%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % % % Scientific Word Wrap/Unwrap Version 2.5 % % Scientific Word Wrap/Unwrap Version 3.0 % % % % If you are separating the files in this message by hand, you will % % need to identify the file type and place it in the appropriate % % directory. The possible types are: Document, DocAssoc, Other, % % Macro, Style, Graphic, PastedPict, and PlotPict. Extract files % % tagged as Document, DocAssoc, or Other into your TeX source file % % directory. Macro files go into your TeX macros directory. Style % % files are used by Scientific Word and do not need to be extracted. % % Graphic, PastedPict, and PlotPict files should be placed in a % % graphics directory. % % % % Graphic files need to be converted from the text format (this is % % done for e-mail compatability) to the original 8-bit binary format. % % % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % % % Files included: % % % % "/document/lec_2_22_00.tex", Document, 35990, 2/23/2000, 14:51:24, ""% % "/document/graphics/maroon0.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/bulletblue.bmp", ImportPict, 1862, 12/27/1999, 21:34:56, ""% % "/document/graphics/maroon2.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/maroon3.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/maroon7.wmf", ImportPict, 4444, 12/27/1999, 20:53:14, ""% % "/document/graphics/maroon4.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/maroon6__1.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%% Start /document/lec_2_22_00.tex %%%%%%%%%%%%%%%%%%% %\newtheorem{theorem}{Theorem} %\newtheorem{axiom}[theorem]{Axiom} %\newtheorem{conjecture}[theorem]{Conjecture} %\newtheorem{corollary}[theorem]{Corollary} %\newtheorem{definition}[theorem]{Definition} %\newtheorem{example}[theorem]{Example} %\newtheorem{exercise}[theorem]{Exercise} %\newtheorem{lemma}[theorem]{Lemma} %\newtheorem{proposition}[theorem]{Proposition} %\newtheorem{remark}[theorem]{Remark} \documentclass{article} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \usepackage{graphicx} \usepackage{amsmath} %TCIDATA{OutputFilter=LATEX.DLL} %TCIDATA{Created=Sunday, May 16, 1999 12:00:37} %TCIDATA{LastRevised=Wednesday, February 23, 2000 09:51:23} %TCIDATA{} %TCIDATA{} %TCIDATA{CSTFile=webmath.cst} %TCIDATA{PageSetup=72,72,72,72,0} %TCIDATA{AllPages= %F=36,\PARA{038

\hfill \thepage} %} \newtheorem{acknowledgement}[theorem]{Acknowledgement} \newtheorem{algorithm}[theorem]{Algorithm} \newtheorem{case}[theorem]{Case} \newtheorem{claim}[theorem]{Claim} \newtheorem{conclusion}[theorem]{Conclusion} \newtheorem{condition}[theorem]{Condition} \newtheorem{criterion}[theorem]{Criterion} \newtheorem{notation}[theorem]{Notation} \newtheorem{problem}[theorem]{Problem} \newtheorem{solution}[theorem]{Solution} \newtheorem{summary}[theorem]{Summary} \newenvironment{proof}[1][Proof]{\textbf{#1.} }{\ \rule{0.5em}{0.5em}} \input{tcilatex} \begin{document} \section{Ma 116 Lecture 2/22/00} \vspace{1pt} \section{Introduction to Power Series\qquad \qquad} \vspace{1pt}Please note that there is material on power series at \hyperref{% Visual Calculus}{}{}{http://archives.math.utk.edu/visual.calculus/6/}. Some of this material was used as part of the presentation of the topics that follow. \subsection{ What is a Power Series?} Recall that the geometric series $\dsum\limits_{n=0}^{\infty }ar^{n}$ converges to $\dfrac{a}{1-r}$ provided $\left| r\right| <1$. \ In this section we will take the ratio $r$ to be a variable $x$. \ In particular, the geometric series \begin{equation*} \dsum\limits_{n=0}^{\infty }ax^{n}=a+ax+ax^{2}+ax^{3}+\cdots \end{equation*} is an example of a power series. \ It converges on the interval $-11$ and converges if $L<1$. \ So, the series diverges for all $x$ except $x=-1$ and converges for $x=-1$. \ Thus, the radius of convergence is $R=0$. \FRAME{dtbpF}{4.2653in}{0.0701in}{0pt}{}{}{maroon.wmf}{\special{language "Scientific Word";type "GRAPHIC";display "PICT";valid_file "F";width 4.2653in;height 0.0701in;depth 0pt;original-width 290.5625pt;original-height 1pt;cropleft "0";croptop "0.5008";cropright "1";cropbottom "0.4992";filename 'graphics/maroon2.wmf';file-properties "XNPEU";}} \begin{center} \end{center} And some more exercises: \begin{exercise} Find the center and radius of convergence of the series $\dsum% \limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n}}{n!