In

mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...

, when the elements of some set $S$ have a notion of equivalence (formalized as an equivalence relation
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented i ...

), 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
In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false.
The connective is bicondi ...

, they are equivalent.
Formally, given a set $S$ and an equivalence relation $\backslash ,\backslash sim\backslash ,$ on $S,$ the of an element $a$ in $S,$ denoted by $;\; href="/html/ALL/s/.html"\; ;"title="">$ is the set
$$\backslash $$
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 $\backslash ,\backslash sim\backslash ,,$ and is denoted by $S\; /\; \backslash sim.$
When the set $S$ has some structure (such as a group operation
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in m ...

or a topology
In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric object that are preserved under Continuous function, continuous Deformation theory, deformations, such ...

) and the equivalence relation $\backslash ,\backslash sim\backslash ,$ 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 (mathematics), 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 "factor ...

s, homogeneous space
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in m ...

s, quotient ring
In ring theory, a branch of abstract algebra, a quotient ring, also known as factor ring, difference ring or residue class ring, is a construction quite similar to the quotient group in group theory and to the quotient space (linear algebra), quo ...

s, quotient monoids, and quotient categories.
Examples

* If $X$ is the set of all cars, and $\backslash ,\backslash sim\backslash ,$ is theequivalence relation
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented i ...

"has the same color as", then one particular equivalence class would consist of all green cars, and $X\; /\; \backslash sim$ could be naturally identified with the set of all car colors.
* Let $X$ be the set of all rectangles in a plane, and $\backslash ,\backslash sim\backslash ,$ 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 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 of ...

s, $\backslash Z,$ such that $x\; \backslash sim\; y$ if and only if their difference $x\; -\; y$ is an even number
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

. 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, $;\; href="/html/ALL/s/.html"\; ;"title="">$ and $;\; href="/html/ALL/s/.html"\; ;"title="">$ordered pair
In mathematics, an ordered pair (''a'', ''b'') is a pair of objects. The order in which the objects appear in the pair is significant: the ordered pair (''a'', ''b'') is different from the ordered pair (''b'', ''a'') unless ''a'' = ''b''. (In con ...

s of integers $(a,\; b)$ with non-zero $b,$ and define an equivalence relation $\backslash ,\backslash sim\backslash ,$ on $X$ such that $(a,\; b)\; \backslash sim\; (c,\; d)$ if and only if $a\; d\; =\; b\; c,$ then the equivalence class of the pair $(a,\; b)$ can be identified with the rational number
In mathematics, a rational number is a number that can be expressed as the quotient or fraction (mathematics), fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ) ...

$a\; /\; b,$ 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
In abstract algebra
In mathematics, more specifically algebra, abstract algebra or modern algebra is the study of algebraic structures. Algebraic structures include group (mathematics), groups, ring (mathematics), rings, field (mathematics) ...

of any integral domain
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

.
* If $X$ consists of all the lines in, say, the Euclidean plane
In mathematics, the Euclidean plane is a Euclidean space of dimension two. That is, a geometry, geometric setting in which two real number, real quantities are required to determine the position (geometry), position of each point (mathematics), ...

, and $L\; \backslash sim\; M$ means that $L$ and $M$ are parallel lines
In geometry, parallel lines are coplanar straight line (geometry), lines that do not intersecting lines, intersect at any point. Parallel planes are plane (geometry), planes in the same three-dimensional space that never meet. ''Parallel curve ...

, 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
In geometry, a point at infinity or ideal point is an idealized limiting point at the "end" of each line.
In the case of an affine plane (including the Euclidean plane), there is one ideal point for each Pencil (mathematics), pencil of parallel l ...

.
Definition and notation

Anequivalence relation
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented i ...

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 ...

$\backslash ,\backslash sim\backslash ,$ on $X$ satisfying the three properties:
* $a\; \backslash sim\; a$ for all $a\; \backslash in\; X$ ( reflexivity),
* $a\; \backslash sim\; b$ implies $b\; \backslash sim\; a$ for all $a,\; b\; \backslash in\; X$ (symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...

