3.1E: Exercises

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Exercise \(\PageIndex{1}\)

For each power series use Theorem \((3.1.3)\) to find the radius of convergence \(R\). If \(R>0\), find the open interval of convergence.

(a) \(\displaystyle{\sum_{n=0}^\infty {(-1)^n\over2^nn}(x-1)^n}\)

(b) \(\displaystyle{\sum_{n=0}^\infty 2^nn(x-2)^n}\)

(d) \(\displaystyle{\sum_{n=0}^\infty{n(n+1)\over16^n}(x-2)^n}\)

(e) \(\displaystyle{\sum_{n=0}^\infty (-1)^n{7^n\over n!}x^n}\)

(f) \(\displaystyle{\sum_{n=0}^\infty {3^n\over4^{n+1}(n+1)^2}(x+7)^n}\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{2}\)

Suppose there's an integer \(M\) such that \(b_m\ne0\) for \(m\ge M\), and

\begin{eqnarray*}
\lim_{m\to\infty}\left|b_{m+1}\over b_m\right|=L,
\end{eqnarray*}

where \(0\le L\le\infty\). Show that the radius of convergence of

\begin{eqnarray*}
\sum_{m=0}^\infty b_m(x-x_0)^{2m}
\end{eqnarray*}

is \(R=1/\sqrt L\), which is interpreted to mean that \(R=0\) if \(L=\infty\) or \(R=\infty\) if \(L=0\).

Hint: Apply Theorem \((3.1.3)\) to the series \(\sum_{m=0}^\infty b_mz^m\) and then let \(z=(x-x_0)^2\).

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{3}\)

For each power series, use the result of Exercise \((3.1E.2)\) to find the radius of convergence \(R\). If \(R>0\), find the open interval of convergence.

(a) \(\displaystyle{\sum_{m=0}^\infty (-1)^m(3m+1)(x-1)^{2m+1}}\)

(b) \(\displaystyle{\sum_{m=0}^\infty (-1)^m{m(2m+1)\over2^m}(x+2)^{2m}}\)

(d) \(\displaystyle{\sum_{m=0}^\infty (-1)^m{m!\over9^m}(x+8)^{2m}}\)

(e) \(\displaystyle{\sum_{m=0}^\infty(-1)^m{(2m-1)\over3^m}x^{2m+1}}\)

(f) \(\displaystyle{\sum_{m=0}^\infty(x-1)^{2m}}\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{4}\)

Suppose there's an integer \(M\) such that \(b_m\ne0\) for \)m\ge M\), and

\begin{eqnarray*}
\lim_{m\to\infty}\left|b_{m+1}\over b_m\right|=L,
\end{eqnarray*}

where \(0\le L\le\infty\). Let \(k\) be a positive integer. Show that the radius of convergence of

\begin{eqnarray*}
\sum_{m=0}^\infty b_m(x-x_0)^{km}
\end{eqnarray*}

is \(R=1/\sqrt[k]L\), which is interpreted to mean that \(R=0\) if \(L=\infty\) or \(R=\infty\) if \(L=0\).

Hint: Apply Theorem \((3.1.3)\) to the series \(\sum_{m=0}^\infty b_mz^m\) and then let \(z=(x-x_0)^k\).

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{5}\)

For each power series use the result of Exercise \((3.1E.4)\) to find the radius of convergence \(R\). If \(R>0\), find the open interval of convergence.

(a) \(\displaystyle{\sum_{m=0}^\infty{(-1)^m\over(27)^m}(x-3)^{3m+2}}\)

(b) \(\displaystyle{\sum_{m=0}^\infty{x^{7m+6}\over m}}\)

(d) \(\displaystyle{\sum_{m=0}^\infty(-1)^m{2^m\over m!}x^{4m+3}}\)

(e) \(\displaystyle{\sum_{m=0}^\infty{m!\over(26)^m}(x+1)^{4m+3}}\)

(f) \(\displaystyle{\sum_{m=0}^\infty{(-1)^m\over8^mm(m+1)}(x-1)^{3m+1}}\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{6}\)

Graph \(y=\sin x\) and the Taylor polynomial

\begin{eqnarray*}
T_{2M+1}(x)=\sum_{n=0}^M{(-1)^nx^{2n+1}\over(2n+1)!}
\end{eqnarray*}

on the interval \((-2\pi,2\pi)\) for \(M=1\), \(2\), \(3\), \(\dots\), until you find a value of \(M\) for which there's no perceptible difference between the two graphs.

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{7}\)

Graph \(y=\cos x\) and the Taylor polynomial

\begin{eqnarray*}
T_{2M}(x)=\sum_{n=0}^M{(-1)^nx^{2n}\over(2n)!}
\end{eqnarray*}

on the interval \((-2\pi,2\pi)\) for \(M=1\), \(2\), \(3\), \(\dots\), until you find a value of \(M\) for which there's no perceptible difference between the two graphs.

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{8}\)

Graph \(y=1/(1-x)\) and the Taylor polynomial

\begin{eqnarray*}
T_N(x)=\sum_{n=0}^Nx^n
\end{eqnarray*}

on the interval \([0,.95]\) for \(N=1\), \(2\), \(3\), \(\dots\), until you find a value of \(N\) for which there's no perceptible difference between the two graphs. Choose the scale on the \(y\)-axis so that \(0\le y\le20\).

