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Residue Theorem Integral

Mathematics Asked by Anubhav Nanavaty on December 5, 2021

Studying for qualifying exams, I came across the following problem: using complex analysis, compute
$$int_{-infty}^{infty}frac{x^2sin(pi x)}{x^3-1}dx
$$

I decided to use the integrand $f(z)=frac{z^2e^{ipi z}}{z^3-1}$ (the goal is to take the imaginary part in the end), and I found that $f$ has a removable singularity at $1$. Now, it seems to me that $|f(Re^{itheta})|=O(e^R/R)$, so a semicircle countour won’t work. I also tried to use a rectangular contour (with height $2pi$) and the sides do vanish as $Rtoinfty$, but since there are quadratic terms in the integrand, I am not able to get a simple result and conclude via the residue theorem. Any ideas?

Edit: With help from the comment, I think I have a solution. Taking $f(z)$ as before, we see that $f(z)=e^{ipi z}cdot g(z)$, where $g(z)=frac{z^2}{z^3-1}$, and since $|g(Re^{itheta})|leq frac{C}{R}$, we invoke Jordan’s lemma to say that $lim_{Rtoinfty}int_{Gamma_R}f(z)=0$, where $Gamma_R$ is the upper semi-circle of radius $R$ centered at $0$. Therefore, we get:
$$int_{-infty}^{infty}frac{xe^{ipi x}}{x^3-1}=2pi i Res_{e^{2pi i/3}}f
$$

Now,
$$Res_{e^{2pi i/3}}f=lim_{zto e^{2pi i/3}}frac{ze^{ipi z}}{(z-1)(z-e^{4ipi /3})}=frac{e^{2pi i/3}exp{ipi e^{2pi i/3}}}{(e^{2pi i/3}-1)(e^{2pi i/3}-e^{4pi i/3})}
$$

This will simplify (using the fact that the roots of unity sum to $0$) to $exp{ipi( e^{2pi i/3}-2/3)}$
Now, $Im{2pi i exp{ipi( e^{2pi i/3}-2/3)}}=2pi e^{-pi sin(2pi/3)}cos[cos(2pi/3)-2/3]$. I don’t think this should be the answer, but I am unable to figure out where I went wrong.

One Answer

As suggested by Mark Viola, the contour to consider is (for small $r>0$ and large $R>0$) $$C_{r,R}=[-R,1-r]cupgamma_rcup[1+r,R]cupGamma_R,$$ where $gamma_r$ is the upper semicircle of radius $r$ centered at $1$ (oriented clockwise), and $Gamma_R$ is the upper semicircle of radius $R$ centered at $0$ (oriented counterclockwise).

As you (already) know, for $f(z)=z^2 e^{mathrm{i}pi z}/(z^3-1)$, begin{align} int_{C_{r,R}}f(z),dz&=2mathrm{i}pioperatorname*{Res}_{z=e^{2pimathrm{i}/3}}f(z);\ lim_{Rtoinfty}int_{Gamma_R}f(z),dz&=0;qquad(text{Jordan's lemma})\ lim_{rto 0}int_{gamma_r}f(z),dz&=-mathrm{i}pioperatorname*{Res}_{z=1}f(z), end{align} which gives the value of your integral, and the v.p. of the corresponding cosine integral for free: $$int_{-infty}^inftyfrac{x^2sinpi x}{x^3-1},dx=-fracpi3,qquadmathrm{v.p.}int_{-infty}^inftyfrac{x^2cospi x}{x^3-1},dx=frac{2pi}{3}e^{-pisqrt{3}/2}.$$

And, as noted by Claude Leibovici, your integral is easy to compute via real methods: begin{align} int_{-infty}^inftyfrac{x^2sinpi x}{x^3-1},dx&=frac13left(underbrace{int_{-infty}^inftyfrac{sinpi x}{x-1},dx}_{x-1=t}+underbrace{int_{-infty}^inftyfrac{(2x+1)sinpi x}{x^2+x+1},dx}_{2x+1=t}right) \&=-frac13left(underbrace{int_{-infty}^inftyfrac{sinpi t}{t},dt}_{=pi}+2underbrace{int_{-infty}^inftyfrac{tcos(pi t/2)}{t^2+3},dt}_{=0}right)=-fracpi3. end{align}

Answered by metamorphy on December 5, 2021

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