Axiom of Foundation: The Limitations and Implications for Sets in Mathematics

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In summary, the axiom of foundation, also known as the axiom of regularity, states that every non-empty set contains an element disjoint from itself. This means that in the case of an infinite sequence, such as a_1 \in a_2 \in a_3 \in a_4 \in a_1, it is not possible for all elements to be contained within each other, as this would violate the axiom. This can also be expanded to infinite sequences, as shown in the provided link, which further proves its validity. The first argument presented, using A = \{a_1,a_2,a_3,a_4\}, is particularly elegant and effective in demonstrating the impossibility of an infinite descending sequence
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quantum123
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Why does the axiom of foundation not allow this?

[tex] a_1 \in a_2 \in a_3 \in a_4 \in a_1 [/tex] ?
 
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The axiom of regularity (foundation) states that every non-empty set A contains an element disjoint from A.

If it's possible, then [tex]A = \{a_1,a_2,a_3,a_4\}[/tex] is a set, so A contains an element disjoint from A. It's not [tex]a_1[/tex], since [tex]a_4 \in a_1[/tex], it's not [tex]a_2[/tex], since [tex]a_1 \in a_2[/tex], it's not [tex]a_3[/tex], since [tex]a_2 \in a_3[/tex], and it's not [tex]a_4[/tex], since [tex]a_3 \in a_4[/tex]; a contradiction. Hence it's not possible.

This can be generalized to infinite sequences as shown below:
http://en.wikipedia.org/wiki/Axiom_of_regularity#No_infinite_descending_sequence_of_sets_exists
Alternatively for you case then, this expands to an infinite sequence [tex]... \in a_3 \in a_4 \in a_1 \in a_2 \in a_3 \in a_4 \in a_1[/tex] which is impossible by the reasons given in the link.
 
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  • #3
wow, I like the first argument using [tex]A = \{a_1,a_2,a_3,a_4\}[/tex] !
thank you so much.
ingenius and beautiful :)
 

1. What is the Axiom of Foundation?

The Axiom of Foundation, also known as the Axiom of Regularity, is one of the axioms of Zermelo-Fraenkel set theory, which is the standard foundation for modern mathematics. It states that every non-empty set must contain an element that is disjoint from the set itself. In simpler terms, this means that a set cannot contain itself as an element.

2. Why is the Axiom of Foundation important in mathematics?

The Axiom of Foundation plays a crucial role in preventing paradoxes such as Russell's paradox, which arises when a set contains itself as an element. By prohibiting such sets, the Axiom of Foundation ensures that the sets used in mathematical proofs are well-defined and do not lead to contradictions.

3. What are the limitations of the Axiom of Foundation?

One limitation of the Axiom of Foundation is that it does not fully address the issue of infinite descending chains of sets. This means that there are some sets that cannot be constructed within Zermelo-Fraenkel set theory using the Axiom of Foundation alone. Other axioms, such as the Axiom of Choice, are needed to fully describe these sets.

4. Can the Axiom of Foundation be proven?

No, the Axiom of Foundation cannot be proven within Zermelo-Fraenkel set theory. It is considered to be an axiom, or a fundamental assumption, that cannot be derived from other axioms. However, the Axiom of Foundation can be consistent with other axioms, meaning that it does not lead to any contradictions.

5. Are there any implications of the Axiom of Foundation in mathematics?

Yes, the Axiom of Foundation has implications for various fields of mathematics, such as topology, logic, and set theory. It helps to ensure the consistency and coherence of mathematical systems, and is a fundamental principle that underlies many important theorems and proofs in mathematics.

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