Equivalence class

Given a set X and an equivalence relation ~ over X, an equivalence class is a subset of X of the form

{ x in X | x ~ a }

where a is an element in X. This equivalence class is usually denoted as [a].

Because of the properties of an equivalence relation it holds that a is in [a] and that any two equivalence classes are either equal or disjoint. It follows that the set of all equivalence classes of X forms a partition of X: every element of X belongs to one and only one equivalence class. Conversely every partition of X also defines an equivalence relation over X.

It also follows from the properties of an equivalence relation that

a ~ b if and only if [a] = [b].

The notion of equivalence classes is useful for constructing sets out of already constructed ones. The set of all equivalence classes in X given an equivalence relation ~ is usually denoted as X / ~ and called the quotient set of X by ~. In cases where X has some additional structure preserved under ~, the quotient becomes an object of the same type in a natural fashion; the map that sends a to [a] is then a homomorphism.

Examples:

• If X is the set of all cars, and ~ is the equivalence relation of "having the same color", then one particular equivalence class consists of all green cars. X / ~ could be naturally identified with the set of all car colors.
• The rational numbers can be constructed as the set of equivalence classes of pairs of integers (a,b) where the equivalence relation is defined by

(a,b) ~ (c,d) if and only if ad = bc.

• The real numbers can be constructed as the set of equivalence classes on the set of Cauchy sequences of rational numbers, where the equivalence relation is defined by

(x_n)_{n=1...infinity} ~ (y_n)_{n=1...infinity} if and only if (x_n - y_n) -> 0 as n -> infinity

• Every group can be partitioned into equivalence classes called conjugacy classes.
• The homotopy class of a continuous map f is the equivalence class of all maps homotopic to f.

See also:

-- rational numbers -- multiplicatively closed set -- real numbers -- homotopy theory -- up to