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# University of California Los Angeles Isolated Point Gradescope Questions

1. Let A ⊆ R. We say that an element c ∈ A is isolated if there is an ε > 0 such that
A ∩ (c − ε, c + ε) = {c}.
(a) Show that c ∈ A is an isolated point if and only if it is not a limit point of A.
(b) Show that every function f : A → R is continuous at each isolated c ∈ A.
(c) Show that every function f : Z → R is continuous on its domain Z.

2. For each of the following functions f : A → R, find f(A) and hence decide whether f
(equivalently its range) has an upper bound, a lower bound, a maximum or a minimum.
(a) f(x) = x
3
, A = (−3, 2).
(b) f(x) = x
2
, A = (−3, 2)
(c) f(x) = (
x if x ∈ Q
0 if x /∈ Q
, A = [0, a] where a > 0.

3. Assume f : R → R is continuous on R and let K = {x : f(x) = 0}. Show that K is a
closed set.

4. Give an example of each of the following, or state that such a request is impossible. For
any that are impossible, supply a short explanation for why this is the case.
(a) A continuous function f : (0, 1) → R and a Cauchy sequence (xn) in (0, 1) such
that (f(xn)) is not a Cauchy sequence.
(b) A continuous function f : [0, 1] → R and a Cauchy sequence (xn) in [0, 1] such that
(f(xn)) is not a Cauchy sequence.
(c) A continuous function f : [0, 1] → R which has a maximum but no minimum.
(d) A continuous bounded function f : (0, 1) → R that attains a maximum value but
not a minimum value.

5. (a) Let f be a continuous real-valued function with domain (a, b). Show that if f(x) = 0
for each rational number x in (a, b), then f(x) = 0 for all x ∈ (a, b).
(b) If f and g are continuous real-valued functions with domain (a, b) and f(x) = g(x)
for all rational x ∈ (a, b), must f and g be the same function?