MHB Prove that A and B are disjoint

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To prove that two sets A and B are disjoint, one must show that if an element x belongs to A, then it does not belong to B, which is sufficient for disjointness. It is not necessary to prove the converse, as establishing one direction suffices. The proof requires that the choice of x be arbitrary, ensuring that the conclusion applies to all elements in A. A common mistake is to assume that showing a specific element is not in B proves disjointness, which is incorrect. For a rigorous approach, referencing a text like Munkres' "Topology" may provide additional insights on the topic.
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How do I prove that 2 sets A and B are disjoint?

Do I let $\displaystyle x\in A$ and prove that $\displaystyle x\notin B$? Do I also have to prove the converse?
 
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Alexmahone said:
How do I prove that 2 sets A and B are disjoint?
Do I let $\displaystyle x\in A$ and prove that $\displaystyle x\notin B$? Do I also have to prove the converse?
As stated, the question is too vague to give a clear answer.
That said, you might suppose that \exists x\in A\cap B.
Prove that leads to a contradiction.
 
Plato said:
As stated, the question is too vague to give a clear answer.
That said, you might suppose that \exists x\in A\cap B.
Prove that leads to a contradiction.

Thanks for your answer, but why is the question too vague?
 
Alexmahone said:
Thanks for your answer, but why is the question too vague?
Because you told us nothing about $A\text{ or }B$.
 
Plato said:
Because you told us nothing about $A\text{ or }B$.

A and B are 2 arbitrary sets.
 
Alexmahone said:
A and B are 2 arbitrary sets.
\left( {\forall x \in \mathcal{ U}} \right)\left[ x \notin A\cap B \right].
 
if for ALL x in A, x is not in B, that would be sufficient. why? because that asserts that A is a subset of A - B = A - A∩B. since A - B is automatically a subset of A, we get immediately that A = A - B = A - A∩B, hence A∩B = Ø (if y was in A∩B, then y would not be in A-B, a contradiction since A = A-B).

however, you have to be careful. just picking "some" x in A, and showing it is not in B, just shows x is in A-B, which can happen when A and B are not disjoint. the choice of x has to be completely arbitrary (a "for all" choice, not a "there exists" choice).

for example, let's use "your method" to prove (0,1) and (3,4) (these are real intervals) are disjoint. let x be any real number in (0,1). then 0 < x < 1. since 1 < 3, by the transitivity of < we have x < 3. hence (x > 3)&(x < 4) is false (trichotomy property: exactly one of x < 3, x = 3 or x > 3 can be true...note this is a specific property of real numbers, we are using the fact that R is an ordered field, here), that is: 3 < x < 4 is false, so x is not in (3,4). since x is arbitrary, we conclude (0,1) and (3,4) are disjoint.

the following is a "bad proof": let x be in (0,1). then x is not in (3,4), so (0,1) and (3,4) are disjoint. why is this bad? because x might be 1/2, and all we have shown is 1/2 is not in (3,4).

it's a subtle difference, and Plato's posts are meant to underscore the important part: disjoint sets don't "overlap". put another way: quantification matters (logically speaking).
 
It is sufficient to prove "if x is in A then it is NOT in B" or "if x is in B then it is NOT in A". You do not have to prove both.
 
Alexmahone said:
How do I prove that 2 sets A and B are disjoint?

Do I let $\displaystyle x\in A$ and prove that $\displaystyle x\notin B$? Do I also have to prove the converse?

In Munkres (Topology), there is a proof about showing sets are disjoint which is probably along the lines you are looking for. However, I can't remember off hand but it may not be too set theoretic. Chapter 2 or 3 I believe. It will be on a left hand side page at the bottom continuing to right page on the top.
 
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