In set theory and its applications throughout mathematics, a class is a collection of sets (or sometimes other mathematical objects) that can be unambiguously defined by a property that all its members share. Some classes are sets, for instance the class of all integers that are even, but others are not, for instance the class of all ordinal numbers or the class of all sets. Classes that are not sets are called proper classes.
The standard Zermelo-Fraenkel set theory axioms do not talk about classes; classes exist only in the metalanguage as equivalence classes of logical formulas. Another approach is taken by the von Neumann-Bernays-Gödel axioms; classes are the basic objects in this theory, and a set is then defined to be a class that is an element of some other class. The proper classes, then, are those classes that are not elements of any other class.
Several objects in mathematics are too big for sets and need to be described with classes, for instance large categories or the class-field of surreal numbers.
The word "class" is sometimes used synonymously with "set," most notably in the term "equivalence class." This usage dates from a historical period where classes and sets were not distinguished as they are in modern terminology. Many discussions of "classes" in the 19th century and earlier are really referring to sets, or perhaps to a more ambiguous concept.
Some classes are sets (for instance, the class of all integers that are even), but others are not (for instance, the class of all ordinal numbers or the class of all sets).
A proper class cannot be an element of a set or a class and is not subject to the Zermelo-Fraenkel axioms of settheory; thereby a number of paradoxes of naive settheory are avoided.
The word "class" is sometimes used synonymously with "set," most notably in the term "equivalence class." This usage dates from a historical period where classes and sets were not distinguished as they are in modern terminology.
Share your thoughts, questions and commentary here
Want to know more? Search encyclopedia, statistics and forums:
Press Releases |
The Wikipedia article included on this page is licensed under the
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m