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Why Read This Book? - Index of

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7.3 More on Limits 217<br />

values <strong>of</strong> 0 and 1 are sprinkled up and down the domain like grains <strong>of</strong> salt and<br />

pepper. No one knows for sure which is the salt and which is the pepper. �<br />

EXERCISE 7.2.20 The signum function, sgn x, is defined in the following way:<br />

Show limx→0 sgn x does not exist.<br />

⎧<br />

⎪⎨ −1, if x0<br />

(7.10)<br />

EXERCISE 7.2.21 Use familiar values <strong>of</strong> sin x to show that limx→0 sin(1/x) does<br />

not exist.<br />

7.3 More on Limits<br />

7.3.1 One-Sided Limits<br />

In defining limx→a f(x) = L, we insisted f be defined on both sides <strong>of</strong> a and all<br />

points to the nearby left and right <strong>of</strong> a map into Nɛ(L).Iff is only defined on one<br />

side <strong>of</strong> a, or perhaps if values <strong>of</strong> f to the immediate left <strong>of</strong> a behave differently<br />

from those to the immediate right (as in sgn x), we can discuss the limit <strong>of</strong> f(x) as<br />

x approaches a from the left or from the right separately. Instead <strong>of</strong> using entire<br />

deleted neighborhoods <strong>of</strong> a, we use only the left or right half <strong>of</strong> them.<br />

Definition 7.3.1 Suppose f is defined on the interval (a, b). Then we say<br />

lim x→a + f(x) = L (read “as x approaches a from the right”) if for all ɛ>0, there<br />

exists δ>0 such that a

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