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Equivalence class
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Equivalence class

In mathematics, given a set X and an equivalence relation ~ on X, the equivalence class of an element a in X is the subset of all elements in X which are equivalent to a:
[a] = { x in X | x\ ~ a }

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 ~. This operation can be thought of (very informally indeed) as the act of "dividing" the input set by the equivalence relation, hence both the name "quotient", and the notation, which are both reminiscent of division.

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 an epimorphism. See congruence relation.

Table of contents
1 Examples
2 Properties
3 Related topics

Examples

(a,b) ~ (c,d) if and only if ad = bc.
Here the equivalence class of the pair (a,b) can be identified with rational number a/b. Is this the origin of the term quotient set?

Properties

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].

If ~ is an equivalence relation on X, and P(x) is a property of elements of x, such that whenever x ~ y, P(x) is true iff P(y) is true, then the property P is said to be a class invariant under the relation ~. A frequent particular case occurs when f is a function from X to another set Y; if x ~ y implies f(x) = f(y) then f is said to be a class invariant under ~, or simply invariant under ~. This occurs, e.g. in the character theory of finite groups. The latter case with the function f can be expressed by a commutative triangle. See also invariant.

Related topics


In music see octave equivalency, transpositional equivalency, inversional equivalency, enharmonic equivalency. Musical set theory takes advantage of all of these, to varying degrees, while other theories take more or less advantage of a selection.