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Why Read This Book? - Index of

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Functions <strong>of</strong> a Real Variable<br />

7<br />

Functions whose domain is a subset <strong>of</strong> the real numbers are called functions <strong>of</strong> a<br />

real variable, and functions whose range is a subset <strong>of</strong> the real numbers are called<br />

real-valued. Some especially powerful and interesting results can be deduced if<br />

the domain <strong>of</strong> such a function is compact. Because this chapter deals exclusively<br />

with real-valued functions <strong>of</strong> a real variable, we will write f : S → R, where it is<br />

understood that S is a subset <strong>of</strong> the real numbers.<br />

7.1 Bounded and Monotone Functions<br />

First we discuss boundedness and monotonicity <strong>of</strong> a function f : S → R. With<br />

these and other ideas from Sections 7.2–7.4, we will point out many links to<br />

properties <strong>of</strong> sequences we discussed in Chapter 6.<br />

7.1.1 Bounded Functions<br />

The definitions <strong>of</strong> the boundedness terms for functions resemble those for<br />

sequences.<br />

Definition 7.1.1 Suppose f : S → R is a function and A is a subset <strong>of</strong> S. We say<br />

f is bounded from above on A provided there exists a real number M1 such that<br />

f(x) ≤ M1 for all x ∈ A. Similarly, we say f is bounded from below on A provided<br />

there exists a real number M2 such that f(x) ≥ M2 for all x ∈ A. If there exists<br />

M>0 such that |f(x)| ≤ M for all x ∈ A, we say f is bounded on A. For each <strong>of</strong><br />

these characteristics, if it applies on the entire domain, we say simply that f is<br />

bounded from above, bounded from below, or bounded.<br />

EXERCISE 7.1.2 Use the logic <strong>of</strong> Definition 7.1.1 to state what the following<br />

terms mean.<br />

(a) f is not bounded from above on A.<br />

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