%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % % % 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_23_00.tex", Document, 30561, 2/23/2000, 15:16:14, ""% % "/document/graphics/maroon7__1.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/maroon8.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/maroon9.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % "/document/graphics/maroonA.wmf", ImportPict, 4444, 12/27/1999, 20:52:56, ""% % % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%% Start /document/lec_2_23_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=Wednesday, February 23, 2000 09:46:56} %TCIDATA{LastRevised=Wednesday, February 23, 2000 10:16:12} %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{\protect\vspace{1pt}Ma 116 Lecture 2/23/00} \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{ Operations on Power Series --- Addition and Subtraction} \vspace{1pt} \begin{theorem} Let $f\left( x\right) =\dsum\limits_{n=0}^{\infty }c_{n}\left( x-a\right) ^{n}$ and $g\left( x\right) =\dsum\limits_{n=0}^{\infty }d_{n}\left( x-a\right) ^{n}$ be power series centered at $x=a$ and let $R_{f}$ and $% R_{g} $\ be their radii of convergence, respectively. \ Further, let $R=\min \left( R_{f},R_{g}\right) $ be the smaller of these two radii. \ Then the sum and difference of $f$ and $g$ may be computed term by term and the radius of convergence is at least $R$ \end{theorem} \textsl{Addition:} \begin{equation*} f\left( x\right) +g\left( x\right) =\dsum\limits_{n=0}^{\infty }\left( c_{n}+d_{n}\right) \left( x-a\right) ^{n} \end{equation*} \textsl{Subtraction:} \begin{equation*} f\left( x\right) -g\left( x\right) =\dsum\limits_{n=0}^{\infty }\left( c_{n}-d_{n}\right) \left( x-a\right) ^{n} \end{equation*} \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/maroon7__1.wmf';file-properties "XNPEU";}} \vspace{1pt} \begin{example} Find the power series expansion for $\dfrac{5x}{6x^{2}-x-1}$ centered about $% x=0$, and find its radius of convergence. \end{example} \emph{Solution:} \ We first factor the denominator and do a partial fraction expansion to get \begin{eqnarray*} \dfrac{5x}{6x^{2}-x-1} &=&\dfrac{5x}{\left( 3x+1\right) \left( 2x-1\right) } \\ &=&\frac{1}{1+3x}-\dfrac{1}{1-2x}\quad \text{(Lots of work here)} \end{eqnarray*} Each of these fractions is the sum of a geometric series with a different ratio: \begin{eqnarray*} \frac{1}{1+3x} &=&\dsum\limits_{n=0}^{\infty }\left( -3x\right) ^{n}=\dsum\limits_{n=0}^{\infty }\left( -3\right) ^{n}x^{n}\quad \text{for }% \left| 3x\right| <1\text{ or }\left| x\right| <\dfrac{1}{3} \\ \frac{1}{1-2x} &=&\dsum\limits_{n=0}^{\infty }\left( 2x\right) ^{n}=\dsum\limits_{n=0}^{\infty }2^{n}x^{n}\quad \text{for }\left| 2x\right| <1\text{ or }\left| x\right| <\dfrac{1}{2} \end{eqnarray*} Subtracting these, we find \begin{eqnarray*} \dfrac{5x}{6x^{2}-x-1} &=&\dsum\limits_{n=0}^{\infty }\left( -3\right) ^{n}x^{n}-\dsum\limits_{n=0}^{\infty }2^{n}x^{n} \\ &=&\dsum\limits_{n=0}^{\infty }\left[ \left( -3\right) ^{n}-2^{n}\right] x^{n}\quad \text{for }\left| x\right| <\dfrac{1}{3} \end{eqnarray*} At this point, we know