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Set theory is a branch of mathematics that studies sets, their operations, and their properties.

• A set is a collection of disjoint items.

Basic symbols

Operators

• the union operator, ∪, pronounced “cup”, means “or”;
• the intersection operator, ∩, pronounced “cap”, means “and”;
• the exclusion operator, \, means “without”;
• the complement operator, ', means “the inverse of”;
• the cross operator, ×, means “the Cartesian product of”.

Qualifiers

• the colon, :, or the vertical bar | qualifiers are interchangeable and mean “such that”;
• the membership qualifier, ∈, means “belongs to”;
• the subset qualifier, ⊆, means “is a subset of”;
• the proper subset qualifier, ⊂, means “is a subset of but is not equal to”.

Canonical sets

• ∅, the empty set, i.e. the set containing no items;
• ℕ, the set of all natural numbers;
• ℤ, the set of all integers;
• ℚ, the set of all rational numbers;
• ℝ, the set of all real numbers.

There are a few caveats to mention regarding the canonical sets: 1. Even though the empty set contains no items, the empty set is a subset of itself (and indeed every other set); 2. Mathematicians generally do not universally agree on whether zero is a natural number, and textbooks will typically explicitly state whether or not the author considers zero to be a natural number.

Cardinality

The cardinality, or size, of a set is determined by the number of items in the set. The cardinality operator is given by a double pipe, |...|.

For example, if S = { 1, 2, 4 }, then |S| = 3.

The Empty Set

• The empty set can be constructed in set builder notation using impossible conditions, e.g. ∅ = { x : x ≠ x }, or ∅ = { x : x ∈ N, x < 0 };
• the empty set is always unique (i.e. there is one and only one empty set);
• the empty set is a subset of all sets;
• the cardinality of the empty set is 0, i.e. |∅| = 0.

Representing sets

Literal Sets

A set can be constructed literally by supplying a complete list of objects contained in the set. For example, S = { a, b, c, d }.

Long lists may be shortened with ellipses as long as the context is clear. For example, E = { 2, 4, 6, 8, ... } is clearly the set of all even numbers, containing an infinite number of objects, even though we’ve only explicitly written four of them.

Set Builder

Set builder notation is a more descriptive way of constructing a set. It relies on a subject and a predicate such that S = { subject : predicate }. For example,

A = { x : x is a vowel } = { a, e, i, o, u }
B = { x : x ∈ N, x < 10 } = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }
C = { x : x = 2k, k ∈ N } = { 0, 2, 4, 6, 8, ... }

Sometimes the predicate may “leak” into the subject, e.g.

D = { 2x : x ∈ N } = { 0, 2, 4, 6, 8, ... }

Relations

Membership

• If the value a is contained in the set A, then we say a belongs to A and represent this symbolically as a ∈ A.
• If the value a is not contained in the set A, then we say a does not belong to A and represent this symbolically as a ∉ A.

Equality

• If two sets contain the same items then we say the sets are equal, e.g. A = B.
• Order does not matter when determining set equality, e.g. { 1, 2, 3, 4 } = { 2, 3, 1, 4 }.
• Sets are disjoint, meaning elements cannot be repeated, e.g. { 1, 2, 2, 3, 4, 3, 4, 2 } = { 1, 2, 3, 4 }.
• Two sets A and B are equal if and only if A ⊆ B and B ⊆ A.

Special Sets

The Power Set

• Let A be any set. The set that contains all possible subsets of A is called a “power set” and is written as P(A). If the set A contains n elements, then P(A) contains 2^n elements.

P(A) = { x : x ⊆ A }

Set operations among two sets

Union

Given two sets A and B, the union of the two sets are the items that appear in either A or B, written as A ∪ B.

A ∪ B = { x : x ∈ A ∪ x ∈ B }

Intersection

Given two sets A and B, the intersection of the two sets are the items that appear in both A and B, written as A ∩ B.

A ∩ B = { x : x ∈ A, x ∈ B }

Difference

Given two sets A and B, the set difference of A with B is every item in A that does not belong to B.

A \ B = { x : x ∈ A, x ∉ B }

Symmetrical difference

Given two sets A and B, the symmetrical difference is all items among A and B that doesn’t appear in their intersections.

A △ B = { x : ((x ∈ A) ∩ (x ∉ B)) ∪ ((x ∈ B) ∩ (x ∉ A)) }

A △ B = (A \ B) ∪ (B \ A)

Cartesian product

Given two sets A and B, the cartesian product between A and B consists of a set containing all combinations of items of A and B.

A × B = { (x, y) | x ∈ A, y ∈ B }

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Originally contributed by Andrew Ryan Davis, and updated by 9 contributors.