4 0. Background
Note that a sequence of rational numbers tending to

2 is a Cauchy sequence,√
but does not not have a limit within the system of rational numbers, because 2
is irrational. In a set-theoretic sense, the real numbers are constructed by filling in
the holes found among the rational numbers. Because all Cauchy sequences have a
limit, we say that the real numbers are complete.
A function f is continuous at a if limx→a f(x) = f(a). That is, for every ε 0
there is a δ 0 such that |f(x) f(a)| ε if |x a| δ. Here the choice of δ
depends on both ε and a. However, if in a domain D we have |f(x) f(y)| ε
whenever x D, y D and |x−y| δ, then we say that f is uniformly continuous
on D.
Theorem 0.5. If a real-valued function f(x) is continuous on a closed bounded
interval [a, b], then it is uniformly continuous on that interval, and attains its max-
imum and minimum values.
The same theorem also holds for real-valued continuous functions defined on
closed bounded sets in the plane
R2.
Theorem 0.6. (The Squeeze Theorem) Suppose that δ 0, that f−, f, and f+ are
functions such that f−(x) f(x) f+(x) for a−δ x a+δ. If limx→a f−(x) = c
and limx→a f+(x) = c, then limx→a f(x) = c.
Theorem 0.7. (Rolle’s Theorem) If f(x) is a continuous real-valued function on
the interval a x b with f(x) differentiable for a x b, and if f(a) = f(b),
then there exists a ξ (a, b) such that f (ξ) = 0.
Theorem 0.8. (The Mean Value Theorem of Differential Calculus) If f(x) is a
continuous real-valued function on the interval a x b and f(x) is differentiable
for a x b, then there exists a ξ (a, b) such that
f (ξ) =
f(b) f(a)
b a
.
Theorem 0.9. If
∑∞
n=1
|an| ∞, then the sum
∑∞
n=1
an converges.
Theorem 0.10. Suppose that amn 0 for all m and n. We form two sums:
(0.6)

m=1

n=1
amn ,

n=1

m=1
amn .
If either of these sums is finite, then the other one is also finite, and they are equal.
Theorem 0.11. If |amn| Amn for all m and n, and if

m=1

n=1
Amn ∞,
then the sums (0.6) converge and are equal.
Theorem 0.12. Let f(z) =
∑∞
k=0
akzk be a power series, and let R be defined by
the relation
1
R
= lim sup
n→∞
|an|1/n
.
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