the radius of convergence is at least $\dfrac{1}{3}$, but we need to find it explicitly using the ratio test: \begin{eqnarray*} L &=&\lim_{n\rightarrow \infty }\dfrac{\left| a_{n+1}\right| }{\left| a_{n}\right| }=\lim_{n\rightarrow \infty }\dfrac{\left| \left( -3\right) ^{n+1}-2^{n+1}\right| \left| x\right| ^{n+1}}{\left| \left( -3\right) ^{n}-2^{n}\right| \left| x\right| ^{n}} \\ &=&\left| x\right| \lim_{n\rightarrow \infty }\dfrac{\left| \left( -3\right) ^{n+1}-2^{n+1}\right| }{\left| \left( -3\right) ^{n}-2^{n}\right| }\cdot \dfrac{\dfrac{1}{3^{n}}}{\dfrac{1}{3^{n}}} \\ &=&\left| x\right| \lim_{n\rightarrow \infty }\dfrac{\left| \dfrac{\left( -3\right) ^{n+1}}{3^{n}}-\dfrac{2^{n+1}}{3^{n}}\right| }{\left| \dfrac{% \left( -3\right) ^{n}}{3^{n}}-\dfrac{2^{n}}{3^{n}}\right| } \\ &=&\left| x\right| \lim_{n\rightarrow \infty }\dfrac{\left| \left( -1\right) ^{n+1}3-0\right| }{\left| \left( -1\right) ^{n}-0\right| }=3\left| x\right| \end{eqnarray*} The series converges when $L=3\left| x\right| <1$ or $\left| x\right| <% \dfrac{1}{3}$. \ So the the radius of convergence is $R=\dfrac{1}{3}$ after all. \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/maroon7__1.wmf';file-properties "XNPEU";}} \fbox{\underline{% \CustomNote[When is the radius of convergence not the minimum?]{Margin Hint}{% In the previous example, we saw \begin{equation*} \frac{1}{1+3x}=\dsum\limits_{n=0}^{\infty }\left( -3\right) ^{n}x^{n} \end{equation*}% \par and \begin{eqnarray*} \dfrac{5x}{6x^{2}-x-1} &=&\frac{1}{1+3x}-\dfrac{1}{1-2x} \\ &=&\dsum\limits_{n=0}^{\infty }\left[ \left( -3\right) ^{n}-2^{n}\right] x^{n} \end{eqnarray*}% \par both have radius of convergence $R=\dfrac{1}{3}$. \ So their difference \begin{eqnarray*} \frac{1}{1+3x}-\dfrac{5x}{6x^{2}-x-1} &=&\dfrac{1}{1-2x} \\ &=&\dsum\limits_{n=0}^{\infty }2^{n}x^{n} \end{eqnarray*}% \par has radius of convergence at least $\dfrac{1}{3}$. \ In fact, the radius is $% \dfrac{1}{2}$ which is larger.}}} \vspace{1pt} \begin{exercise} Find the power series expansion for $\dfrac{4x}{1-4x^{2}}$ centered about $% x=0$, and find its radius of convergence.\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETProbSolvHint}{0.1505in}{0.1833in}{0in% }}{}{}{}]{Margin Hint}{% The partial fraction expansion is \par \quad $\dfrac{4x}{1-4x^{2}}=\dfrac{1}{1-2x}-\dfrac{1}{1+2x}$}...% \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{$\dfrac{4x}{1-4x^{2}}=\dsum\limits_{n=0}^{\infty }\left[ 2^{n}-\left( -2\right) ^{n}\right] x^{n}=\dsum\limits_{k=0}^{\infty }2^{2k+2}x^{2k+1}$% \par The radius of convergence is $R=\dfrac{1}{2}$.}...% \CustomNote[\hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{% }{}{}]{Margin Hint}{% The partial fraction expansion is \par \quad $\dfrac{4x}{1-4x^{2}}=\dfrac{1}{1-2x}-\dfrac{1}{1+2x}$% \par Each term is a geometric series: \par $\quad \dfrac{1}{1-2x}=\dsum\limits_{n=0}^{\infty }2^{n}x^{n}\quad $for $% \left| x\right| <\dfrac{1}{2}$% \par $\quad \dfrac{1}{1+2x}=\dsum\limits_{n=0}^{\infty }\left( -2\right) ^{n}x^{n}\quad $for $\left| x\right| <\dfrac{1}{2}$% \par So the difference is \par $\quad \dfrac{4x}{1-4x^{2}}=\dsum\limits_{n=0}^{\infty }\left[ 2^{n}-\left( -2\right) ^{n}\right] x^{n}\quad $for $\left| x\right| <\dfrac{1}{2}$% \par Notice that the coefficients of the even order terms are all zero. \ So we only need the odd terms. \ These may be specified by setting $n=2k+1$. \ Thus the series may be written as \par $\quad \dfrac{4x}{1-4x^{2}}=\dsum\limits_{k=0}^{\infty }\left[ 2^{2k+1}-\left( -2\right) ^{2k+1}\right] x^{2k+1}$% \par \quad $=\dsum\limits_{k=0}^{\infty }\left[ 2^{2k+1}+2^{2k+1}\right] x^{2k+1}\allowbreak =\dsum\limits_{k=0}^{\infty }2^{2k+2}x^{2k+1}$% \par Using the ratio test we find the radius of convergence is in fact $R=\dfrac{1% }{2}$.} \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/maroon7__1.wmf';file-properties "XNPEU";}} \subsection{ Operations on Power Series --- Multiplication by Polynomials} \vspace{1pt} Let $f\left( x\right) =\dsum\limits_{n=0}^{\infty }c_{n}\left( x-a\right) ^{n}$ be a power series centered at $x=a$ and whose radius of convergence is $R$. \ Also let $k$ be a constant and let $p$ be a non-negative integer. \ Then: \textsl{Muliplication by a Polynomial:} \begin{equation*} k\left( x-a\right) ^{p}f\left( x\right) =\dsum\limits_{n=0}^{\infty }kc_{n}\left( x-a\right) ^{n+p}=\dsum\limits_{i=p}^{\infty }kc_{i-p}\left( x-a\right) ^{i} \end{equation*} which also has radius of convergence $R.$ \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/maroon8.wmf';file-properties "XNPEU";}} \vspace{1pt} The following series was found above, \ but here is an easier derivation.: \begin{example} Find the power series expansion for $\dfrac{4x}{1-4x^{2}}$ centered about $% x=0$, and find its radius of convergence. \end{example} \emph{Solution:} \ We first use substitution to write $\dfrac{1}{1-4x^{2}}$ as a geometric series: \begin{equation*} \dfrac{1}{1-4x^{2}}=\dsum\limits_{n=0}^{\infty }\left( 4x^{2}\right) ^{n}=\dsum\limits_{n=0}^{\infty }4^{n}x^{2n}\quad \text{for }\left| 4x^{2}\right| <1\text{ or }\left| x\right| <\dfrac{1}{2} \end{equation*} Finally we multiply by $4x$: \begin{equation*} \dfrac{4x}{1-4x^{2}}=\dsum\limits_{n=0}^{\infty }4^{n+1}x^{2n+1}\quad \text{% for }\left| x\right| <\dfrac{1}{2} \end{equation*} \begin{remark} This agrees with the previous result \begin{equation*} \dfrac{4x}{1-4x^{2}}=\dsum\limits_{k=0}^{\infty }2^{2k+2}x^{2k+1} \end{equation*} \end{remark} \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/maroon8.wmf';file-properties "XNPEU";}} \vspace{1pt} \begin{exercise} Find the power series expansion for $\dfrac{3x^{2}}{1+x^{3}}$ centered about $x=0$, and find its radius of convergence.\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{$\dfrac{3x^{2}}{1+x^{3}}=\dsum\limits_{n=0}^{\infty }3x^{3n+2}\quad $for $\left| x\right| <1$}...