# Show that A + B = {x + y : x ∈ A, y ∈ B} is closed in E

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ANALYSIS II, Homework 7

1. (a) Let A and B be two subsets of a normed space E. Suppose that A is closed in E and B is compact in E. Show that

A + B = {x + y : x ∈ A, y ∈ B}

is closed in E.

(b) Let en = (0, ...0, 1, 0, ....) ∈ l2 for n = 1, 2, ..., and A = {en : n ∈ N}

and B = {−en+n1e1 : n ∈ N}. Show that A and B are closed and bounded sets in the space l2 but A + B is not closed in l2.

2. Let f ∈ C([0, 1], R). Suppose that for all x ∈ [0, 1] we have that |f(x)| ≤ Rx

0 f (t) dt. Show that f(x) = 0 for all x ∈ [0, 1].

3. A metric space X is called separable, if there exists a countable set {x1, x2, ....} ⊂ X such that {x1, x2, ....} = X. Show that every precompact metric space is separable.

4. Show that the intersection of arbitrary many compacts sets in a metric space X is compact.

5. Let f :]0, 1[→]0, 1[. True or false?

(a)If f is continuous and (xn)n is a Cauchy sequence, then (f(xn))n is a Cauchy sequence?

(b) If f maps every Cauchy sequence into a Cauchy sequence, then f is continuous?

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