Chapter 3: Problem 2
Give an example of two divergent sequences \(X\) and \(Y\) such that: (a) their sum \(X+Y\) converges, (b) their product \(X Y\) converges.
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Chapter 3: Problem 2
Give an example of two divergent sequences \(X\) and \(Y\) such that: (a) their sum \(X+Y\) converges, (b) their product \(X Y\) converges.
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If \(\sum a_{n}\) with \(a_{n}>0\) is convergent, and if \(b_{n}:=\left(a_{1}+\cdots+a_{n}\right) / n\) for \(n \in \mathbb{N}\), then show that \(\sum b_{n}\) is always divergent.
Suppose that \(x_{n} \geq 0\) for all \(n \in \mathbb{N}\) and that \(\lim \left((-1)^{n} x_{n}\right)\) exists. Show that \(\left(x_{n}\right)\) converges.
Show that \(\lim \left(2^{n} / n !\right)=0 .\) [Hint: if \(n \geq 3\), then \(0<2^{n} / n ! \leq 2\left(\frac{2}{3}\right)^{n-2}\).]
Use the Cauchy Condensation Test to discuss the \(p\) -series \(\sum_{n=1}^{\infty}\left(1 / n^{p}\right)\) for \(p>0\).
Show that the following sequences are divergent. (a) \(\left(1-(-1)^{n}+1 / n\right)\), (b) \((\sin n \pi / 4)\).
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