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In mathematics, when the elements of some set S have a notion of equivalence (formalized as an
equivalence relation In mathematics, an equivalence relation is a binary relation that is reflexive, symmetric and transitive. The equipollence relation between line segments in geometry is a common example of an equivalence relation. Each equivalence relatio ...
), then one may naturally split the set S into equivalence classes. These equivalence classes are constructed so that elements a and b belong to the same equivalence class if, and only if, they are equivalent. Formally, given a set S and an equivalence relation \,\sim\, on S, the of an element a in S, denoted by is the set \ of elements which are equivalent to a. It may be proven, from the defining properties of equivalence relations, that the equivalence classes form a partition of S. This partition—the set of equivalence classes—is sometimes called the quotient set or the quotient space of S by \,\sim\,, and is denoted by S / \sim. When the set S has some structure (such as a
group operation In mathematics, a group is a set and an operation that combines any two elements of the set to produce a third element of the set, in such a way that the operation is associative, an identity element exists and every element has an inverse. Thes ...
or a
topology In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ho ...
) and the equivalence relation \,\sim\, is compatible with this structure, the quotient set often inherits a similar structure from its parent set. Examples include quotient spaces in linear algebra, quotient spaces in topology,
quotient group A quotient group or factor group is a mathematical group obtained by aggregating similar elements of a larger group using an equivalence relation that preserves some of the group structure (the rest of the structure is "factored" out). For exam ...
s, homogeneous spaces, quotient rings, quotient monoids, and quotient categories.


Examples

* If X is the set of all cars, and \,\sim\, is the
equivalence relation In mathematics, an equivalence relation is a binary relation that is reflexive, symmetric and transitive. The equipollence relation between line segments in geometry is a common example of an equivalence relation. Each equivalence relatio ...
"has the same color as", then one particular equivalence class would consist of all green cars, and X / \sim could be naturally identified with the set of all car colors. * Let X be the set of all rectangles in a plane, and \,\sim\, the equivalence relation "has the same area as", then for each positive real number A, there will be an equivalence class of all the rectangles that have area A. * Consider the
modulo In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the '' modulus'' of the operation). Given two positive numbers and , modulo (often abbreviated as ) is t ...
2 equivalence relation on the set of
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
s, \Z, such that x \sim y if and only if their difference x - y is an even number. This relation gives rise to exactly two equivalence classes: one class consists of all even numbers, and the other class consists of all odd numbers. Using square brackets around one member of the class to denote an equivalence class under this relation, and /math> all represent the same element of \Z / \sim. * Let X be the set of ordered pairs of integers (a, b) with non-zero b, and define an equivalence relation \,\sim\, on X such that (a, b) \sim (c, d) if and only if a d = b c, then the equivalence class of the pair (a, b) can be identified with the rational number a / b, and this equivalence relation and its equivalence classes can be used to give a formal definition of the set of rational numbers. The same construction can be generalized to the field of fractions of any integral domain. * If X consists of all the lines in, say, the Euclidean plane, and L \sim M means that L and M are parallel lines, then the set of lines that are parallel to each other form an equivalence class, as long as a line is considered parallel to itself. In this situation, each equivalence class determines a point at infinity.


Definition and notation

An
equivalence relation In mathematics, an equivalence relation is a binary relation that is reflexive, symmetric and transitive. The equipollence relation between line segments in geometry is a common example of an equivalence relation. Each equivalence relatio ...
on a set X is a
binary relation In mathematics, a binary relation associates elements of one set, called the ''domain'', with elements of another set, called the ''codomain''. A binary relation over Set (mathematics), sets and is a new set of ordered pairs consisting of ele ...
\,\sim\, on X satisfying the three properties: * a \sim a for all a \in X ( reflexivity), * a \sim b implies b \sim a for all a, b \in X ( symmetry), * if a \sim b and b \sim c then a \sim c for all a, b, c \in X ( transitivity). The equivalence class of an element a is often denoted /math> or , and is defined as the set \ of elements that are related to a by \,\sim. The word "class" in the term "equivalence class" may generally be considered as a synonym of " set", although some equivalence classes are not sets but proper classes. For example, "being isomorphic" is an equivalence relation on groups, and the equivalence classes, called isomorphism classes, are not sets. The set of all equivalence classes in X with respect to an equivalence relation R is denoted as X / R, and is called X
modulo In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the '' modulus'' of the operation). Given two positive numbers and , modulo (often abbreviated as ) is t ...
R (or the of X by R). The surjective map x \mapsto /math> from X onto X / R, which maps each element to its equivalence class, is called the , or the canonical projection. Every element of an equivalence class characterizes the class, and may be used to ''represent'' it. When such an element is chosen, it is called a representative of the class. The choice of a representative in each class defines an injection from X / R to . Since its composition with the canonical surjection is the identity of X / R, such an injection is called a section, when using the terminology of category theory. Sometimes, there is a section that is more "natural" than the other ones. In this case, the representatives are called . For example, in modular arithmetic, for every
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
greater than , the congruence modulo is an equivalence relation on the integers, for which two integers and are equivalent—in this case, one says ''congruent'' —if divides a-b; this is denoted a\equiv b \pmod m. Each class contains a unique non-negative integer smaller than m, and these integers are the canonical representatives. The use of representatives for representing classes allows avoiding to consider explicitly classes as sets. In this case, the canonical surjection that maps an element to its class is replaced by the function that maps an element to the representative of its class. In the preceding example, this function is denoted a \bmod m, and produces the remainder of the Euclidean division of by .


