How to Prove this Interesting Integral Equation?

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Discussion Overview

The discussion revolves around proving the integral equation $$\int^1_0 \frac{dx}{\sqrt[3]{x^2-x^3}} = \frac{2\pi }{\sqrt{3}}$$ Participants explore various methods for proving this equation, including the use of the Beta function, contour integration, and transformations of the integral.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using the Beta function and the complement formula as a hint for the proof.
  • Another participant mentions that there is a potentially more complicated method available.
  • A different participant proposes solving the integral using contour integration, indicating that this approach is lengthy and may require further elaboration later.
  • The contour integration method involves a substitution that transforms the integral into a different form, leading to a complex analysis approach.
  • Details are provided about the contour integration process, including the choice of branch cuts and the use of a key-hole contour.
  • Participants discuss the evaluation of integrals along different paths and the calculation of residues, particularly at the simple pole at -1.
  • One participant concludes that the principal value of the integral converges to the desired result, although this is presented without consensus from others.

Areas of Agreement / Disagreement

There is no consensus on a single method for proving the integral. Multiple approaches are proposed, and participants express varying levels of complexity and detail in their methods.

Contextual Notes

The discussion includes various mathematical steps and transformations that are not fully resolved, and assumptions regarding convergence and the validity of the methods used are present but not explicitly stated.

alyafey22
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Prove that

$$\int^1_0 \frac{dx}{\sqrt[3]{x^2-x^3}} = \frac{2\pi }{\sqrt{3}}$$
 
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Hint: Use the Beta function and the complement formula

$$\int_0^1\frac{dx}{\sqrt[3]{x^2-x^3}}=\int_0^1x^{-2/3}(1-x)^{-1/3}dx=B\left(\frac{1}{3},\frac{2}{3}\right)= \frac{\Gamma\left( \frac{1}{3}\right)\Gamma\left(\frac{2}{3}\right)}{\Gamma\left(\frac{1}{3}+\frac{2}{3}\right)}=\frac{\pi\;/\sin (\pi/3)}{1}=\frac{2\pi}{\sqrt{3}}$$
 
Yep , still there is another way . Possibly more complicated .
 
It can be solved using contour integration . The approach is quite long I might post it later .

On the other hand we can transform to another contour integration using a substitution .

$$t = \frac{1}{x}-1 \,\,\, dt = - \frac{1}{x^2}dx$$

$$\int^{\infty}_0 \frac{dt}{ \sqrt[3]{t}(t+1)}$$

we can use the following function to integrate

$$f(z) = \frac{e^{\frac{-1}{3}\text{Log}_0(z)}}{z+1}$$

Notice we are choosing a branch cut along the positive x-axis .

We will use a key-hole contour as indicated in the picture .

View attachment 909

$$\oint_{\Gamma_2} f(z) \, dz +\oint_{\Gamma_4} f(z) \, dz+ \int_{\Gamma_3} f(z) \, dz + \int_{\Gamma_1} f(z) \, dz = 2\pi i \text{Res}(f(z) ; -1) $$

Integration along the big circle :

$$\oint_{\Gamma_2}\frac{e^{\frac{-1}{3}\text{Log}_0(z)}}{z+1}\, dz $$

We will use the following paramaterization $$z = Re^{it}\,\,\, 0< t < 2\pi $$

$$iR\int_{0}^{2\pi }\frac{e^{it}\, e^{\frac{-1}{3}\text{Log}_{0}(Re^{it})}}{Re^{it}+1}\, dt \leq 2 \pi \frac{R^{1-\frac{1}{3}}}{R-1} $$

Now take $R$ to be arbitrarily large

$$\lim_{R \to \infty}2 \pi \frac{R^{\frac{2}{3}}}{R-1} = 0 $$

Similarity the integration along the smaller circle goes to $0$ as $$r \to 0$$ .

Integration along the x-axis :

$$\text{P.V} \int^{\infty}_{0} \frac{e^{\frac{-1}{3}\text{Log}_0(z)}}{z+1}\, dz $$

Along the positive x-axis we get the following

$$\text{P.V} \int^{\infty}_{0} \frac{e^{\frac{-1}{3}\ln(x)}}{x+1}\, dx$$

For the other integral on the opposite direction

$$\text{P.V} \int^{0}_{\infty} \frac{e^{\frac{-1}{3}(\ln(x)+2\pi i)}}{x+1}\, dx$$

The residue at $$-1$$

Since we have a simple pole

$$\text{Res}(f;-1)= e^{-\frac{1}{3}\text{Log}_0(-1)} = e^{-\frac{\pi}{3} i} $$

Final step

$$\text{P.V}(1-e^{-\frac{2\pi}{3} i } )\int^{\infty}_{0} \frac{1}{\sqrt[3]{x}(x+1)}\, dx = 2\pi i e^{-\frac{\pi}{3} i} $$

$$\text{P.V}\int^{\infty}_{0} \frac{1}{\sqrt[3]{x}(x+1)}\, dx = 2\pi i \frac{e^{-\frac{\pi}{3} i}}{1-e^{-\frac{2\pi}{3} i } } $$

We can easily prove that $$2\pi i \frac{e^{-\frac{\pi}{3} i}}{1-e^{-\frac{2\pi}{3} i } }=\frac{\pi }{\sin \left( \frac{\pi}{3}\right)}=\frac{2\pi }{\sqrt{3}}$$

Hence the result

$$\text{P.V}\int^{\infty}_{0} \frac{1}{\sqrt[3]{x}(x+1)}\, dx = \frac{2\pi }{\sqrt{3}} $$

We can easily prove that the integral converges hence the principle value is equal to the integral $$\square$$ .
 

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