Chapter 1: Problem 31
Give an example of a nonconstant function \(f:(-1,1) \rightarrow \mathbb{R}\) such that \(f\) has a local extremum at 0, and 0 is a point of inflection for \(f\).
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Chapter 1: Problem 31
Give an example of a nonconstant function \(f:(-1,1) \rightarrow \mathbb{R}\) such that \(f\) has a local extremum at 0, and 0 is a point of inflection for \(f\).
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Prove that every rational number \(r\) can be written as \(r=\frac{p}{q}\), where \(p, q \in \mathbb{Z}, q>0\) and \(p, q\) are relatively prime, and moreover the integers \(p\) and \(q\) are uniquely determined by \(r .\)
Use the Cauchy-Schwarz inequality to prove the A.M.-H.M. inequality, namely, if \(n \in \mathbb{N}\) and \(a_{1}, \ldots, a_{n}\) are positive real numbers, then prove that $$ \frac{a_{1}+\cdots+a_{n}}{n} \geq \frac{n}{r}, \quad \text { where } \quad r:=\frac{1}{a_{1}}+\cdots+\frac{1}{a_{n}} . $$
Given any function \(f: \mathbb{R} \rightarrow \mathbb{R}\), prove the following: (i) If \(f(x y)=f(x)+f(y)\) for all \(x, y \in \mathbb{R}\), then \(f(x)=0\) for all \(x \in \mathbb{R}\). [Note: It is, however, possible that there are nonzero functions defined on subsets of \(\mathbb{R}\), such as \((0, \infty)\) that satisfy \(f(x y)=f(x)+f(y)\) for all \(x, y\) in the domain of \(f\). A prominent example of this is the logarithmic function, which will be discussed in Section 7.1.] (ii) If \(f(x y)=f(x) f(y)\) for all \(x, y \in \mathbb{R}\), then either \(f(x)=0\) for all \(x \in \mathbb{R}\), or \(f(x)=1\) for all \(x \in \mathbb{R}\), or \(f(0)=0\) and \(f(1)=1\). Further, \(f\) is either an even function or an odd function. Give examples of even as well as odd functions \(f: \mathbb{R} \rightarrow \mathbb{R}\) satisfying \(f(x y)=f(x) f(y)\) for all \(x, y \in \mathbb{R}\)
Let \(p(x), q(x) \in \mathbb{R}[x]\). Show that \(p(x)\) and \(q(x)\) are relatively prime if and only if \(u(x) p(x)+v(x) q(x)=1\) for some \(u(x), v(x) \in \mathbb{R}[x] .\) Is it true that if a nonzero polynomial \(d(x) \in \mathbb{R}[x]\) satisfies \(u(x) p(x)+v(x) q(x)=d(x)\) for some \(u(x), v(x) \in \mathbb{R}[x]\), then \(d(x)\) is a GCD of \(p(x)\) and \(q(x)\) ?
Show that a nonzero polynomial in \(\mathbb{R}[x]\) of degree \(n\) has at most \(n\) roots in \(\mathbb{R}\). (Hint: Exercise 59.)
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