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The dumbbell contour is shown in the annex and here is my question:

How to evaluate this integral: \begin{equation} \int_0^1 \frac{\sqrt[4]{x(1-x)^3}}{(1+x)^3}dx \end{equation}


$\textbf{My Attempt:}$

step 1. \begin{equation} Let :\ I = \int_0^1 \frac{\sqrt[4]{x(1-x)^3}}{(1+x)^3}dx\\ f(z)=\frac{|z|^{\frac{1}{4}}|1-z|^{\frac{3}{4}}e^{\frac{i}{4}[arg(z)+3arg(1-z)]}}{(1+z)^3} \end{equation} step 2. \begin{equation} Let :-\pi\le arg(z) \le \pi,\ 0\le arg(1-z)\le 2\pi \end{equation} step 3. \begin{equation} \int_C =\int_{C_1}+\int_{C_2}+\int_{\gamma_1}+\int_{\gamma_2}=\int_{\Gamma^+}-\int_{\gamma^+_3} \end{equation} step 4. \begin{equation} let:\ z =t +i\epsilon\\ \int_{\gamma_1} = -\int_0^1 \frac{|t+i\epsilon|^{\frac{1}{4}}|1-t-i\epsilon|^{\frac{3}{4}}e^{\frac{i}{4}[arg(t+i\epsilon)+3arg(1-t-i\epsilon)]}}{(1+t+i\epsilon)^3}dt =\lim_{\epsilon \to 0}-e^{\frac{3\pi}{2}} \int_0^1 \frac{|t+i\epsilon|^{\frac{1}{4}}|1-t-i\epsilon|^{\frac{3}{4}}}{(1+t+i\epsilon)^3}dt = iI\\ Let: z = \epsilon e^{i\theta}\\ 0\le |\int_{C_1}| \le \epsilon ^{\frac{5}{4}}\int_0^{2\pi}\frac{|1-\epsilon e^{i\theta}|^{\frac{3}{4}}}{|1+\epsilon e^{i\theta} |^3}d\theta \le =\lim_{\epsilon \to 0} 2\pi \epsilon ^{\frac{5}{4}}\frac{|1-\epsilon|^{\frac{3}{4}}}{|1+\epsilon |^3}=0, so \int_{C_1} = 0 \\Similarly:\ \int_{\gamma_2} = I \ \ \int_{C_2}=0 \end{equation} step 5. \begin{equation} \int_{\Gamma^+}=2\pi iRes[f(z),\infty]=2\pi iRes[\frac{1}{z^2}f(\frac{1}{z}),0]=0 \\ \int_{\gamma_3^+}=2\pi iRes[f(z),-1]=\pi i\lim_{z\to -1}\frac{d^2}{dz^2}(z^{\frac{1}{4}}(1-z)^{\frac{3}{4}})=-\frac{3}{16}e^{\frac{\pi}{4}i}2^{-\frac{5}{4}}\pi i \end{equation} Hence:\begin{equation} (1+i)I =\sqrt 2 e^{\frac{\pi i}{4}}I=-(-\frac{3}{16}e^{\frac{\pi}{4}i}2^{-\frac{5}{4}}\pi i)\\ so :\ I = 3\pi 2^{-\frac{23}{4}}i \end{equation} But ,What? Why the result have "i"? Correct answer does not contain "i", is $$I = 3\pi 2^{-\frac{23}{4}}$$ Could anybody give me some hints? Thanks!

