Definition:Universe (Set Theory)

From ProofWiki
Jump to navigation Jump to search

This page is about Universe in the context of Set Theory. For other uses, see Universe.

Definition

Sets are considered to be subsets of some large universal set, also called the universe.

Exactly what this universe is will vary depending on the subject and context.

When discussing particular sets, it should be made clear just what that universe is.

However, note that from There Exists No Universal Set, this universe cannot be everything that there is.


The traditional symbol used to signify the universe is $\mathfrak A$.

However, this is old-fashioned and inconvenient, so some newer texts have taken to using $\mathbb U$ or just $U$ instead.


With this notation, this definition can be put into symbols as:

$\forall S: S \subseteq \mathbb U$


The use of $\mathbb U$ or a variant is not universal: some sources use $X$.


Zermelo-Fraenkel Theory

If the universal class is allowed to be a set in ZF(C) set theory, then a contradiction results.

One equivalent of the axiom of specification states that:

$\forall z: \forall A: \paren {A \subseteq z \implies A \in U}$


However, we may conservatively extend the ZFC axioms to incorporate classes, which is done in von Neumann–Bernays–Gödel (NBG) set theory.

The basic gadget we work with is a class, and a set is defined to be an element of a class. Some refer to a proper class to be one which is not contained in any class (so an "improper" class would be one that is contained in some class, thus it is a set).

We avoid the contradiction mentioned by modifying the axiom of specification through restricting quantifiers to range over sets, but not all classes. (We also demand that sets are not bijective to the class of all ordinals.)

Michael Shulman's "Set theory for category theory" (arXiv:0810.1279v2 [math.CT]) studies various esoteric foundational issues relevant for category theory, and gives the definition of a Universe in any set theory as: a model for the ZFC axioms. This covers the von Neumann universe, the Grothendieck universe, the class of all sets in NBG set theory, etc.

However, some alternative set theories, such as Quine's New Foundations, allow the universal set to be a value of a variable, and reject certain instances of the axiom of specification.


All the elements of the universal set are precisely the Universe of Discourse of quantification.


Also known as

Some sources refer to the universal set as the universe of discourse, which name is also used for a similar concept in the field of logic.

Some sources (particularly in the context of probability theory) refer to it as the space.


Also see

  • Results about the universal set can be found here.


Sources


This page may be the result of a refactoring operation.
As such, the following source works, along with any process flow, will need to be reviewed.
When this has been completed, the citation of that source work (if it is appropriate that it stay on this page) is to be placed above this message, into the usual chronological ordering.
In particular: Determine whether the discussion in the Zermelo-Fraenkel Theory section appears here in any form
If you have access to any of these works, then you are invited to review this list, and make any necessary corrections.
To discuss this page in more detail, feel free to use the talk page.
When this work has been completed, you may remove this instance of {{SourceReview}} from the code.


Retrieved from "https://proofwiki.org/w/index.php?title=Definition:Universe_(Set_Theory)&oldid=573470"