}\left( x-3\right) ^{n}$.\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{}...% \CustomNote[\hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{% }{}{}]{Margin Hint}{% The center is $a=2$. \ To find the radius we apply the ratio test \par \begin{equation*} a_{n}=\left( -3\right) ^{n}\left( x-2\right) ^{n}\quad \text{and\quad }% a_{n+1}=\left( -3\right) ^{n+1}\left( x-2\right) ^{n+1} \end{equation*}% \par \begin{eqnarray*} L=\lim_{n\rightarrow \infty }\dfrac{\left| a_{n+1}\right| }{\left| a_{n}\right| }=\lim_{n\rightarrow \infty }\left| \dfrac{\left( -3\right) ^{n+1}\left( x-2\right) ^{n+1}}{\left( -3\right) ^{n}\left( x-2\right) ^{n}}% \right| \\ =\lim_{n\rightarrow \infty }\left| \left( -3\right) \left( x-2\right) \right| =3\left| x-2\right| \end{eqnarray*}% \par So the series converges absolutely when $3\left| x-2\right| <1$ or $\left| x-2\right| <\dfrac{1}{3}$ and the radius of convergence is $R=\dfrac{1}{3}$. \ In conclusion, the series converges absolutely on the interval $\left( \dfrac{5}{3},\dfrac{7}{3}\right) $.} \end{exercise} \begin{exercise} Find the center and radius of convergence of the series $\dsum% \limits_{n=0}^{\infty }\dfrac{n!}{2^{n}}\left( x-1\right) ^{n}$.\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{}...% \CustomNote[\hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{% }{}{}]{Margin Hint}{% The center is $a=2$. \ To find the radius we apply the ratio test \par \begin{equation*} a_{n}=\left( -3\right) ^{n}\left( x-2\right) ^{n}\quad \text{and\quad }% a_{n+1}=\left( -3\right) ^{n+1}\left( x-2\right) ^{n+1} \end{equation*}% \par \begin{eqnarray*} L=\lim_{n\rightarrow \infty }\dfrac{\left| a_{n+1}\right| }{\left| a_{n}\right| }=\lim_{n\rightarrow \infty }\left| \dfrac{\left( -3\right) ^{n+1}\left( x-2\right) ^{n+1}}{\left( -3\right) ^{n}\left( x-2\right) ^{n}}% \right| \\ =\lim_{n\rightarrow \infty }\left| \left( -3\right) \left( x-2\right) \right| =3\left| x-2\right| \end{eqnarray*}% \par So the series converges absolutely when $3\left| x-2\right| <1$ or $\left| x-2\right| <\dfrac{1}{3}$ and the radius of convergence is $R=\dfrac{1}{3}$. \ In conclusion, the series converges absolutely on the interval $\left( \dfrac{5}{3},\dfrac{7}{3}\right) $.} \end{exercise} \FRAME{dtbpF}{4.2653in}{0.0701in}{0pt}{}{}{maroon.wmf}{\special{language "Scientific Word";type "GRAPHIC";display "PICT";valid_file "F";width 4.2653in;height 0.0701in;depth 0pt;original-width 290.5625pt;original-height 1pt;cropleft "0";croptop "0.5008";cropright "1";cropbottom "0.4992";filename 'graphics/maroon2.wmf';file-properties "XNPEU";}} \subsection{ Interval of Convergence} \vspace{1pt} \begin{definition} The Power Series Convergence Theorem implies that a power series \begin{equation*} S\left( x\right) =\dsum\limits_{n=0}^{\infty }c_{n}\left( x-a\right) ^{n}=c_{0}+c_{1}\left( x-a\right) +c_{2}\left( x-a\right) ^{2}+c_{3}\left( x-a\right) ^{3}+\cdots \end{equation*} converges on an interval called its \emph{interval of convergence}. \ This interval may consist of a single point $\left[ a\right] $, the set of all real numbers $\left( -\infty ,\infty \right) $, or a finite interval which may be open: $\left( a-R,a+R\right) $, closed: $\left[ a-R,a+R\right] $, or half open: $\left[ a-R,a+R\right) $ or $\left( a-R,a+R\right] $. \end{definition} \FRAME{dtbpF}{4.2653in}{0.0701in}{0pt}{}{}{maroon.wmf}{\special{language "Scientific Word";type "GRAPHIC";display "PICT";valid_file "F";width 4.2653in;height 0.0701in;depth 0pt;original-width 290.5625pt;original-height 1pt;cropleft "0";croptop "0.6126";cropright "1";cropbottom "0.3874";filename 'graphics/maroon3.wmf';file-properties "XNPEU";}} \vspace{1pt} We have seen