Is the sample space not a set under ZFL?

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Discussion Overview

The discussion revolves around the concept of sample space in set theory, particularly in the context of Zermelo-Fraenkel set theory with the Axiom of Choice (ZFC) and its implications for defining complements of sets. Participants explore the definitions and interpretations of complements and sample spaces, raising questions about their status as sets.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes confusion regarding the status of the sample space as a set, referencing an exercise from Jech & Hrbacek that suggests the complement of a set is not a set, leading to paradoxes.
  • Another participant clarifies that there are two notions of complement: one defined within a sample space and another as a set of elements not in a given set, which may not be a set itself.
  • Some participants express uncertainty about the meaning of "the state for all set operations," seeking clarification on its implications for the sample space.
  • A participant suggests that the sample space is a collection of sets for which operations like union and subtraction can be performed, while acknowledging that there are sets outside this space that are less relevant for the discussion.
  • One participant provides an example of a sample space related to a coin toss, illustrating the concept with a specific scenario.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the sample space is considered a set under ZFL, and there are competing interpretations of the definitions and implications of complements.

Contextual Notes

The discussion highlights the ambiguity in the definitions of complements and sample spaces, as well as the potential for misunderstanding when different interpretations are applied. There is also a lack of clarity regarding the implications of these definitions on set operations.

scinoob
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I am reading Introduction to Set Theory (Jech & Hrbacek) and in one of the exercises we're asked to prove that the complement of a set is not a set. I get that if it were a set that would imply that "a set of all sets" (the union of the set and its complement, by the axiom of pairing) exists and that leads to paradoxes. However, does that mean that the sample space is not considered a set? I always thought it was a set (and a quick check on Wikipedia confirms that). So, understandably I'm confused.

Any help? Thanks!
 
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scinoob said:
I am reading Introduction to Set Theory (Jech & Hrbacek) and in one of the exercises we're asked to prove that the complement of a set is not a set. I get that if it were a set that would imply that "a set of all sets" (the union of the set and its complement, by the axiom of pairing) exists and that leads to paradoxes. However, does that mean that the sample space is not considered a set? I always thought it was a set (and a quick check on Wikipedia confirms that). So, understandably I'm confused.

Any help? Thanks!

The issue is that there are two (related) notions of a complement in use here.

When you pick out a sample space, that means that you pick out a set ##\Omega## which is the setting for all set operations. In particular, the complement of ##A## is then defined as ##\Omega\setminus A##.

On the other hand, Hrbacek and Jech do not pick out a sample space. Their complement of ##A## is defined as ##\{x~\vert~x\notin A\}## which is not a set.

So you should really be careful which kind of complement you're working with.
 
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Thanks micromass, I wasn't aware there were two different meanings of the term 'complement'. One last thing on this, could you tell me what is meant by 'the state for all set operations'?
 
scinoob said:
Thanks micromass, I wasn't aware there were two different meanings of the term 'complement'. One last thing on this, could you tell me what is meant by 'the state for all set operations'?

Can you please give the context?
 
Oh, it's from your post (second sentence) :)
 
scinoob said:
Oh, it's from your post (second sentence) :)

Is it clear now? I used the wrong translation, sorry.
 
I am not sure. Do you mean that the sample space is the set of all sets for which set operations (like union, subtraction, etc.) can be performed? If so, could you give an example of sets for which these operations are inadmissible?
 
scinoob said:
I am not sure. Do you mean that the sample space is the set of all sets for which set operations (like union, subtraction, etc.) can be performed? If so, could you give an example of sets for which these operations are inadmissible?

It's just that when we specify a sample space, then we agree that these will be the only sets we'll work with. Of course, there will be sets outside the sample space, but we find them less important for our purpose.
In particular, the complement is defined as all elements in the sample space not in the set.
 
Sample space is usually associated to some specific (informal) experiment , and it is a listing of all possible outcomes of the experiment/situation at hand. Maybe the simplest example is that of throwing a coin once and observing what face shows once the coin settles . The sample space is then {H,T}.
 
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