\int_0^1 \frac{2\phi t+1}{t^2+2\phi t+1}dt\\ = &\ \phi \ln( t^2+2\phi t+1 )|_0^1 + \frac{1-2\phi^2}{\sqrt{1-\phi^2}} \tan^{-1}\frac{t+\phi}{\sqrt{1-\phi^2}}\bigg|_0^1 \end{align}

Then \begin{align} I= &\ \sqrt5 [\phi_-\ln( 2+2\phi_-) - \phi_+\ln( 2+2\phi_+)]\\ &\>\>\> + \frac{\sqrt5\pi}{10} \bigg( 3\frac{1-2\phi_-^2}{\sqrt{1-\phi_-^2}} - \frac{1-2\phi_+^2}{\sqrt{1-\phi_+^2}} \bigg)\\ = &\ \frac{\sqrt5-5}4\ln\frac{\sqrt5-1}{2\sqrt5} - \frac{\sqrt5+5}4\ln\frac{\sqrt5+1}{2\sqrt5}\\ &\>\>\> + \frac{\sqrt5\pi}{10} \bigg( \frac{3(1+\sqrt5)}{\sqrt{10+2\sqrt5}} - \frac{1-\sqrt5}{\sqrt{10-2\sqrt5}} \bigg) \\= &-\frac{\sqrt5}2\ln\frac{\sqrt5+1}2 -\frac{5}4\ln5 + \frac{\sqrt5\pi}{10}\sqrt{5+2\sqrt5}\\ =&\ \frac{\pi}{2} \sqrt{1+\frac{2 \sqrt{5}}{5}}-\frac{5}{4} \ln5-\frac{\sqrt{5}}{2} \ln \frac{\sqrt{5}+1}2 \end{align}

Quanto
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1

My working is quite long but I got the following in the end:

$$\begin{align}F(x) = \int\dfrac{1 - \frac1{\sqrt[5]x}}{1 - x}\,\mathrm dx =&\dfrac{-\left(5 + \sqrt 5\right)\ln\left(2\sqrt[5]{x^2} + \left(1 + \sqrt{5}\right)\sqrt[5]x + 2\right) - \left(5 - \sqrt5\right)\ln\left(2\sqrt[5]{x^2} + \left(1 - \sqrt{5}\right)\sqrt[5]x + 2\right)}{4} \\ &+ \dfrac{\left(5 - \sqrt5\right)\arctan\left(\frac{4\sqrt[5]x + \sqrt5 + 1}{\sqrt2\sqrt{5 - \sqrt5}}\right)}{\sqrt2\sqrt{5 - \sqrt5}} + \dfrac{\sqrt{5 + \sqrt5}\arctan\left(\frac{4\sqrt[5]x - \sqrt5 + 1}{\sqrt2\sqrt{5 + \sqrt5}}\right)}{\sqrt2} + C\end{align}$$

And with the help of Matlab, $$F(1) - F(0) = -0.387792901804606$$

Matlab code (in case you want to verify the computation):


a = @(x)((-1*(5+sqrt(5))*log(2*x^(2/5) + (1 + sqrt(5)) * x^(1/5) + 2) - (5 - sqrt(5))*log(2*x^(2/5) + (1 - sqrt(5)) * x^(1/5) + 2))/4);
b = @(x)((5 - sqrt(5))*atan((4*x^(1/5) + sqrt(5) + 1)/(sqrt(2)*sqrt(5-sqrt(5)))) / (sqrt(2)*sqrt(5-sqrt(5))));
c = @(x)(sqrt(5 + sqrt(5))*atan((4*x^(1/5) - sqrt(5) + 1)/(sqrt(2)*sqrt(5+sqrt(5)))) / sqrt(2));

format long;
ans = (a(1) + b(1) + c(1)) - (a(0) + b(0) + c(0))
an4s
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It might be interesting to follow my derivation of the result using Mathematica here https://mathematica.stackexchange.com/questions/215089/possible-bug-integrate1-x-1-5-1-x-x-0-1-0/215093#215093

We can see that Mathematica saves us from doing some fine structured work but, as is common, the simplifying procedure is not so easy.

Actually I wanted the execute this example to get experience for other more complicated problems like What's the generating function for $\sum_{n=1}^\infty\frac{\overline{H}_n}{n^3}x^n\ ?$

Dr. Wolfgang Hintze
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Use the Taylor expansion of $\dfrac1{1-x}$ to recover a digamma expression:

$$\begin{align*} & \int_0^1 \frac{1-x^{-1/5}}{1-x} \, dx \\ &= \sum_{n\ge0} \int_0^1 \left(x^n - x^{n-1/5}\right) \, dx \\ &= \sum_{n\ge0} \left(\frac1{n+1} - \frac1{n+\frac45}\right) \\ &= \gamma + \psi\left(\frac45\right) \end{align*}$$

Gauss's digamma theorem gives an equivalent closed form; in Mathematica, see

FunctionExpand[EulerGamma + PolyGamma[0, 4/5]]

user170231
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