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ankit.jain
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It states that no set can be an element of itself..what does it exactly mean?why can't. We consider the whole set as a single element?
fresh_42 said:You have to exclude the possibility that a set can contain itself as an element because otherwise you will run into paradoxa: "The barber in this village shaves everyone except those who shave themselves."
So does the barber shave himself or not?
Thank you for clarification.micromass said:It is the strengthening of the comprehension axiom that avoids the barber paradox.
At this point I got stuck. If the axiom of regularity reads ∀x (x ≠ ∅ ⇒ ∃y ∈ x (y∩x = ∅) ) so why isn't the construction S = {S} forbidden by it, since x = y = S are the only elements and therefore in contradiction to the axiom. At which point am I wrong?The axiom of regularity does not avoid any paradoxes. The barber paradox cannot be avoided by the axiom of regularity.
fresh_42 said:At this point I got stuck. If the axiom of regularity reads ∀x (x ≠ ∅ ⇒ ∃y ∈ x (y∩x = ∅) ) so why isn't the construction S = {S} forbidden by it, since x = y = S are the only elements and therefore in contradiction to the axiom. At which point am I wrong?
What exactly is still not clear?ankit.jain said:Still not clear.
Yes, the whole set is considered as a single element. The axiom of regularity implies that for any set S, S∈S is not allowed.ankit.jain said:It states that no set can be an element of itself..what does it exactly mean?why can't. We consider the whole set as a single element?
FactChecker said:How can you ever say that you have precisely defined a set that contains itself? I am curious if there is a well-defined example of such a thing. If not, I think it is irrelevant to mathematics.
From my understanding, you don't have to define them. You simply omit the axiom of regularity/foundation and study the hypothetical "sets" that would violate it. [itex]\in[/itex] is just a binary predicate relating two objects we happen to call "sets" in set theory. Every object in the domain of the first-order system of ZFC is called a "set". It is the axioms themselves that demand "sets" operate as we would envision collections of "things" in the real world. Of course, we can have other things in the universe for which the extensionality axiom doesn't apply. You then have to add an "is a set" predicate and preface the extensionality axiom with itFactChecker said:How can you ever say that you have precisely defined a set that contains itself? I am curious if there is a well-defined example of such a thing. If not, I think it is irrelevant to mathematics.
mbs said:From my understanding, you don't have to define them. You simply omit the axiom of regularity/foundation and study the hypothetical "sets" that would violate it. [itex]\in[/itex] is just a binary predicate relating two objects we happen to call "sets" in set theory. Every object in the domain of the first-order system of ZFC is called a "set". It is the axioms themselves that demand "sets" operate as we would envision collections of "things" in the real world. Of course, we can have other things in the universe for which the extensionality axiom doesn't apply. You then have to add an "is a set" predicate and preface the extensionality axiom with it
I think I understand. So leaving it out does not necessarily mean that there is, in fact, a set that contains itself. It just means that you can not use that axiom (no set contains itself) in proofs.mbs said:From my understanding, you don't have to define them. You simply omit the axiom of regularity/foundation and study the hypothetical "sets" that would violate it. [itex]\in[/itex] is just a binary predicate relating two objects we happen to call "sets" in set theory. Every object in the domain of the first-order system of ZFC is called a "set". It is the axioms themselves that demand "sets" operate as we would envision collections of "things" in the real world. Of course, we can have other things in the universe for which the extensionality axiom doesn't apply. You then have to add an "is a set" predicate and preface the extensionality axiom with it
FactChecker said:I think I understand. So leaving it out does not mean that there is, in fact, a set that contains itself, it just means that you can not use that axiom (no set contains itself) in proofs.
I guess you could hypothetically define an example, just by saying that the set X has itself as an element. Although I would like to say that X is not well-defined, I would need to use the axiom to say that.micromass said:You're correct of course, but I interpreted his post as assuming the negation of regularity.
Okay. I don't think I said anything that contradicts anything you're saying. A set for which the axiom of foundation fails is simply a (non-empty) set S in which every member of S contains another member of S. Simply removing the foundation axiom keeps such sets purely hypothetical. You would need addition axioms to formally construct the existence of all such sets (much like the axiom of infinity constructs the integers). Simply having one non-regular set isn't enough to prove anything.micromass said:Of course we can just leave out the regularity axiom, but that doesn't get us far. All that negating the foundation axiom does, is saying there is one set for which the foundation axiom fails. You don't even have any information regarding the set. And even if you do. For example, take a set ##x## for which ##x=\{x\}##. Is such a set ##x## unique? How do we check equality with such sets? The extensionality axiom is useless here.
So just leaving out the regularity axiom is not useful, to have something useful, we should replace it with a better axiom for which we can actually prove things with. For example https://en.wikipedia.org/wiki/Aczel's_anti-foundation_axiom
What a fascinating area of study - I couldn't understand why I had not seen this before, then realized that I finished full-time academic study two years before Aczel's paper. Can you point me to a good, relatively discursive book? I was never very fond of books containing long formal proofs of theorem after theorem with no apparent motivation and 30 years out of study has not made me any more so.micromass said:
MrAnchovy said:What a fascinating area of study - I couldn't understand why I had not seen this before, then realized that I finished full-time academic study two years before Aczel's paper. Can you point me to a good, relatively discursive book? I was never very fond of books containing long formal proofs of theorem after theorem with no apparent motivation and 30 years out of study has not made me any more so.
Many thanks, ordered from the train on the way home and it will be with me on Sunday apparently - how did life work before Amazon?micromass said:The book by Hrbacek and Jech contains a nice chapter on this anti-foundation thing. It's a really great book to read. https://www.amazon.com/dp/0824779150/?tag=pfamazon01-20
The axiom of regularity, also known as the axiom of foundation, is a fundamental principle in set theory that states that every non-empty set contains an element that is disjoint from the set itself. In other words, a set cannot be a member of itself.
The axiom of regularity is important because it ensures that sets do not contain circular or self-referential elements, which can lead to paradoxes and contradictions in mathematics. It also allows for a well-defined hierarchy of sets, which is essential for building a strong foundation for mathematical reasoning.
The axiom of regularity is one of the axioms in the Zermelo-Fraenkel set theory, which is the standard system of axioms used in modern mathematics. It is closely related to other axioms such as the axiom of extensionality, which states that sets with the same elements are equal, and the axiom of choice, which allows for the creation of new sets from existing ones.
One example is the set {1, {1}}, where the element {1} is also a subset of the set itself. Another example is the set of all sets, which contains itself as a member. These examples violate the axiom of regularity because they contain elements that are not disjoint from the set itself.
The axiom of regularity can be used in mathematical proofs to ensure that sets are well-defined and do not contain paradoxical elements. It can also be used to prove the existence of certain objects, such as the smallest element in a set, by showing that it is disjoint from the set itself.