% \CustomNote[\hyperref{\TCIIcon{BITMAPSETSolution}{0.1609in}{0.1609in}{0in}}{% }{}{}]{Margin Hint}{% We first use substitution to write $\dfrac{1}{1+x^{3}}$ as a geometric series: \begin{equation*} \dfrac{1}{1+x^{3}}=\dsum\limits_{n=0}^{\infty }\left( -x^{3}\right) ^{n}=\dsum\limits_{n=0}^{\infty }\left( -1\right) ^{n}x^{3n}\quad \text{for }% \left| x\right| <1 \end{equation*}% \par Finally we multiply by $3x^{2}$: \begin{equation*} \dfrac{3x^{2}}{1+x^{3}}=\dsum\limits_{n=0}^{\infty }3\left( -1\right) ^{n}x^{3n+2}\quad \text{for }\left| x\right| <1 \end{equation*}% } \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/maroon8.wmf';file-properties "XNPEU";}} \subsection{ Operations on Power Series --- Differentiation} \vspace{1pt} Let $f\left( x\right) =\dsum\limits_{n=0}^{\infty }c_{n}\left( x-a\right) ^{n}$ be a power series centered at $x=a$ and whose radius of convergence is $R$. \ Then the derivative of $f$ may be computed by differentiating the terms of the series for $f$: \textsl{Differentiation:} \begin{eqnarray*} f\,^{\prime }\left( x\right) &=&\dsum\limits_{n=0}^{\infty }nc_{n}\left( x-a\right) ^{n-1}=\dsum\limits_{n=1}^{\infty }nc_{n}\left( x-a\right) ^{n-1} \\ &=&\dsum\limits_{k=0}^{\infty }\left( k+1\right) c_{k+1}\left( x-a\right) ^{k} \end{eqnarray*} which also has radius of convergence $R.$ \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/maroon9.wmf';file-properties "XNPEU";}} \vspace{1pt} \begin{example} Find the power series expansion for $\dfrac{3x^{2}}{\left( 1-x^{3}\right) ^{2}}$ centered about $x=0$, and find its radius of convergence. \end{example} \emph{Solution:} \ We first notice that $\dfrac{3x^{2}}{\left( 1-x^{3}\right) ^{2}}$ is the derivative of $\dfrac{1}{1-x^{3}}$ whose series is \begin{equation*} \dfrac{1}{1-x^{3}}=\dsum\limits_{n=0}^{\infty }\left( x^{3}\right) ^{n}=\dsum\limits_{n=0}^{\infty }x^{3n}\quad \text{for }\left| x\right| <1 \end{equation*} So we differentiate this series term by term: \begin{equation*} \dfrac{3x^{2}}{\left( 1-x^{3}\right) ^{2}}=\dsum\limits_{n=0}^{\infty }3nx^{3n-1}=\dsum\limits_{n=1}^{\infty }3nx^{3n-1}\quad \text{for }\left| x\right| <1 \end{equation*} Notice that we can drop the $n=0$ term because it is zero. \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/maroon9.wmf';file-properties "XNPEU";}} \vspace{1pt} \begin{exercise} Find the power series expansion for $\dfrac{1}{\left( 1+2x\right) ^{2}}$ centered about $x=0$, and find its radius of convergence.\dotfill \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/maroon9.wmf';file-properties "XNPEU";}} \subsection{ Operations on Power Series --- Integration} \vspace{1pt} Let $f\left( x\right) =\dsum\limits_{n=0}^{\infty }c_{n}\left( x-a\right) ^{n}$ be a power series centered at $x=a$ and whose radius of convergence is $R$. \ Then the integral of $f$ may be computed by integrating the terms of the series for $f$ and adding a constant of integration: \textsl{Integration:} \begin{eqnarray*} \dint f\left( x\right) \,dx &=&\dsum\limits_{n=0}^{\infty }c_{n}\dint \left( x-a\right) ^{n}\,dx \\ &=&\dsum\limits_{n=0}^{\infty }c_{n}\dfrac{\left( x-a\right) ^{n+1}}{n+1}+C \\ &=&\dsum\limits_{k=1}^{\infty }\dfrac{c_{k-1}}{k}\left( x-a\right) ^{k}+C \end{eqnarray*} which also has radius of convergence $R.