Is P(A) ⊆ P(B) a Sufficient Condition for A ⊆ B?

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Discussion Overview

The discussion revolves around the question of whether the inclusion of power sets, specifically P(A) ⊆ P(B), is a sufficient condition to conclude that A ⊆ B. The scope includes mathematical reasoning and proof structure.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asserts that if P(A) ⊆ P(B), then A must be a subset of B, providing a proof attempt based on the definitions of power sets.
  • Another participant challenges the proof by requesting an additional justification for a step that concludes A is in P(B) based on A being in P(A).
  • A different participant suggests a clearer proof structure, recommending starting with an arbitrary element x in A and demonstrating its membership in B step-by-step.
  • One participant argues that if x is in A, then the singleton set {x} is in P(A), and thus must also be in P(B), leading to the conclusion that x is in B.
  • Another participant corrects the previous claim, stating that the reason {x} is in P(A) is due to it being a subset of A, not because it is a singleton.
  • A later reply acknowledges the correction and thanks the participant for the clarification.

Areas of Agreement / Disagreement

Participants express differing views on the proof structure and the validity of the initial claims. There is no consensus on the sufficiency of the condition P(A) ⊆ P(B) for concluding A ⊆ B, and the discussion remains unresolved.

Contextual Notes

Participants highlight the need for clearer logical steps in the proof and the importance of definitions in the context of power sets. There are unresolved issues regarding the proof's organization and the implications of the definitions used.

YamiBustamante
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There's what I have so far.
We assume that P(A) ⊆ P(B). This means that every element x that exists in P(A), also exits in P(B). By definition of a power set, x∈P(A) if x ⊆ A. Therefore, A∈P(A). Since P(A) ⊆ P(B), A∈P(B), meaning all x ⊆ A, x ∈ P(B). Furthermore, B∈P(B), meaning all x ⊆ B, x ∈ P(B). Since x ⊆ A and x ⊆ B and P(A) ⊆ P(B), A ⊆ B.

Is my proof correct?
 
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YamiBustamante said:
x∈P(A) if x ⊆ A. Therefore, A∈P(A).
You need to put in an extra step to justify this 'therefore'.

I can't make anything of what is written after that. It doesn't seem to prove the required conclusion.

You've shown (after adding the extra step) that A is in the powerset of A.
Can you prove that it's therefore in the powerset of B? (very easy)
If you can prove that, just use the definition of 'powerset' to get to your conclusion.
 
Your statements are not organized in the way a proof should be. So it is hard to follow your logic. Start with an arbitrary x in A and show, step-by-step that x is in B:
x∈A
Then what does that say about x and P(A)?
Then what does that say about x and P(B)?
Then what does that say about x and B?
 
Sorry but if ##x\in A## then ##\{x\} \in \mathcal{P}(A)## because is a singleton and by original assumption ##\{x\}\in \mathcal{P}(B)## so ##\{x\}## is a subset of ##B## that is (by transitivity of the order ##\subseteq ##) ##x\in B##. This hold for every ##x\in A## so the conclusion. I have lost something?
 
Ssnow said:
Sorry but if ##x\in A## then ##\{x\} \in \mathcal{P}(A)## because is a singleton
No. The reason ##\{x\} \in \mathcal{P}(A)## is that ##\{x\}## is a subset of ##A##. Being a singleton has nothing to do with it.
 
ok, I used a bad expression sorry, sure the reason is what you said @andrewkirk ... thks
 

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