Thursday, December 29, 2016

complex analysis - Residue Calculus (Computing an Improper Integral)


Use residue calculus to compute the integral $\int_{-\infty}^{\infty}\frac{1}{(z^{2}+25)(z^{2}+16)}dz$





My solution



If we add to the interval $I_{R}=[-R,R]$ add the semicircle $\gamma_{R}$ in the upper half plane with centre at the origin and radius $R$, then we obtain a closed contour $\Gamma_{R}$ (which we consider to be oriented in the positive direction over which the integral $f(z)=\frac{1}{(z^{2}+25)(z^{2}+16)}$ can be computed using the Cauchy's Residue theorem.



\begin{align}
\int_{-\infty}^{\infty}\frac{1}{(z^{2}+25)(z^{2}+16)}dz&=\lim_{R \to \infty}\int_{\Gamma_{R}}f(z)dz \\ &=2\pi i \sum_{k=1}^{2}\text{Res}(z_{k}), \quad \Im(z_{k})>0
\end{align}
In this case, we have two simple poles at the points $z_{1}=4i$ and $z_{2}=5i$. The residues are computed as
\begin{align}
\text{Res}(z_{1})&=\frac{1}{(z^{2}+25)D(z^{2}+16)}\big|_{z=4i} \\

&=\frac{1}{(z^{2}+25)2z}\big|_{z=4i} \\
&=\frac{1}{9\cdot8i} \\
&=\frac{1}{72i} \\
&=-\frac{i}{72}
\end{align}
Similarly, we get for $z_{2}$ that
\begin{align}
\text{Res}(z_{2})&=\frac{1}{D(z^{2}+25)(z^{2}+16)}\big|_{z=5i} \\
&=\frac{1}{2z(z^{2}+16)}\big|_{z=5i} \\
&=\frac{1}{10i \cdot (-9)} \\

&=-\frac{1}{90i} \\
&=\frac{i}{90}
\end{align}
Thus, we get
\begin{align}
\int_{-\infty}^{\infty}\frac{1}{(z^{2}+25)(z^{2}+16)}dz &=2\pi i \left(\text{Res}(z_{1})+\text{Res}(z_{2})\right) \\
&=2 \pi i \left(-\frac{i}{360}\right) \\
&=\frac{\pi}{180}
\end{align}




Are there other methods one could use to arrive at this answer?

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