set (in mathematics)

The Columbia Encyclopedia, 6th ed.

set (in mathematics)

set, in mathematics, collection of entities, called elements of the set, that may be real objects or conceptual entities. Set theory not only is involved in many areas of mathematics but has important applications in other fields as well, e.g., computer technology and atomic and nuclear physics.

Definition of Sets

A set must be well defined; i.e., for any given object, it must be unambiguous whether or not the object is an element of the set. For example, if a set contains all the chairs in a designated room, then any chair can be determined either to be in or not in the set. If there were no chairs in the room, the set would be called the empty, or null, set, i.e., one containing no elements. A set is usually designated by a capital letter. If A is the set of even numbers between 1 and 9, then A={2, 4, 6, 8}. The braces, {}, are commonly used to enclose the listed elements of a set. The elements of a set may be described without actually being listed. If B is the set of real numbers that are solutions of the equation x2=9, then the set can be written as B={x:x2=9} or B={x|x2=9}, both of which are read: B is the set of all x such that x2=9; hence B is the set {3,-3}.

Membership in a set is indicated by the symbol ∈ and nonmembership by ∉; thus, xA means that element x is a member of the set A (read simply as "x is a member of A" ) and yA means y is not a member of A. The symbols ⊂ and ⊃ are used to indicate that one set A is contained within or contains another set B;AB means that A is contained within, or is a subset of, B; and AB means that A contains, or is a superset of, B.

Operations on Sets

There are three basic set operations: intersection, union, and complementation. The intersection of two sets is the set containing the elements common to the two sets and is denoted by the symbol ∩. The union of two sets is the set containing all elements belonging to either one of the sets or to both, denoted by the symbol ∪. Thus, if C={1, 2, 3, 4} and D={3, 4, 5}, then CD={3, 4} and CD={1, 2, 3, 4, 5}. These two operations each obey the associative law and the commutative law, and together they obey the distributive law.

In any discussion the set of all elements under consideration must be specified, and it is called the universal set. If the universal set is U={1, 2, 3, 4, 5} and A={1, 2, 3}, then the complement of A (written A′) is the set of all elements in the universal set that are not in A, or A′={4, 5}. The intersection of a set and its complement is the empty set (denoted by ∅), or AA′=∅; the union of a set and its complement is the universal set, or AA′=U. See also symbolic logic.

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