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    The claim holds only within ZFC set theory; in alternativ... — Carmelics
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    Challenges→Two infinite well-ordered sets with the same ordinal number have the same cardinal number.

    The claim holds only within ZFC set theory; in alternative foundations like NFU or Aczel's non-well-founded set theory, the relationship between ordinal and cardinal structure is not preserved in the same way.

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    Key Terms

    Aczel's non-well-founded set theory(as another example of an alternative foundation)
    A mathematical system created by Peter Aczel that allows sets to contain themselves (directly or indirectly), unlike in ZFC where this is forbidden.
    Alternative foundations(as competing systems mentioned in contrast)
    Different mathematical rule systems that mathematicians could use instead of ZFC to organize their work.
    NFU(in set theory)
    A modified version of Quine's NF system that Jensen created; it adds extra objects called 'urelements' to make the system work more flexibly.
    Ordinal(as a mathematical structure being preserved or not)
    A type of number used in mathematics to describe the order or position of things (like 1st, 2nd, 3rd), not just how many.
    Preserved

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    (describing whether ordinal and cardinal structure maintain their relationships)
    In this context, it means the relationships between two ideas work the same way or stay consistent.
    Set theory(as used in mathematics)
    A branch of mathematics that studies collections of objects (called 'sets') and the rules for how they relate to each other.
    ZFC set theory(as the primary formal system being discussed)
    A standard system of rules for organizing and working with collections of things (called sets) in mathematics; most mathematicians use this system as their foundation.
    cardinal(in mathematics and set theory)
    A number that represents the size of a set (how many things are in it), as opposed to the order they're in.

    Connections

    2 topics

    Proof of definition segments1 linkedModality & Possibility1 linked

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    Two infinite well-ordered sets with the same ordinal number have the same cardin...

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