$ \begin{remark} If you use a specific antiderivative on the left, then you must evaluate the constant on the right usually by evaluating both sides at the center $x=a$. \end{remark} \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/maroonA.wmf';file-properties "XNPEU";}} \vspace{1pt} \begin{example} Find the power series expansion for $\arctan x$ centered about $x=0$, and find its interval of convergence. \end{example} \emph{Solution:} \ We first notice that $\arctan x$ is the integral of $% \dfrac{1}{1+x^{2}}$ whose series is \begin{equation*} \dfrac{1}{1+x^{2}}=\dsum\limits_{n=0}^{\infty }\left( -x^{2}\right) ^{n}=\dsum\limits_{n=0}^{\infty }\left( -1\right) ^{n}x^{2n}\quad \text{for }% \left| -x^{2}\right| <1\text{ or }\left| x\right| <1 \end{equation*} So we integrate this series term by term: \ (Don't forget the constant!) \begin{equation*} \arctan x=\dsum\limits_{n=0}^{\infty }\left( -1\right) ^{n}\dint x^{2n}\,dx=\dsum\limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n}}{2n+1}% x^{2n+1}+C\quad \text{for }\left| x\right| <1 \end{equation*} To find the constant, we evaluate both sides at $x=0$. \begin{equation*} \arctan 0=\dsum\limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n}}{2n+1}% 0^{2n+1}+C \end{equation*} Recall $\arctan 0=0$ and $0^{2n+1}=0$. \ Thus $C=0$. \ We substitute back to conclude \begin{equation*} \arctan x=\dsum\limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n}}{2n+1}% x^{2n+1}\quad \text{for }\left| x\right| <1 \end{equation*} We check the convergence at the endpoints. \ At $x=1$, the series becomes $% \dsum\limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n}}{2n+1}$ which converges by the Alternating Series Test. \ At $x=-1$, the series becomes $% \dsum\limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n}}{2n+1}\left( -1\right) ^{2n+1}=\dsum\limits_{n=0}^{\infty }\dfrac{\left( -1\right) ^{n+1}% }{2n+1}$ which also converges by the Alternating Series Test. \ So the interval of convergence is $-1\leq x\leq 1$. \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 "0.2561";cropright "1";cropbottom "0.7439";filename 'graphics/maroonA.wmf';file-properties "XNPEU";}} \vspace{1pt} \begin{example} Find a power series representation for% \begin{equation*} f(x)=\dfrac{1}{(1+x)^{3}}. \end{equation*} \end{example} We begin with the fact that \begin{equation*} \dfrac{1}{1+x}=\dfrac{1}{1-(-x)}=1-x+x^{2}-x^{3}+x^{4}+\cdots \end{equation*} Differentiating, we obtain: \begin{equation*} -\dfrac{1}{(1+x)^{2}}=-1+2x-3x^{2}+4x^{3}+\cdots \end{equation*} and hence $\vspace{1pt}$ \begin{equation*} \dfrac{1}{(1+x)^{2}}=1-2x+3x^{2}-4x^{3}+\cdots \end{equation*} Differentiating again, yields \begin{equation*} \dfrac{-2}{(1+x)^{3}}=-2+6x-12x^{2}+\cdots \end{equation*} Finally, dividing by $-2$, we obtain: \begin{equation*} \dfrac{1}{(1+x)^{3}}=1-3x+6x^{2}+\cdots \end{equation*} \vspace{1pt}\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 "0.2561";cropright "1";cropbottom "0.7439";filename 'graphics/maroonA.wmf';file-properties "XNPEU";}} \begin{exercise} Find the power series expansion for $\ln \left( 1+x\right) $ centered about $% x=0$, and find its interval of convergence.\dotfill \CustomNote[\hyperref{\TCIIcon{BITMAPSETAnswer}{0.1609in}{0.1487in}{0in}}{}{% }{}]{Margin Hint}{$\ln \left( 1+x\right) =\dsum\limits_{k=1}^{\infty }\dfrac{% \left( -1\right) ^{k-1}}{k}x^{k}\quad $for $-1