Chapter 2: Problem 5
Give a formal definition of the statement \(\left(a_{n}\right) \rightarrow-\infty\)
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Chapter 2: Problem 5
Give a formal definition of the statement \(\left(a_{n}\right) \rightarrow-\infty\)
These are the key concepts you need to understand to accurately answer the question.
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A celebrated theorem due to Riemann \(^{8}\) shows that a conditionally convergent series can be rearranged so as to sum to any real number, or to diverge to \(\infty\), or to diverge to \(-\infty\). This exercise has the more modest aim of showing that a rearrangement may have a different sum. Consider the alternating harmonic series $$ 1-\frac{1}{2}+\frac{1}{3}-\cdots $$ with sum \(S\), and denote its sum to \(n\) terms \((n=1,2,3, \ldots)\) by \(S_{n}\). Consider also the rearranged series $$ 1+\frac{1}{3}-\frac{1}{2}+\frac{1}{5}+\frac{1}{7}-\frac{1}{4}+\frac{1}{9}+\frac{1}{11}-\frac{1}{6}+\cdots $$ and denote its sum to \(n\) terms by \(T_{n}\). For each \(n \geq 1\), let $$ H_{n}=1+\frac{1}{2}+\frac{1}{3}+\cdots+\frac{1}{n} $$ a) Show that \(S_{2 n}=H_{2 n}-H_{n}\) for all \(n \geq 1\). b) Show that $$ T_{3 n}=H_{4 \mathrm{n}}-\frac{1}{2} H_{2 n}-H_{n}=S_{4 n}+\frac{1}{2} S_{2 n} $$ and deduce that the rearranged series has sum \(3 S / 2\).
Determine the sum of the series \(\sum_{n=1}^{\infty} 1 / n(n+2)\).
Determine \(\lim _{n \rightarrow \infty}\left(2^{n}+3^{n}\right)^{1 / n}\)
Let \(\sum_{n=1}^{\infty} a_{n}\) and \(\sum_{n=1}^{\infty} b_{n}\) be convergent series with sums \(A\) and \(B\) respectively. Show that \(\sum_{n=1}^{\infty}\left(a_{n}+b_{n}\right)=A+B\) and, for every constant \(k, \sum_{n=1}^{\infty}\left(k a_{n}\right)=k A\).
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