# Evaluate $\int _0^{\infty }\frac{x^6}{\left(x^4+a^4\right)^2}dx$

The function

$$f\left(z\right)=\frac{z^6}{\left(z^4+a^4\right)^2}$$

Has the following poles of order 2:

$$z(k)=a \exp\left( \frac{\left(2k+1\right)}4 i\pi \right)$$

$$f$$ is even, therefore: $$\int _0^{+\infty }\frac{x^6}{\left(x^4+a^4\right)^2}dx =\frac{1}{2}\int _{-\infty }^{+\infty \:}\frac{x^6}{\left(x^4+a^4\right)^2}dx$$

$$\int _0^{+\infty }\frac{x^6}{\left(x^4+a^4\right)^2}dx=i\pi \sum _k\:Res\left(f,\:z\left(k\right)\right)$$

$$Res\left(f,\:z\left(k\right)\right)=\lim _{z\to z\left(k\right)}\left(\frac{1}{\left(2-1\right)!}\left(\frac{d}{dz}\right)^{2-1}\frac{z^6\left(z-z\left(k\right)\right)^2}{\left(z^4+a^4\right)^2}\right)$$

$$z^4+a^4=z^4-z_k^4\implies\dfrac{z^6(z-z_k)^2}{(z^4+a^4)^2}=\dfrac{z^6}{(z^3+z_k z^2+z_k^2 z+z_k^3)^2}$$

$$Res\left(f,\:z_k\right)=\lim _{z\to \:z_k}\left(\frac{d}{dz}\left(\frac{z^6}{\left(z^3+z_kz^2+z_k^2z+z_k^3\right)^2}\right)\right)$$

$$Res\left(f,\:z_k\right)=\frac{2z_kz^5\left(z^2+2z_kz+3z_k^2\right)}{\left(z^3+z_kz^2+z_k^2z+z_k^3\right)^3}=\frac{2z_k^6\cdot 6z_k^2}{\left(4z_k^3\right)^3}$$

$$Res\left(f,\:z_k\right)=\frac{12z_k^8}{64z_k^9}=\frac{3}{16z_k}$$

$$\int _0^{+\infty }\frac{x^6}{\left(x^4+a^4\right)^2}dx=\frac{3i\pi }{16a}\sum _{k=0}^n\:e^{-\frac{\left(2k+1\right)}{4}i\pi }$$

We consider only the residues within the upper half plane, that is to say those corresponding to $$k=0$$ and $$k=1$$.

$$\int _0^{+\infty \:}\frac{x^6}{\left(x^4+a^4\right)^2}dx=\frac{3i\pi \:}{16a}\left(e^{-\frac{i\pi }{4}\:\:}+e^{-\frac{3i\pi \:}{4}\:\:}\right)$$

$$\int _0^{+\infty \:}\frac{x^6}{\left(x^4+a^4\right)^2}dx=\frac{3i\pi \:}{16a}\left(\frac{\sqrt{2}}{2}\:-i\frac{\sqrt{2}}{2}-\frac{\sqrt{2}}{2}-i\frac{\sqrt{2}}{2}\right)$$

$$\int _0^{+\infty \:}\frac{x^6}{\left(x^4+a^4\right)^2}dx=\frac{3\pi \sqrt{2}\:}{16a}$$

• In case it helps wolframalpha.com/input/… – Syrtis Major Apr 29 at 10:00
• I'm afraid whipping out the result from an online calculator won't satisfy my maths teacher. – Velyth Apr 29 at 10:02
• I hope and wish that this would not satisfy yourself ! Cheers – Claude Leibovici Apr 29 at 10:11
• Are you open to solutions that don't use contour integration? – J.G. Apr 29 at 10:19
• I swear there was this exact same question in the review queue within the past couple of days. However I cannot find it via Approach$0$. Maybe it's closed and deleted.... or how about this older post? – Lee David Chung Lin Apr 29 at 10:26

## 2 Answers


In the last line, the last integral is equivalent to the first one: It just arises from a $$\ds{x \mapsto 1/x}$$ change of variable.

Then, \begin{align} &\bbox[10px,#ffd]{\int_{0}^{\infty}{x^{6} \over \pars{x^{4} + a^{4}}^{2}}\,\dd x} = {3 \over 8\verts{a}}\int_{0}^{\infty} {1 + 1/x^{2} \over \pars{x - 1/x}^{2} + 2}\,\dd x \\[5mm] \stackrel{x - 1/x\ \mapsto\ x}{=}\,\,\, &\ {3 \over 8\verts{a}}\int_{-\infty}^{\infty} {\dd x \over x^{2} + 2} = {3 \over 8\verts{a}}\,{1 \over 2}\,\root{2} \int_{-\infty}^{\infty}{\dd x/\root{2} \over \pars{x/\root{2}}^{2} + 1} \\[5mm] \stackrel{x/\root{2}\ \mapsto\ x}{=}\,\,\, &\ {3\root{2} \over 16\verts{a}}\ \underbrace{\int_{-\infty}^{\infty}{\dd x \over x^{2} + 1}} _{\ds{=\ \pi}}\ =\ \bbx{{3\root{2}\pi \over 16}\,{1 \over \verts{a}}} \end{align}

• nice work!!!!!!! – logo Apr 30 at 21:02

If you want an approach that doesn't require as much differentiation, first use partial fractions to write $$\dfrac{z^3}{z^4+a^4}=\sum_{k=0}^3\dfrac{c_k}{z-z(k)}$$ so $$\frac{z^6}{(z^4+a^4)^2}=\sum_k\frac{c_k^2}{(z-z(k))^2}+2\sum_{kThe first sum doesn't contribute to $$\int_{\Bbb R}\dfrac{x^6 dx}{(x^4+a^4)^2}$$, but some of the latter sum's terms do, viz. $$\oint\frac{dz}{(z-w)^{n+1}}=2\pi i\delta_{n0}$$for enclosed $$w$$. Hence$$\int_0^\infty\frac{x^6 dx}{(x^4+a^4)^2}=\pi i\sum_{kwhere $$\{z(k)|k\in S\}$$ is the set of residues your contour encloses and $$[]$$ is the Iverson bracket, i.e. $$[k\in S]$$ is $$1$$ if $$k\in S$$ or $$0$$ otherwise. If you're experienced with Beta functions, you should try calculating the integral separately with $$x=a\tan^{1/2}t$$ to double-check you get the same answer twice. (I get $$\frac{3\pi}{8a\sqrt{2}}$$.)

• Thanks for putting forth this approach but I'm really supposed to proceed as I already did, can you tell me if my work up there is close to the answer? – Velyth Apr 29 at 14:26
• I haven't checked your residues yet, but the approach I describe is basically the same; the reason residues have the formula you know they do is because of the loop integral I gave. If on the other hand you're sure your teacher doesn't want such a "first principles" approach, I guess you'll have to do things the way you already do them. – J.G. Apr 29 at 15:03