Laura Olatex
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    You might want to take a look at this – Quý Nhân Jan 18 '25 at 00:02
  • Why are you using both the dogbone and dumbell contours? – FShrike Jan 18 '25 at 00:12
  • In fact, I have seen the example problem of dumbbell contour on Wiki. However, the residue at infinity in the problem I asked is a bit strange, and the result has an extra i that is inexplicable. – Laura Olatex Jan 18 '25 at 20:02
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    FWIW, you can simplify the work slightly and avoid the residue at $-1$ altogether by substituting $x=\frac{1-y}{1+y}$:$$2^{-5/4} \int_0^1 (1-y)^{1/4} y^{3/4} , dy$$ – user170231 Jan 21 '25 at 19:00
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    Assuming you transformed this contour into this contour using the deformation principle of contours, your mistake lies within calculating the residue at $-1$ because you used $-\pi<\arg z\le\pi$ and $0\le\arg\left(1-z\right)<2\pi$. Try either (1) using $-\pi\le\arg z<\pi$ and $0\le\arg\left(1-z\right)<2\pi$, or (2) using $-\pi<\arg z\le\pi$ and $0<\arg\left(1-z\right)\le2\pi$. IDK how you should correctly guess what endpoints to include without trial and error. I can write an answer if you want. – Accelerator Jan 23 '25 at 09:16
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    @Accelerator Thank you. I would greatly appreciate it if you could write down the steps. – Laura Olatex Jan 25 '25 at 17:37

1 Answers1

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This first part of my answer is me expanding on my comment. TLDR: the mistake is within your residue calculation. Using the formula for Residues at Mulitple-Order Poles, we get

$$ \begin{align} -\frac{2\pi i}{1+i}\mathop{\mathrm{Res}}_{z=-1}f(z) &= -\frac{2\pi i}{1+i}\left(\frac{1}{(3-1)!}\right)\lim_{z\to -1}\frac{d^{3-1}}{dz^{3-1}}(z-(-1))^3f(z) \\ &= -\frac{\pi i}{1+i} \lim_{z\to -1}\frac{d^{2}}{dz^{2}}z^{\frac{1}{4}}\left(1-z\right)^{\frac{3}{4}} \\ &= \frac{3\pi i}{16\left(1+i\right)}\lim_{z\to -1}z^{-\frac{7}{4}}\left(1-z\right)^{-\frac{5}{4}} \\ &= \frac{3\pi i}{16\left(1+i\right)}\lim_{z\to -1}\exp\left(-\frac{7}{4}\ln\left|z\right|-\frac{7i}{4}\arg z\right)\exp\left(-\frac{5}{4}\ln\left|1-z\right|-\frac{5i}{4}\arg\left(1-z\right)\right) \\ &= \frac{3\pi i}{16\left(1+i\right)}\exp\left(-\frac{7}{4}\ln\left(1\right)-\frac{7i}{4}\arg\left(-1\right)\right)\exp\left(-\frac{5}{4}\ln\left(2\right)-\frac{5i}{4}\arg\left(2\right)\right)\,. \\ \end{align} $$

You defined $-\pi\le\arg z\le\pi$ and $0\le\arg\left(1-z\right)\le2\pi$. Including both endpoints of the intervals may seem like it is not that big of an issue. But to calculate $\arg(-1)$, it would equal both $-\pi$ and $\pi$. Same idea goes for calculating $\arg(2)$. Keeping functions single-valued in this situation is important.

Let's go with $-\pi<\arg z\le\pi$ and $0\le\arg\left(1-z\right)<2\pi$ for now and see what happens. The residue becomes

$$ \begin{align} \frac{3\pi i}{16\left(1+i\right)}\exp\left(-\frac{7}{4}\ln\left(1\right)-\frac{7i}{4}\arg\left(-1\right)\right)\exp\left(-\frac{5}{4}\ln\left(2\right)-\frac{5i}{4}\arg\left(2\right)\right) &= \frac{3\pi i}{16\left(1+i\right)}\exp\left(-\frac{7i}{4}\cdot\pi\right)\exp\left(-\frac{5}{4}\ln\left(2\right)-\frac{5i}{4}\cdot0\right) \\ &= 3\pi i\cdot2^{-\frac{23}{4}}\,. \\ \end{align} $$

This is where the extra $i$ factor comes from, which is obviously incorrect because the original integral equals a real number.

