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XIV - \thepage } %} \input{tcilatex} \begin{document} \section{Ma 681} \vspace{1pt} \section{Lecture 14} \vspace{1pt} \subsection{Evaluation of Real Integrals} The residue theorem can be used to evaluate certain kinds of real integrals. We give a number of examples: \vspace{1pt} \subsubsection{Integrals of the Form $I=\int_{0}^{2\pi }R(\cos \theta ,\sin \theta )d\theta $} \vspace{1pt} Here $R\left( s,t\right) $ is a rational function of $s$ and $t$. We assume that $R(\cos \theta ,\sin \theta )$ is finite on $0\leq \theta \leq 2\pi $. Let $z=e^{i\theta }$. Then the interval $0\leq \theta \leq 2\pi $ yields the curve $C:\left| z\right| =1$ . Now $\cos \theta =\frac{1}{2}\left( e^{i\theta }+e^{-i\theta }\right) =\frac{% 1}{2}\left( z+\frac{1}{z}\right) $ and $\sin \theta =\frac{1}{2i}\left( e^{i\theta }-e^{-i\theta }\right) =\frac{1}{% 2i}\left( z-\frac{1}{z}\right) $ ; \vspace{1pt}also $\vspace{1pt}$ \begin{center} $dz=ie^{i\theta }d\theta =izd\theta $ or $d\theta =\left( \frac{1}{i}\right) \left( \frac{dz}{z}\right) $. \vspace{1pt} \end{center} Thus \vspace{1pt} \begin{center} $I=\oint_{C}R\left[ \frac{1}{2}\left( z+\frac{1}{z}\right) ,\frac{1}{2i}% \left( z-\frac{1}{z}\right) \right] \dfrac{1}{iz}dz$ \end{center} where $C:\left| z\right| =1$. $I$ may be evaluated by residues. Example: Evaluate $\int_{0}^{2\pi }\dfrac{\cos 2\theta }{1-2p\cos \theta +p^{2}}d\theta \qquad -1r_{0}$, \end{center} where $r_{0}$ and $k$ are sufficiently large. Hence \vspace{1pt} \begin{center} $\left| \int_{S}f\left( z\right) dz\right| <\dfrac{k}{r^{2}}\left( \pi r\right) =\dfrac{k\pi }{r}\qquad $for $r>r_{0}$ \end{center} and this $\rightarrow 0$ as $r\rightarrow \infty $, so that \vspace{1pt} \begin{center} $\int_{-\infty }^{\infty }f\left( x\right) dx=2\pi i\sum_{j=1}^{n}$Res$% \left( f,z_{j}\right) $ \end{center} Example: Evaluate $\dint_{-\infty }^{\infty }\dfrac{x^{2}}{\left( x^{2}+a^{2}\right) \left( x^{2}+b^{2}\right) }dx$ where $a,b>0$. The poles of $\frac{x^{2}}{\left( x^{2}+a^{2}\right) \left( x^{2}+b^{2}\right) }$ are at $z=\pm ai,\pm bi$. Of these only $z=ai$ and $% z=bi$ lie in the upper half plane. At $z=ai$ the residue is: \vspace{1pt} \begin{center} $\lim_{z\rightarrow ai}\left( z-ai\right) \dfrac{z^{2}}{\left( z-ai\right) \left( z+ai\right) \left( z^{2}+b^{2}\right) }=\dfrac{-a^{2}}{2ai\left( -a^{2}+b^{2}\right) }=\dfrac{a}{2i\left( a^{2}-b^{2}\right) }$ \end{center} Similarly, at $z=bi$ we have $\frac{b}{2i\left( b^{2}-a^{2}\right) }$. Hence the value of the integral is \vspace{1pt} \begin{center} $2\pi i\left[ \dfrac{a}{2i\left( a^{2}-b^{2}\right) }-\dfrac{b}{2i\left( a^{2}-b^{2}\right) }\right] =\dfrac{\pi }{a+b}$. \end{center} \vspace{1pt} \subsubsection{$\int_{-\infty }^{\infty }f\left( x\right) \cos axdx$ and $% \int_{-\infty }^{\infty }f\left( x\right) \sin axdx$} \vspace{1pt} Here $f\left( x\right) $ is a rational functional with the properties listed above. We note that \begin{center} \vspace{1pt}$\int_{-\infty }^{\infty }f\left( x\right) \cos axdx+i\int_{-\infty }^{\infty }f\left( x\right) \sin axdx=\int_{-\infty }^{\infty }e^{iax}dx$ \end{center} \vspace{1pt} The $\int_{-\infty }^{\infty }e^{iax}dx$ may be treated as above. Thus \vspace{1pt} \begin{center} $\int_{-\infty }^{\infty }e^{iax}dx=2\pi i\sum $residues of $e^{iaz}f\left( z\right) $ at its poles in the upper half plane \end{center} Therefore \vspace{1pt} $\int_{-\infty }^{\infty }f\left( x\right) \cos axdx$ \begin{center} $\vspace{1pt}$ $=-2\pi \sum $imaginary parts of residues of $e^{iaz}f\left( z\right) $ at its poles in the upper half plane \end{center} Similarly \vspace{1pt} $\int_{-\infty }^{\infty }f\left( x\right) \sin axdx$ \begin{center} $=2\pi \sum $real parts of the residues of $e^{iaz}f\left( z\right) $ at its poles in the upper half plane \end{center} Example. $\dint_{-\infty }^{\infty }\dfrac{\cos ax}{k^{2}+x^{2}}dx=\dfrac{% \pi }{k}e^{-ak}$; $\dint_{-\infty }^{\infty }\dfrac{\sin ax}{k^{2}+x^{2}}% dx=0 $ $\ \ a>0,$ \ $k>0.$ Now $f\left( z\right) =\frac{e^{iaz}}{k^{2}+z^{2}}$ has only one pole in the upper half plane, namely a single pole at $z=ik$. Therefore \begin{center} Res$\left( f\left( z\right) ,ik\right) =\lim_{z\rightarrow ik}\left( z-ik\right) \dfrac{e^{iaz}}{\left( z-ik\right) \left( z+ik\right) }=\dfrac{% e^{-ka}}{2ik}$. \end{center} Therefore \vspace{1pt} \begin{center} $\dint_{-\infty }^{\infty }\dfrac{e^{iax}}{k^{2}+x^{2}}dx=2\pi i\dfrac{% e^{-ka}}{2ik}=\dfrac{\pi }{k}e^{-ka}$ \end{center} and this yields the result. \subsubsection{\protect\vspace{1pt}Integrals $\int_{0}^{\infty }x^{a}p\left( x\right) /q\left( x\right) dx$} \vspace{1pt} Let $a$ be a real number with $0