This article is about sets in mathematics. For other meanings, see Set (disambiguation).

Sets are one of the most important and fundamental concepts in modern mathematics. Basic set theory, having only been invented at the end of the 19th century, is now a ubiquitous part of mathematics education, being introduced as early as elementary school. This article gives a brief and basic introduction to what mathematicians call "intuitive" or "naive" set theory; for a more detailed account see Naive set theory. For a rigorous axiomatic treatment of sets see Axiomatic set theory.

Introduction

Informally, a set is just a well-defined collection of objects considered as a whole. The objects of a set are called elements or members. The elements of a set can be anything: numbers, people, letters of the alphabet, other sets, and so on. Sets are conventionally denoted with capital letters, A, B, C, etc. Two sets A and B are said to be equal, written A = B, if they have the same members.

Describing Sets

Descriptions using words or lists

A set may be defined by specifying in words the property which characterizes it, and enclosing this description in curly braces. For example:

A = {the first four positive whole numbers}

B = {the colors of the Tricolore (the French flag)}

Alternatively, a set can be defined by explicitly listing its elements between curly braces, for example:

C = {4, 2, 1, 3}

D = {red, white, blue}

Even though two sets may be described differently, they still may be identical as sets. For example, for the sets described above, A = C and B = D, since they have precisely the same members. It makes no difference in what order the elements are listed, nor whether there are repetitions in the list. For example, the three sets {6, 11}, {11, 6}, and {11, 11, 6, 11} are identical, since each has just two distinct members 6 and 11.

Descriptions using mathematical notation

For large sets (that is to say, sets in which there are many elements), it becomes highly impractical to explicitly write out the full list of contents. For example, E = {the first one thousand positive whole numbers} would, as a list, be as tedious to write as it would be to read. However, a mathematician would seldom describe E in words as above, preferring instead to use a symbolic shorthand:

E = {1, 2, 3, ..., 1000}

An abbreviated list can be used to describe a set such as E, where the elements can follow a pattern that is obvious to the reader. The full list is abbreviated using the ellipsis (...) symbol.

If, on the other hand, the characterizing property describes a less obvious pattern, then it is ill-advised to use an abbreviated list, which could serve to confuse the reader. For example, upon reading

F = {–4, –3, 0, ..., 357}

it is unclear that

F = {the first twenty numbers which are four less than a square number}.

In such circumstances, mathematicians describe the characterizing property of the set using mathematical notation. For example:

F = {n² – 4 : n is integer and 0 n 19}

In this description, the colon (:) means such that, and the mathematician interprets this description as

F is the set of numbers of the form n² – 4, such that n is a whole number between 0 and 19.

An explicit list of the contents of F can be found by evaluating the expression n² – 4 for each value of n from 0 to 19.

For more information on describing sets see Set-builder notation.

Cardinality of a set

Each of the sets described above have a definite number of members; for example, the set A has four members, while the set B has three members.

A set can also have zero members. Such a set is called the empty set (or the null set) and is denoted by the symbol ∅. For example, as of 2004, the set A of all people living on the moon, has zero members, and thus, A = ∅. Like the number zero, seemingly trivial, the empty set turns out to be quite important in mathematics.

For more information on the empty set see Empty set.

A set can also have an infinite number of members; for example, the set of natural numbers is infinite.

For more information on infinity and the size of sets see Cardinality and Cardinal number.

Subsets

If every member of the set A is also a member of the set B, then A is said to be a subset of B, written A ⊆ B, or equivalently B ⊇ A, which can be read as B is a superset of A, B includes A or B contains A. The relationship between sets established by ⊆ is called inclusion or containment.

If A is a subset of but not equal to B, then A is called a proper subset of B, written A ⊂ B.

Examples:

• The set of all men is a proper subset of the set of all people.
• The set of all natural numbers is a proper subset of all integers.
• {1, 3} ⊂ {1, 2, 3, 4}
• {1, 2, 3, 4} ⊆ {1, 2, 3, 4}

The empty set is a subset of every set and every set is a subset of itself:

• ∅ ⊆ A
• A ⊆ A

For more information about subsets, see Subset.

Unions

There are several ways to construct new sets from existing ones. Two sets can be "added" together. The union of A and B, denoted by A ∪ B, is the set of all things which are members of either A or B.

Examples:

• {1, 2} ∪ {red, white} = {1, 2, red, white}
• {1, 2, green} ∪ {red, white, green} = {1, 2, red, white, green}
• {1, 2} ∪ {1, 2} = {1, 2}

Some basic properties of unions:

• A ∪ B   =   B ∪ A
• A  ⊆  A ∪ B
• A ∪ A   =  A
• A ∪  ∅   =  A

For more information about unions of sets, see Union (set theory).

Intersections

A new set can also be constructed by determining which members two sets have "in common". The intersection of A and B, denoted by A ∩ B, is the set of all things which are members of both A and B. If A ∩ B  =   ∅, then A and B are said to be disjoint.

Examples:

• {1, 2} ∩ {red, white} = ∅
• {1, 2, green} ∩ {red, white, green} = {green}
• {1, 2} ∩ {1, 2} = {1, 2}

Some basic properties of intersections:

• A ∩ B   =   B ∩ A
• A ∩ B   ⊆   A
• A ∩ A   =   A
• A ∩  ∅   =    ∅

For more information about intersections of sets, see Intersection (set theory).

Complements

Two sets can also be "subtracted". The relative complement of A in B (also called the set theoretic difference of B and A), denoted by B − A, is the set of all elements which are members of B, but not members of A.

In certain settings all sets under discussion are considered to be subsets of a given universal set U. In such cases, U − A, is called the absolute complement or simply complement of A, and is denoted by A′.

Examples:

• {1, 2} − {red, white} = {1, 2}
• {1, 2, green} − {red, white, green} = {1, 2}
• {1, 2} − {1, 2} = ∅
• If U is the set of integers, then the complement of the even integers is the odd integers

Some basic properties of complements:

• A ∪ A′ = U
• A ∩ A′ = ∅
• (A′ )′ = A
• A − B = A ∩ B′

For more information about complements of sets, see Complement (set theory).

Further reading

For more information on the basic properties of sets, subsets, intersections, unions and complements, see The algebra of sets. For a more general development of these ideas and others in set theory, see Naive set theory.

References

• Halmos, Paul R., Naive Set Theory, Princeton, N.J.: Van Nostrand (1960) ISBN 0387900926
• Stoll, Robert R., Set Theory and Logic, Mineola, N.Y.: Dover Publications (1979) ISBN 0486638294