On the other hand, if we choose $-\pi<\arg z\le\pi$ and $0<\arg\left(1-z\right)\le2\pi$, then we get

$$ \begin{align} \frac{3\pi i}{16\left(1+i\right)}\exp\left(-\frac{7}{4}\ln\left(1\right)-\frac{7i}{4}\arg\left(-1\right)\right)\exp\left(-\frac{5}{4}\ln\left(2\right)-\frac{5i}{4}\arg\left(2\right)\right) &= \frac{3\pi i}{16\left(1+i\right)}\exp\left(-\frac{7i}{4}\cdot\pi\right)\exp\left(-\frac{5}{4}\ln\left(2\right)-\frac{5i}{4}\cdot2\pi\right)\\ &= 3\pi\cdot2^{-\frac{23}{4}}\,. \end{align} $$

Side note... I suppose that you would need to experiment with choosing which endpoints of the intervals to include and to not include when doing the calculations, because it's not clear to me how someone can guess the correct intervals immediately without trial and error.

Finally, the YouTuber "qncubed3" has a solution to a similar problem you may find useful: Complex Analysis: Dogbone Contour Example #2.


For my enjoyment and hopefully yours, here is the full solution.

Let $\mathcal{I}$ be the integral in question. Define $f: \mathbb{C}\,\backslash ([0,1]\cup\left\{-1\right\}) \to \mathbb{C}$ where

$$z \mapsto \frac{\left|z\right|^{\frac{1}{4}}\left|1-z\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg z\right)\exp\left(\frac{3i}{4}\arg\left(1-z\right)\right)}{\left(1+z\right)^{3}}\,.$$

Here, we define $-\pi<\arg z\le\pi$ and $0<\arg\left(1-z\right)\le2\pi$. We exclude $[0,1]$ from the domain. This is because when the branch of $\arg z$ lies on $(\infty,0]$ and when the branch of $\arg(1-z)$ lies on $(-\infty,1]$ (since the negative sign in front of the $z$ would "flip" the branch pointing right to the left instead), the branches "cancel out" on $(-\infty,0)$.

To prove they cancel out, we'll show that the numerator of $f$ is continuous on $(-\infty,0)$. Let $a>0$, let $\varepsilon > 0$, and $g(z)$ equal the numerator of $f(z)$. When we approach the negative real axis from above, we get

$$ \begin{align} \lim_{\varepsilon\to0^+}g(-a+i\varepsilon)&=\lim_{\varepsilon\to0^+}\left|-a+i\varepsilon\right|^{\frac{1}{4}}\left|1-(-a+i\varepsilon)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg (-a+i\varepsilon)\right)\exp\left(\frac{3i}{4}\arg\left(1-(-a+i\varepsilon)\right)\right) \\ &=\left|-a+i\cdot0\right|^{\frac{1}{4}}\left|1-\left(-a+i\cdot0\right)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\cdot\pi\right)\exp\left(\frac{3i}{4}\cdot2\pi\right) \\ &= a^{\frac{1}{4}}\left(1+a\right)^{\frac{3}{4}}\exp\left(\frac{7\pi i}{4}\right)\,. \\ \end{align} $$

When we approach the real axis from below, we get

$$ \begin{align} \lim_{\varepsilon\to0^+}g(-a-i\varepsilon)&=\lim_{\varepsilon\to0^+}\left|-a-i\varepsilon\right|^{\frac{1}{4}}\left|1-(-a-i\varepsilon)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg (-a-i\varepsilon)\right)\exp\left(\frac{3i}{4}\arg\left(1-(-a-i\varepsilon)\right)\right) \\ &=\left|-a-i\cdot0\right|^{\frac{1}{4}}\left|1-\left(-a-i\cdot0\right)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\cdot(-\pi)\right)\exp\left(\frac{3i}{4}\cdot0\right) \\ &= a^{\frac{1}{4}}\left(1+a\right)^{\frac{3}{4}}\exp\left(-\frac{\pi i}{4}\right)\,.\\ \end{align} $$

Since $\exp\left(\frac{7\pi i}{4}\right) = \exp\left(-\frac{\pi i}{4}\right)$, we have continuity across $(-\infty,0)$ for $g$.

