Chapter 2: Problem 4
Show that \(|x-a|<\varepsilon\) if and only if \(a-\varepsilon
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Chapter 2: Problem 4
Show that \(|x-a|<\varepsilon\) if and only if \(a-\varepsilon
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(a) If \(a
Let \(X\) and \(Y\) be nonempty sets and let \(h: X \times Y \rightarrow \mathbb{R}\) have bounded range in \(\mathbb{R}\). Let \(F: X \rightarrow \mathbb{R}\) and \(G: Y \rightarrow \mathbb{R}\) be defined by $$ F(x):=\sup \\{h(x, y): y \in Y\\}, \quad G(y):=\sup \\{h(x, y): x \in X\\} $$ Establish the Principle of the Iterated Suprema: $$ \sup \\{h(x, y): x \in X, y \in Y\\}=\sup \\{F(x): x \in X\\}=\sup \\{G(y): y \in Y\\} $$ We sometimes express this in symbols by $$ \sup _{x, y} h(x, y)=\sup _{x} \sup _{y} h(x, y)=\sup _{y} \sup _{x} h(x, y) . $$
Let \(S\) be a nonempty bounded set in \(\mathbb{R}\). (a) Let \(a>0\), and let \(a S:=\\{\) as: \(s \in S\\}\). Prove that $$ \inf (a S)=a \inf S, \quad \sup (a S)=a \sup S $$ (b) Let \(b<0\) and let \(b S=\\{b s: s \in S\\}\). Prove that $$ \inf (b S)=b \sup S, \quad \sup (b S)=b \inf S $$
Let \(J_{n}:=(0,1 / n)\) for \(n \in \mathbb{N}\). Prove that \(\bigcap_{n=1}^{\infty} J_{n}=\emptyset\).
Modify the argument in Theorem \(2.4 .7\) to show that there exists a positive real number \(u\) such that \(u^{3}=2\).
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