In set theory, a set A is transitive, if
- whenever x ∈ A, and y ∈ x, then y ∈ A, or, equivalently,
- whenever x ∈ A, and x is not an urelement, then x is a subset of A.
Similarly, a class M is transitive if every element of M is a subset of M.
Contents |
Examples
Using the definition of ordinal numbers suggested by John von Neumann, ordinal numbers are defined as hereditarily transitive sets: an ordinal number is a transitive set whose members are also transitive (and thus ordinals).
Any of the stages Vα and Lα leading to the construction of the von Neumann universe V and Gödel's constructible universe L are transitive sets. The universes L and V themselves are transitive classes.
Properties
A set X is transitive if and only if 
, where 
is the union of all elements of X that are sets, 
. If X is transitive, then is transitive. If X and Y are transitive, then X∪Y∪{X,Y} is transitive. In general, if X is a class all of whose elements are transitive sets, then 
is transitive.
A set X which does not contain urelements is transitive if and only if it is a subset of its own power set, 
The power set of a transitive set without urelements is transitive.
Transitive closure
The transitive closure of a set X is the smallest (with respect to inclusion) transitive set which contains X. Suppose one is given a set X, then the transitive closure of X is
Transitive models of set theory
Transitive classes are often used for construction of interpretations of set theory in itself, usually called inner models. The reason is that properties defined by bounded formulas are absolute for transitive classes.
A transitive set (or class) that is a model of a formal system of set theory is called a transitive model of the system. Transitivity is an important factor in determining the absoluteness of formulas.
See also
References
- Jech, Thomas (2008) [originally published in 1973]. The Axiom of Choice. Dover Publications. ISBN 0-486-46624-8.
 |
This mathematical logic-related article is a stub. You can help Wikipedia by expanding it. |
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