Next, we construct a dogbone/dumbbell contour like your screenshot. Let $\mathcal{C}$ be the positively oriented contour comprised of the union of sets below:

$$ \begin{align} \gamma_1 &:= \left\{-t+i\delta \in \mathbb{C} : t \in \left[\sqrt{r^{2}-\delta^{2}}-1, -\sqrt{r^{2}-\delta^{2}}\right]\right\}\\ C_1 &:= \left\{re^{it} \in \mathbb{C} : t \in \left[\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right), 2\pi-\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)\right]\right\}\\ \gamma_2 &:= \left\{t-i\delta \in \mathbb{C} : t \in \left[\sqrt{r^{2}-\delta^{2}},1-\sqrt{r^{2}-\delta^{2}}\right]\right\}\\ C_2 &:= \left\{1+re^{it}: t \in \left[-\pi+\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right), \pi\ -\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)\right]\right\}\,.\\ \end{align} $$

Here, we define $0<\delta<r<\frac{1}{2}$ to maintain the shape of the contour. See here for how I came up with the sets above even though the link has a slightly different problem. Down below is a visual of what the union of the sets looks like.

$$ \text{Counterclockwise Dogbone/Dumbbell Contour }\mathcal{C} $$ Dogbone Contour

Here is an animation I made of how the counterclockwise traveling works.

We write the integral over $\mathcal{C}$ as

$$ \oint_{\mathcal{C}}f=\int_{\gamma_1}f+\int_{C_1}f+\int_{\gamma_2}f+\int_{C_2}f\,. $$

Applying $\delta \to 0^+$ then $r \to 0^+$ in that specific order, we get

$$ \lim_{r \to 0^+}\lim_{\delta \to 0^+}\oint_{\mathcal{C}}f=\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\gamma_1}f+\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{C_1}f+\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\gamma_2}f+\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{C_2}f $$

Evaluating the integral over $\gamma_1$ and taking the iterated limits, we get

$$ \begin{align} \lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\gamma_1}f &= \lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\sqrt{r^{2}-\delta^{2}}-1}^{-\sqrt{r^{2}-\delta^{2}}}f(-t+i\delta)\left(-dt\right)\\ &\overset{-t=x}= -\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\sqrt{r^{2}-\delta^{2}}}^{1-\sqrt{r^{2}-\delta^{2}}}f(x+i\delta)\,dx \\ &= -\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\sqrt{r^{2}-\delta^{2}}}^{1-\sqrt{r^{2}-\delta^{2}}}\frac{\left|x+i\delta\right|^{\frac{1}{4}}\left|1-\left(x+i\delta\right)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg\left(x+i\delta\right)\right)\exp\left(\frac{3i}{4}\arg\left(1-\left(x+i\delta\right)\right)\right)}{\left(1+x+i\delta\right)^{3}}\,dx\\ &= -\lim_{r \to 0^+}\int_{r}^{1-r}\frac{x^{\frac{1}{4}}\left(1-x\right)^{\frac{3}{4}}\exp\left(\frac{i}{4}\cdot0\right)\exp\left(\frac{3i}{4}\cdot2\pi\right)}{\left(1+x\right)^{3}}\,dx\\ &= i\mathcal{I}\,. \end{align} $$

I'll leave it up to you to prove that

$$ \lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{\gamma_2}f = \mathcal{I}\,. $$

To evaluate the integral over $C_2$ with the iterated limits, we first bound the modulus of that integral as $\delta \to 0^+$. We have

