 # 1. Suppose that A ⊆ B and that B has a greatest lower bound. S...

## Question

1. Suppose that A ⊆ B and that B has a greatest lower bound. Show that A has a greatest lower bound and that glbB ≤ glbA.

2. Let A, B ⊆ ℝ (ℝ represents the real numbers) and A, B both non-empty. Define A + B = {a+b: a∈A and b∈B}. Suppose A and B both have least upper bounds. Prove that the set A + B then also has a least upper bound and that lub(A + B) = lubA + lubB.

3. Designate each of the following sets as ordered fields or not. Type F if the given set is an ordered field and type N if it is not.
a. ℝ (the real numbers)
b. ℚ (the rational numbers)
c. ℂ (the complex numbers)
d. ℕ (the natural numbers)
e. ℝ - ℚ

4. Suppose set A has a greatest lower bound. Then it is not necessarily true that lub(-A) = -glbA.
True or False

5. Set A is bounded below if and only if set –A is bounded above.
True or False

6. Set A is bounded above if and only if set –A is bounded below.
True or False

7. Let A ⊆ ℝ (ℝ represents the real numbers) and A non-empty. Prove TWO of the following:
a. glbA ≤ lubA
b. Let x,y ∈ A. Then |x – y|≤ lubA – glbA
c. glbA = lubA if and only if A contains only a single element

## Solution Preview

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1. Let a∈A (we assume A is non-empty). Then since A⊆B, then a∈B and thus a≥glb B. By the Greatest Lower Property, A has a greatest lower bound. Since glb B is a lower bound of A, then as desired we must have glb A≥glb B by definition....

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