Properties

Every element x of X is a member of the equivalence class Every two equivalence classes /math> and /math> are either equal or disjoint. Therefore, 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 comes from an equivalence relation in this way, according to which x \sim y if and only if x and y belong to the same set of the partition. It follows from the properties of an equivalence relation that x \sim y if and only if = In other words, if \,\sim\, is an equivalence relation on a set X, and x and y are two elements of X, then these statements are equivalent: * x \sim y * = /math> * \cap \ne \emptyset.


Graphical representation

An undirected graph may be associated to any symmetric relation on a set X, where the vertices are the elements of X, and two vertices s and t are joined if and only if s \sim t. Among these graphs are the graphs of equivalence relations; they are characterized as the graphs such that the connected components are cliques.


Invariants

If \,\sim\, is an equivalence relation on X, and P(x) is a property of elements of X such that whenever x \sim y, P(x) is true if P(y) is true, then the property P is said to be an invariant of \,\sim\,, or well-defined under the relation \,\sim. A frequent particular case occurs when f is a function from X to another set Y; if f\left(x_1\right) = f\left(x_2\right) whenever x_1 \sim x_2, then f is said to be \,\sim\,, or simply \,\sim. This occurs, for example, in the character theory of finite groups. Some authors use "compatible with \,\sim\," or just "respects \,\sim\," instead of "invariant under \,\sim\,". Any function f : X \to Y is ''class invariant under'' \,\sim\,, according to which x_1 \sim x_2 if and only if f\left(x_1\right) = f\left(x_2\right). The equivalence class of x is the set of all elements in X which get mapped to f(x), that is, the class /math> is the inverse image of f(x). This equivalence relation is known as the kernel of f. More generally, a function may map equivalent arguments (under an equivalence relation \sim_X on X) to equivalent values (under an equivalence relation \sim_Y on Y). Such a function is a morphism of sets equipped with an equivalence relation.


Quotient space in topology

In
topology In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ho ...
, a
quotient space Quotient space may refer to a quotient set when the sets under consideration are considered as spaces. In particular: *Quotient space (topology), in case of topological spaces * Quotient space (linear algebra), in case of vector spaces *Quotient ...
is a topological space formed on the set of equivalence classes of an equivalence relation on a topological space, using the original space's topology to create the topology on the set of equivalence classes. In
abstract algebra In mathematics, more specifically algebra, abstract algebra or modern algebra is the study of algebraic structures. Algebraic structures include groups, rings, fields, modules, vector spaces, lattices, and algebras over a field. The te ...
, congruence relations on the underlying set of an algebra allow the algebra to induce an algebra on the equivalence classes of the relation, called a quotient algebra. In linear algebra, a
quotient space Quotient space may refer to a quotient set when the sets under consideration are considered as spaces. In particular: *Quotient space (topology), in case of topological spaces * Quotient space (linear algebra), in case of vector spaces *Quotient ...
is a vector space formed by taking a
quotient group A quotient group or factor group is a mathematical group obtained by aggregating similar elements of a larger group using an equivalence relation that preserves some of the group structure (the rest of the structure is "factored" out). For exam ...
, where the quotient homomorphism is a linear map. By extension, in abstract algebra, the term quotient space may be used for quotient modules, quotient rings,
quotient group A quotient group or factor group is a mathematical group obtained by aggregating similar elements of a larger group using an equivalence relation that preserves some of the group structure (the rest of the structure is "factored" out). For exam ...
s, or any quotient algebra. However, the use of the term for the more general cases can as often be by analogy with the orbits of a group action. The orbits of a group action on a set may be called the quotient space of the action on the set, particularly when the orbits of the group action are the right cosets of a subgroup of a group, which arise from the action of the subgroup on the group by left translations, or respectively the left cosets as orbits under right translation. A normal subgroup of a topological group, acting on the group by translation action, is a quotient space in the senses of topology, abstract algebra, and group actions simultaneously. Although the term can be used for any equivalence relation's set of equivalence classes, possibly with further structure, the intent of using the term is generally to compare that type of equivalence relation on a set X, either to an equivalence relation that induces some structure on the set of equivalence classes from a structure of the same kind on X, or to the orbits of a group action. Both the sense of a structure preserved by an equivalence relation, and the study of invariants under group actions, lead to the definition of invariants of equivalence relations given above.


See also

* Equivalence partitioning, a method for devising test sets in
software testing Software testing is the act of examining the artifacts and the behavior of the software under test by validation and verification. Software testing can also provide an objective, independent view of the software to allow the business to apprecia ...
based on dividing the possible program inputs into equivalence classes according to the behavior of the program on those inputs * Homogeneous space, the quotient space of Lie groups * * * *


Notes


References

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Further reading

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External links

* {{Authority control Algebra Binary relations Equivalence (mathematics) Set theory