Chapter 2: Problem 17
PROPERTY 4. \(E(-x)=\frac{1}{E(x)}=(E(x))^{-1}\)
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Chapter 2: Problem 17
PROPERTY 4. \(E(-x)=\frac{1}{E(x)}=(E(x))^{-1}\)
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Evaluate the following integrals as series. (a) \(\int_{x=0}^{1} e^{x^{2}} \mathrm{~d} x\) (b) \(\int_{x=0}^{1} \frac{1}{1+x^{4}} \mathrm{~d} x\) (c) \(\int_{x=0}^{1} \sqrt[3]{1-x^{3}} \mathrm{~d} x\)
(a) Show that $$ \int_{x=0}^{1 / 2} \sqrt{x-x^{2}} \mathrm{~d} x=\sum_{n=0}^{\infty} \frac{(-1)^{n} \prod_{j=0}^{n-1}\left(\frac{1}{2}-j\right)}{\sqrt{2} n !(2 n+3) 2^{n}} $$ and use this to show that $$ \pi=16\left(\sum_{n=0}^{\infty} \frac{(-1)^{n} \prod_{j=0}^{n-1}\left(\frac{1}{2}-j\right)}{\sqrt{2} n !(2 n+3) 2^{n}}\right) $$ (b) We now have two series for calculating \(\pi:\) the one from part (a) and the one derived earlier, namely $$ \pi=4\left(\sum_{n=0}^{\infty} \frac{(-1)^{n}}{2 n+1}\right) $$
(a) Show that if \(y=\sum_{n=0}^{\infty} a_{n} x^{n}\) satisfies the differential equation \(\frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}=-y\), then $$ a_{n+2}=\frac{-1}{(n+2)(n+1)} a_{n} $$ and conclude that $$ y=a_{0}+a_{1} x-\frac{1}{2 !} a_{0} x^{2}-\frac{1}{3 !} a_{1} x^{3}+\frac{1}{4 !} a_{0} x^{4}+\frac{1}{5 !} a_{1} x^{5}-\frac{1}{6 !} a_{0} x^{6}-\frac{1}{7 !} a_{1} x^{7}+\cdots . $$ (b) Since \(y=\sin x\) satisfies \(\frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}=-y,\) we see that $$ \sin x=a_{0}+a_{1} x-\frac{1}{2 !} a_{0} x^{2}-\frac{1}{3 !} a_{1} x^{3}+\frac{1}{4 !} a_{0} x^{4}+\frac{1}{5 !} a_{1} x^{5}-\frac{1}{6 !} a_{0} x^{6}-\frac{1}{7 !} a_{1} x^{7}+\cdots $$ for some constants \(a_{0}\) and \(a_{1}\). Show that in this case \(a_{0}=0\) and \(a_{1}=1\) and obtain $$ \sin x=x-\frac{1}{3 !} x^{3}+\frac{1}{5 !} x^{5}-\frac{1}{7 !} x^{7}+\cdots=\sum_{n=0}^{\infty} \frac{(-1)^{n}}{(2 n+1) !} x^{2 n+1} $$
(a) Show that if \(y=\sum_{n=0}^{\infty} a_{n} x^{n}\) satisfies the differential equation \(\frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}=-y,\) then $$ a_{n+2}=\frac{-1}{(n+2)(n+1)} a_{n} $$ and conclude that $$ y=a_{0}+a_{1} x-\frac{1}{2 !} a_{0} x^{2}-\frac{1}{3 !} a_{1} x^{3}+\frac{1}{4 !} a_{0} x^{4}+\frac{1}{5 !} a_{1} x^{5}-\frac{1}{6 !} a_{0} x^{6}-\frac{1}{7 !} a_{1} x^{7}+\cdots $$ (b) Since \(y=\sin x\) satisfies \(\frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}=-y,\) we see that $$ \sin x=a_{0}+a_{1} x-\frac{1}{2 !} a_{0} x^{2}-\frac{1}{3 !} a_{1} x^{3}+\frac{1}{4 !} a_{0} x^{4}+\frac{1}{5 !} a_{1} x^{5}-\frac{1}{6 !} a_{0} x^{6}-\frac{1}{7 !} a_{1} x^{7}+\cdots $$ for some constants \(a_{0}\) and \(a_{1}\). Show that in this case \(a_{0}=0\) and \(a_{1}=1\) and obtain $$ \sin x=x-\frac{1}{3 !} x^{3}+\frac{1}{5 !} x^{5}-\frac{1}{7 !} x^{7}+\cdots=\sum_{n=0}^{\infty} \frac{(-1)^{n}}{(2 n+1) !} x^{2 n+1} $$ (b) Let \(s(x, N)=\sum_{n=0}^{N} \frac{(-1)^{n}}{(2 n+1) !} x^{2 n+1}\) and \(c(x, N)=\sum_{n=0}^{N} \frac{(-1)^{n}}{(2 n) !} x^{2 n}\) and use a computer algebra system to plot these for \(-4 \pi \leq x \leq 4 \pi, N=\) \(1,2,5,10,15 .\) Describe what is happening to the series as \(N\) becomes larger.
Without using Taylor's Theorem, represent the following functions as power series expanded about 0 (i.e., in the form \(\left.\sum_{n=0}^{\infty} a_{n} x^{n}\right)\). (a) \(\ln \left(1-x^{2}\right)\) (b) \(\frac{x}{1+x^{2}}\) (c) \(\arctan \left(x^{3}\right)\) (d) \(\ln (2+x)\) [Hint: \(\left.2+x=2\left(1+\frac{x}{2}\right)\right]\)
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