that the center $a$ may be read off the series, and the radius $% R$ may be determined using the ratio test (or the root test). \ It remains to determine the convergence at the endpoints of the interval of convergence. \vspace{1pt} \begin{remark} It is much more important to be able to determine the radius of convergence than it is to be able to determine whether the series converges at the endpoints of the interval of convergence. \end{remark} \vspace{1pt} \begin{remark} You \QTR{red}{cannot use the ratio test or the root test} to determine the convergence at the endpoints, because these tests fail when $L=1$ which is precisely at the endpoints of the interval of convergence. \ You \QTR{green}{% must use some other test}. \end{remark} \vspace{1pt} The following three examples illustrate the three cases of an open, closed or half open interval of convergence. \begin{example} Find the interval of convergence of the series $\dsum\limits_{n=0}^{\infty }% \dfrac{2}{3^{n}}\left( x-1\right) ^{n}$\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{% The interval of convergence is the open interval $\left( -2,4\right) $.}...% \hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{}{}{% epowserintv1.tex} \end{example} \begin{example} Find the interval of convergence of the series $\dsum\limits_{n=0}^{\infty }% \dfrac{2}{3^{n}n^{2}}\left( x-1\right) ^{n}$\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{% The interval of convergence is the closed interval $\left[ -2,4\right] $.}...% \hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{}{}{% epowserintv2.tex} \end{example} You try this one first: \begin{example} Find the interval of convergence of the series $\dsum\limits_{n=0}^{\infty }% \dfrac{2}{3^{n}\sqrt{n}}\left( x-1\right) ^{n}$\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{% The interval of convergence is the half-open interval $\left[ -2,4\right) $.}% ...\hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{}{}{% epowserintv3.tex} \end{example} \FRAME{dtbpF}{4.2653in}{0.0701in}{0pt}{}{}{maroon.wmf}{\special{language "Scientific Word";type "GRAPHIC";display "PICT";valid_file "F";width 4.2653in;height 0.0701in;depth 0pt;original-width 290.5625pt;original-height 1pt;cropleft "0";croptop "0.3679";cropright "1";cropbottom "0.6321";filename 'graphics/maroon3.wmf';file-properties "XNPEU";}} \begin{example} \vspace{1pt}For what values of $x$ is the series \end{example} \begin{equation*} \sum\limits_{n=1}^{\infty }\dfrac{n(2x-1)^{n}}{4^{n}}=\dfrac{\left( 2x-1\right) }{4}+\dfrac{2\left( 2x-1\right) ^{2}}{16}+\dfrac{3\left( 2x-1\right) ^{3}}{64}+\cdots \end{equation*} convergent? We shall use the Ratio Test again. $\vspace{1pt}$ \begin{equation*} L=\lim\limits_{n\rightarrow \infty }\left| \dfrac{(n+1)(2x-1)^{n+1}}{4^{n+1}}% \cdot \dfrac{4^{n}}{n(2x-1)^{n}}\right| =\lim\limits_{n\rightarrow \infty }\left| \dfrac{2x-1}{4}\right| \end{equation*}% $.$ \ \vspace{1pt} For the series to converge we must have $L<1$, that is $\left| \dfrac{2x-1}{4% }\right| <1$ or equivalently $\left| \frac{1}{2}x-\frac{1}{4}\right| <1.$ Thus \begin{equation*} -1<\dfrac{x}{2}-\dfrac{1}{4}<1 \end{equation*} or \begin{equation*} -\dfrac{3}{4}<\dfrac{x}{2}<\dfrac{5}{4} \end{equation*} or finally \vspace{1pt} \begin{equation*} -\dfrac{3}{2}\dfrac{5}{2}$ and $x<-\dfrac{3}{2}$. \ The cases when $% x=-\dfrac{3}{2}$ and $x=\dfrac{5}{2}$ must be tested separately. \vspace{1pt} When $x=-\dfrac{3}{2},$ then $\sum\limits_{n=1}^{\infty }\dfrac{n(2x-1)^{n}}{% 4^{n}}=$ $\sum\limits_{n=1}^{\infty }\dfrac{n(-4)^{n}}{4^{n}}=$ $% \sum\limits_{n=1}^{\infty }n(-1)^{n},$ which diverges since the $n$ th term of this series does not go to zero as $n\rightarrow \infty .$ \vspace{1pt} When $x=\dfrac{5}{2},$ then $\sum\limits_{n=1}^{\infty }\dfrac{n(2x-1)^{n}}{% 4^{n}}=$ $\sum\limits_{n=1}^{\infty }\dfrac{n(4)^{n}}{4^{n}}=$ $% \sum\limits_{n=1}^{\infty }n,$ which again diverges. Why?% \CustomNote{AnswerNote}{% The $nth$ does not go to zero.} \vspace{1pt} \vspace{1pt}Thus this series converges in $-\dfrac{3}{2}