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{9}\)

Graph \(y=\cosh x\) and the Taylor polynomial

\begin{eqnarray*}
T_{2M}(x)=\sum_{n=0}^M{x^{2n}\over(2n)!}
\end{eqnarray*}

on the interval \((-5,5)\) for \(M=1\), \(2\), \(3\), \(\dots\), until you find a value of \(M\) for which there's no perceptible difference between the two graphs. Choose the scale on the \(y\)-axis so that \(0\le y\le75\).

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{10}\)

Graph \(y=\sinh x\) and the Taylor polynomial

\begin{eqnarray*}
T_{2M+1}(x)=\sum_{n=0}^M{x^{2n+1}\over(2n+1)!}
\end{eqnarray*}

on the interval \((-5,5)\) for \(M=0\), \(1\), \(2\), \(\dots\), until you find a value of \(M\) for which there's no perceptible difference between the two graphs. Choose the scale on the \(y\)-axis so that \(-75~\le~y\le~75\).

Answer: Add texts here. Do not delete this text first.

In Exercises \((3.1E.11)\) to \((3.1E.15)\), find a power series solution \(y(x)=\sum_{n=0}^\infty a_nx^n\).

Exercise \(\PageIndex{11}\)

\((2+x)y''+xy'+3y\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{12}\)

\((1+3x^2)y''+3x^2y'-2y\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{13}\)

\((1+2x^2)y''+(2-3x)y'+4y\)

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Exercise \(\PageIndex{14}\)

\((1+x^2)y''+(2-x)y'+3y\)

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Exercise \(\PageIndex{15}\)

\((1+3x^2)y''-2xy'+4y\)

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Exercise \(\PageIndex{16}\)

Suppose \(y(x)=\sum_{n=0}^\infty a_n(x+1)^n\) on an open interval that contains \(x_0=-1\). Find a power series in \(x+1\) for

\begin{eqnarray*}
xy''+(4+2x)y'+(2+x)y.
\end{eqnarray*}

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{17}\)

Suppose \(y(x)=\sum_{n=0}^\infty a_n(x-2)^n\) on an open interval that contains \(x_0=2\). Find a power series in \(x-2\) for

\begin{eqnarray*}
x^2y''+2xy'-3xy.
\end{eqnarray*}

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{18}\)

Do the following experiment for various choices of real numbers \(a_0\) and \(a_1\).

(a) Use differential equations software to solve the initial value problem

\begin{eqnarray*}
(2-x)y''+2y=0,\quad y(0)=a_0,\quad y'(0)=a_1,
\end{eqnarray*}

numerically on \((-1.95,1.95)\). Choose the most accurate method your software package provides. (See Section 3.1 for a brief discussion of one such method.)

(b) For \(N=2\), \(3\), \(4\), \(\dots\), compute \(a_2\), \(\dots\), \(a_N\) from Equation \((3.1.18)\) and graph

\begin{eqnarray*}
T_N(x)=\sum_{n=0}^N a_nx^n
\end{eqnarray*}

and the solution obtained in part (a) on the same axes. Continue increasing \(N\) until it's obvious that there's no point in continuing. (This sounds vague, but you'll know when to stop.)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{19}\)

Follow the directions of Exercise \((3.1.18)\) for the initial value problem

\begin{eqnarray*}
(1+x)y''+2(x-1)^2y'+3y=0,\quad y(1)=a_0,\quad y'(1)=a_1,
\end{eqnarray*}

on the interval \((0,2)\). Use Equations \((3.1.24) \mbox{ and } (3.1.25)\) to compute \(\{a_n\}\).

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{20}\)

Suppose the series \(\sum_{n=0}^\infty a_nx^n\) converges on an open interval \((-R,R)\), let \(r\) be an arbitrary real number, and define

\begin{eqnarray*}
y(x)=x^r\sum_{n=0}^\infty a_nx^n=\sum_{n=0}^\infty a_nx^{n+r}
\end{eqnarray*}

on \((0,R)\). Use Theorem \((3.1.4)\) and the rule for differentiating the product of two functions to show that

\begin{eqnarray*}
y'(x)&=&\displaystyle{\sum_{n=0}^\infty (n+r)a_nx^{n+r-1}},\\
y''(x)&=&\displaystyle{\sum_{n=0}^\infty(n+r)(n+r-1)a_nx^{n+r-2}},\\
&\vdots&\\
y^{(k)}(x)&=&\displaystyle{\sum_{n=0}^\infty(n+r)(n+r-1)\cdots(n+r-k)a_nx^{n+r-k}}
\end{eqnarray*}

on \((0,R)\)

Answer: Add texts here. Do not delete this text first.

In Exercises \((3.1E.21)\) to \((3.1E.26)\), let \(y\) be as defined in Exercise \((3.1E.20)\), and write the given expression in the form \(x^r\sum_{n=0}^\infty b_nx^n\).

Exercise \(\PageIndex{21}\)

\(x^2(1-x)y''+x(4+x)y'+(2-x)y\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{22}\)

\(x^2(1+x)y''+x(1+2x)y'-(4+6x)y\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{23}\)

\(x^2(1+x)y''-x(1-6x-x^2)y'+(1+6x+x^2)y\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{24}\)

\(x^2(1+3x)y''+x(2+12x+x^2)y'+2x(3+x)y\)

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Exercise \(\PageIndex{25}\)

\(x^2(1+2x^2)y''+x(4+2x^2)y'+2(1-x^2)y\)

Answer: Add texts here. Do not delete this text first.

Exercise \(\PageIndex{26}\)

\(x^2(2+x^2)y''+2x(5+x^2)y'+2(3-x^2)y\)

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