$$ \begin{align} \left|\lim_{\delta \to 0^+}\int_{C_{2}}f\right| &= \left|\lim_{\delta \to 0^+}\int_{-\pi+\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}^{\pi-\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}f(1+re^{it})\left(rie^{it}\,dt\right)\right| \\ &= \left|ri\lim_{\delta \to 0^+}\int_{-\pi+\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}^{\pi-\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}\frac{\left|1+re^{it}\right|^{\frac{1}{4}}\left|1-\left(1+re^{it}\right)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg\left(1+re^{it}\right)\right)\exp\left(\frac{3i}{4}\arg\left(1-\left(1+re^{it}\right)\right)\right)}{\left(1+1+re^{it}\right)^{3}}e^{it}\,dt\right| \\ &=r^{\frac{7}{4}}\left|\lim_{\delta \to 0^+}\int_{-\pi+\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}^{\pi-\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}\frac{\left|1+re^{it}\right|^{\frac{1}{4}}\exp\left(\frac{i}{4}\arg\left(1+re^{it}\right)\right)\exp\left(\frac{3i}{4}\arg\left(-re^{it}\right)\right)}{\left(2+re^{it}\right)^{3}}\,dt\right|\\ &= r^{\frac{7}{4}}\left|\int_{-\pi}^{\pi}\frac{\left|1+re^{it}\right|^{\frac{1}{4}}\exp\left(\frac{i}{4}\arg\left(1+re^{it}\right)\right)\exp\left(\frac{3i}{4}\arg\left(-re^{it}\right)\right)}{\left(2+re^{it}\right)^{3}}\,dt\right| \\ &\leq r^{\frac{7}{4}}\int_{-\pi}^{\pi}\left|\frac{\left|1+re^{it}\right|^{\frac{1}{4}}\exp\left(\frac{i}{4}\arg\left(1+re^{it}\right)\right)\exp\left(\frac{3i}{4}\arg\left(-re^{it}\right)\right)}{\left(2+re^{it}\right)^{3}}\right|\,dt \\ &\leq r^{\frac{7}{4}}\int_{-\pi}^{\pi}\frac{\left(r^{2}\sin^{2}t+\left(1+r\cos t\right)^{2}\right)^{\frac{1}{8}}}{\left(2-r\right)^{3}}\,dt\,. \\ \end{align} $$

With that upper bound in mind, we take $r \to 0^+$ and use the Squeeze Theorem to prove that

$$ \lim_{r\to0^+}\left|\lim_{\delta \to 0^+}\int_{C_{2}}f\right| = 0\,. $$

This implies

$$ \lim_{r\to0^+}\lim_{\delta \to 0^+}\int_{C_{2}}f = 0\,. $$

I'll leave it up to you to prove that

$$ \lim_{r\to0^+}\lim_{\delta \to 0^+}\int_{C_{1}}f = 0\,. $$

Gathering these four results together, we go back to our dumbbell/dogbone contour integral and get

$$ \begin{align} \require{cancel} \lim_{r \to 0^+}\lim_{\delta \to 0^+}\oint_{\mathcal{C}}f&=i\mathcal{I}+\cancelto{0}{\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{C_1}f}+\mathcal{I}+\cancelto{0}{\lim_{r \to 0^+}\lim_{\delta \to 0^+}\int_{C_2}f} \\ \frac{1}{1+i}\lim_{r \to 0^+}\lim_{\delta \to 0^+}\oint_{\mathcal{C}}f &= \mathcal{I}\,. \\ \end{align} $$

Next, we use the Principle of Deformation of Contours to transform $\mathcal{C}$ into this keyhole-like contour comprised of the union of sets below:

$$ \begin{align} \Gamma &:= \left\{Re^{it} \in \mathbb{C}: t\in\left[-\pi+\arctan\left(\frac{\delta}{\sqrt{R^{2}-\delta^{2}}}\right), \pi-\arctan\left(\frac{\delta}{\sqrt{R^{2}-\delta^{2}}}\right)\right]\right\} \\ \lambda_1 &:= \left\{t-1+i\delta: t\in\left[1-\sqrt{R^{2}-\delta^{2}},-\sqrt{r^{2}-\delta^{2}}\right]\right\} \\ \gamma_3 &:= \left\{-1-re^{-it}\in\mathbb{C}: t\in\left[\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right), 2\pi-\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)\right]\right\} \\ \lambda_2 &:= \left\{-t-1-i\delta\in\mathbb{C}: t\in\left[\sqrt{r^{2}-\delta^{2}}, \sqrt{R^{2}-\delta^{2}}-1\right]\right\}\,. \\ \end{align} $$

Just like the dumbbell/dogbone, we define $0<\delta<r<\frac{1}{2}$ to maintain the shape of the contour. We also let $R \gg 1$. See here for how I came up with the sets above even though the link has a slightly different problem. Down below is a visual of what the union of the sets looks like.

$$ \text{Counterclockwise Keyhole-Like Contour }\mathcal{C} $$

Keyhole-Like Contour

Click here for an animation for what the contour looks like being traversed in the positive direction.

