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\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*}
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\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.
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\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}
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\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 $R.$
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\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}
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\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}
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\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 $R.$
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\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.
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\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}
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\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}
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\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 $-1\leq x\leq 1$.
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\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*}
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\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