Proving A is either ℝ or ∅

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In summary, the conversation discusses the relationship between open and closed sets in relation to the set A, which is either the entire real number line or the empty set. The equations G ∩ bd(G) = ∅ and bd(F) ⊆ F are mentioned, as well as the fact that bd(S) = bd(ℝ∖S) = bd(S'). There is an attempt at a solution where A is assumed to be both open and closed, but it is concluded that this leads to a contradiction as ℝ is closed, which goes against the definition of a clopen set.
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glebovg
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Homework Statement



Show that if A ⊆ ℝ is both open and closed then A is either ℝ or ∅.

Homework Equations



G ∩ bd(G) = ∅ ⇒ G is open

bd(F) ⊆ F ⇒ F is closed

bd(S) = bd(ℝ∖S) = bd(S')

The Attempt at a Solution



Suppose A is a clopen set such that it is neither ℝ nor ∅ then ℝ∖A = A' is neither ℝ nor ∅. Now, A' is open because A is open (and closed). So ℝ = A ∪ A', where both A and A' are closed, which implies that ℝ is closed, a contradiction.
 
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  • #2
Why is the fact that "ℝ is closed" a contradiction. ℝ is indeed closed.
 
  • #3
Isn't ℝ a clopen set?
 
  • #4
What's the definition of a "clopen" set?
 

1. What does it mean to prove that A is either ℝ or ∅?

Proving that A is either ℝ or ∅ means showing that the set A contains either all real numbers or is an empty set. In other words, A can only have elements that are real numbers or no elements at all.

2. How do you prove that A is either ℝ or ∅?

To prove that A is either ℝ or ∅, you can use a direct proof, a proof by contradiction, or a proof by contrapositive. In a direct proof, you would show that all elements in A are real numbers or that A has no elements. In a proof by contradiction, you would assume that A contains elements that are not real numbers and show that this leads to a contradiction. In a proof by contrapositive, you would show that if A does not contain all real numbers, then it must be an empty set.

3. What are some examples of sets that are either ℝ or ∅?

Examples of sets that are either ℝ or ∅ include the set of all real numbers, the set of rational numbers, the set of integers, and the empty set. These sets contain only real numbers or no elements at all.

4. Can you prove that A is either ℝ or ∅ without using mathematical symbols?

Yes, it is possible to prove that A is either ℝ or ∅ without using mathematical symbols. This can be done by using words to explain the concept and logic behind the proof. However, using mathematical symbols can make the proof more concise and easier to understand.

5. Why is it important to prove that A is either ℝ or ∅?

Proving that A is either ℝ or ∅ is important because it helps establish the properties and characteristics of the set A. It also allows for the set to be categorized and used in various mathematical operations and proofs. Additionally, it ensures that the elements in the set are well-defined and consistent.

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