We write the integral over $\mathcal{C}$ as

$$ \oint_{\mathcal{C}}f=\int_{\Gamma}f+\int_{\lambda_1}f+\int_{\gamma_3}f+\int_{\lambda_2}f\,. $$

Applying $\delta\to-^+$ on both sides, we get

$$ \lim_{\delta\to0^+}\oint_{\mathcal{C}}f=\lim_{\delta\to0^+}\int_{\Gamma}f+\lim_{\delta\to0^+}\int_{\lambda_1}f+\lim_{\delta\to0^+}\int_{\gamma_3}f+\lim_{\delta\to0^+}\int_{\lambda_2}f\,. $$

Evaluating the integral over $\lambda_1$ and taking the limit, we get

$$ \begin{align} \lim_{\delta\to0^+}\int_{\lambda_1}f &= \int_{1-\sqrt{R^{2}-\delta^{2}}}^{-\sqrt{r^{2}-\delta^{2}}}\frac{\left|t-1+i\delta\right|^{\frac{1}{4}}\left|1-\left(t-1+i\delta\right)\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg\left(t-1+i\delta\right)\right)\exp\left(\frac{3i}{4}\arg\left(1-\left(t-1+i\delta\right)\right)\right)}{\left(1+t-1+i\delta\right)^{3}}dt \\ &= \int_{1-R}^{-r}\frac{\left|t-1\right|^{\frac{1}{4}}\left|2-t\right|^{\frac{3}{4}}}{t^{3}}\exp\left(\frac{i}{4}\cdot\pi\right)\exp\left(\frac{3i}{4}\cdot2\pi\right)dt \\ &\overset{t\mapsto -t}= -\exp\left(\frac{7\pi i}{4}\right)\int_{r}^{R-1}\frac{\left|t+1\right|^{\frac{1}{4}}\left|2+t\right|^{\frac{3}{4}}}{t^{3}}dt\,. \\ \end{align} $$

I'll leave it up to you to prove that

$$ \lim_{\delta\to0^+}\int_{\lambda_2}f = \exp\left(-\frac{i\pi}{4}\right)\int_{r}^{R-1}\frac{\left|t+1\right|^{\frac{1}{4}}\left|2+t\right|^{\frac{3}{4}}}{t^{3}}dt\,. $$

We now have

$$ \require{cancel} \lim_{\delta\to0^+}\oint_{\mathcal{C}}f=\lim_{\delta\to0^+}\int_{\Gamma}f+\bcancel{\lim_{\delta\to0^+}\int_{\lambda_1}f}+\lim_{\delta\to0^+}\int_{\gamma_3}f+\bcancel{\lim_{\delta\to0^+}\int_{\lambda_2}f}\,. $$

Applying $r\to0^+$ on both sides and knowing that $\int_{\Gamma}f$ does not depend on $r$, we get

$$ \lim_{r\to0^+}\lim_{\delta\to0^+}\oint_{\mathcal{C}}f=\lim_{\delta\to0^+}\int_{\Gamma}f+\lim_{r\to0^+}\lim_{\delta\to0^+}\int_{\gamma_3}f\,. $$

Going back to the equality involving $\mathcal{I}$, we have

$$ \mathcal{I} = \frac{1}{1+i}\lim_{\delta\to0^+}\int_{\Gamma}f+\frac{1}{1+i}\lim_{r\to0^+}\lim_{\delta\to0^+}\int_{\gamma_3}f\,. $$

Applying $R\to\infty$ on both sides and knowing that $\mathcal{I}$ and $\int_{\gamma_3}f$ do not depend on $R$, we get

$$ \mathcal{I} = \frac{1}{1+i}\lim_{R\to\infty}\lim_{\delta\to0^+}\int_{\Gamma}f+\frac{1}{1+i}\lim_{r\to0^+}\lim_{\delta\to0^+}\int_{\gamma_3}f\,. $$

Next, we evaluate the first iterated limit by bounding its modulus as follows:

$$ \begin{align} \left|\lim_{\delta\to0^+}\int_{\Gamma}f\right| &= \left|\lim_{\delta\to0^+}\int_{-\pi+\arctan\left(\frac{\delta}{\sqrt{R^{2}-\delta^{2}}}\right)}^{\pi-\arctan\left(\frac{\delta}{\sqrt{R^{2}-\delta^{2}}}\right)}f\left(Re^{it}\right)\left(Rie^{it}dt\right)\right|\\ &= \left|\int_{-\pi}^{\pi}\frac{\left|Re^{it}\right|^{\frac{1}{4}}\left|1-Re^{it}\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg\left(Re^{it}\right)\right)\exp\left(\frac{3i}{4}\arg\left(1-Re^{it}\right)\right)}{\left(1+Re^{it}\right)^{3}}Rie^{it}dt\right|\\ &\leq \int_{-\pi}^{\pi}\left|\frac{\left|Re^{it}\right|^{\frac{1}{4}}\left|1-Re^{it}\right|^{\frac{3}{4}}\exp\left(\frac{i}{4}\arg\left(Re^{it}\right)\right)\exp\left(\frac{3i}{4}\arg\left(1-Re^{it}\right)\right)}{\left(1+Re^{it}\right)^{3}}Rie^{it}\right|dt \\ &= \int_{-\pi}^{\pi}\left|\frac{R^{\frac{5}{4}}\left(R^{2}\sin^{2}t+\left(1-R\cos t\right)^{2}\right)^{\frac{3}{8}}}{\left(1+Re^{it}\right)^{3}}\right|dt \\ &\leq \int_{-\pi}^{\pi}\frac{R^{\frac{5}{4}}\left(R^{2}\sin^{2}t+\left(1-R\cos t\right)^{2}\right)^{\frac{3}{8}}}{\left(R-1\right)^{3}}dt \\ \end{align} $$

Then we take $R \to \infty$ and use the Squeeze Theorem to prove that

$$ \lim_{R\to\infty}\left|\lim_{\delta\to0^+}\int_{\Gamma}f\right| = 0\,. $$

This implies

$$ \lim_{R\to\infty}\lim_{\delta\to0^+}\int_{\Gamma}f = 0\,. $$

As for the other integral, we have

$$ \begin{align} \frac{1}{1+i}\lim_{r\to0^+}\lim_{\delta\to0^+}\int_{\gamma_3}f &= \frac{1}{1+i}\lim_{r\to0^+}\lim_{\delta\to0^+}\int_{\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}^{2\pi-\arctan\left(\frac{\delta}{\sqrt{r^{2}-\delta^{2}}}\right)}f\left(-1-re^{-it}\right)d\left(-1-re^{-it}\right) \\ &= \frac{1}{1+i}\lim_{r\to0^+}\int_{0}^{2\pi}f\left(-1-re^{-it}\right)d\left(-1-re^{-it}\right) \\ &=\frac{1}{1+i}\oint_{\gamma_4}f(z)\,dz\,, \end{align} $$

where $\gamma_4$ is a small circle of radius $r$ centered at $z=-1$ traveling clockwise.

By Cauchy's Residue Theorem, we get

$$ \frac{1}{1+i}\oint_{\gamma_4}f(z)\,dz = -\frac{2\pi i}{1+i}\mathop{\mathrm{Res}}_{z=-1}f(z) $$

Just scroll to the top of this answer to see what the answer to the residue calculation is.

Now we have

$$ \require{cancel} \mathcal{I} = \frac{1}{1+i}\cancelto{0}{\lim_{R\to\infty}\lim_{\delta\to0^+}\int_{\Gamma}f}+3\pi\cdot2^{-\frac{23}{4}}\,. $$

We finally conclude with

$$ \bbox[13px,#fffbe9,border:5px inset #670000]{\int_{0}^{1}\frac{x^{\frac{1}{4}}\left(1-x\right)^{\frac{3}{4}}}{\left(1+x\right)^{3}}dx = 3\pi\cdot2^{-\frac{23}{4}}} $$

and we're finished!

Accelerator
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