),
* if $a\; \backslash sim\; b$ and $b\; \backslash sim\; c$ then $a\; \backslash sim\; c$ for all $a,\; b,\; c\; \backslash in\; X$ ( transitivity).
The equivalence class of an element $a$ is often denoted $;\; href="/html/ALL/s/.html"\; ;"title="">$proper class
Proper may refer to:
Mathematics
* Proper map, in topology, a property of continuous function between topological spaces, if inverse images of compact subsets are compact
* Proper morphism, in algebraic geometry, an analogue of a proper map for ...

es. For example, "being isomorphic
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

" 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 $R$ (or the of $X$ by $R$). The surjective map
In mathematics, a surjective function (also known as surjection, or onto function) is a Function (mathematics), function that every element can be mapped from element so that . In other words, every element of the function's codomain is the Im ...

$x\; \backslash mapsto;\; href="/html/ALL/s/.html"\; ;"title="">$section
Section, Sectioning or Sectioned may refer to:
Arts, entertainment and media
* Section (music), a complete, but not independent, musical idea
* Section (typography), a subdivision, especially of a chapter, in books and documents
** Section sign ...

, when using the terminology of category theory
Category theory is a general theory of mathematical structures and their relations that was introduced by Samuel Eilenberg and Saunders Mac Lane in the middle of the 20th century in their foundational work on algebraic topology. Nowadays, categ ...

.
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
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

, 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 of ...

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 $a\backslash equiv\; b\; \backslash pmod\; m.$ 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 $a\; \backslash bmod\; m,$ and produces the remainder of the Euclidean division
In arithmetic
Arithmetic () is an elementary part of mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantitie ...

of by .
Properties

Every element $x$ of $X$ is a member of the equivalence class $;\; href="/html/ALL/s/.html"\; ;"title="">$ Every two equivalence classes $;\; href="/html/ALL/s/.html"\; ;"title="">$Graphical representation

Anundirected graph
In discrete mathematics, and more specifically in graph theory, a graph is a structure amounting to a Set (mathematics), set of objects in which some pairs of the objects are in some sense "related". The objects correspond to mathematical abstra ...

may be associated to any symmetric relation
A symmetric relation is a type of binary relation. An example is the relation "is equal to", because if ''a'' = ''b'' is true then ''b'' = ''a'' is also true. Formally, a binary relation ''R'' over a set ''X'' is symmetric if:
:\forall a, b \in ...

on a set $X,$ where the vertices are the elements of $X,$ and two vertices $s$ and $t$ are joined if and only if $s\; \backslash sim\; t.$ Among these graphs are the graphs of equivalence relations; they are characterized as the graphs such that the connected components are cliques
A clique ( AusE, CanE
Cane or caning may refer to:
*Walking stick or walking cane, a device used primarily to aid walking
*Assistive cane, a walking stick used as a mobility aid for better balance
*White cane, a mobility or safety device used ...

.
Invariants

If $\backslash ,\backslash sim\backslash ,$ is an equivalence relation on $X,$ and $P(x)$ is a property of elements of $X$ such that whenever $x\; \backslash sim\; y,$ $P(x)$ is true if $P(y)$ is true, then the property $P$ is said to be an invariant of $\backslash ,\backslash sim\backslash ,,$ orwell-defined
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in m ...

under the relation $\backslash ,\backslash sim.$
A frequent particular case occurs when $f$ is a function from $X$ to another set $Y$; if $f\backslash left(x\_1\backslash right)\; =\; f\backslash left(x\_2\backslash right)$ whenever $x\_1\; \backslash sim\; x\_2,$ then $f$ is said to be $\backslash ,\backslash sim\backslash ,,$ or simply $\backslash ,\backslash sim.$ This occurs, for example, in the character theory
In mathematics, more specifically in group theory, the character of a group representation is a function (mathematics), function on the group (mathematics), group that associates to each group element the trace (linear algebra), trace of the corres ...

of finite groups. Some authors use "compatible with $\backslash ,\backslash sim\backslash ,$" or just "respects $\backslash ,\backslash sim\backslash ,$" instead of "invariant under $\backslash ,\backslash sim\backslash ,$".
Any function $f\; :\; X\; \backslash to\; Y$ is ''class invariant under'' $\backslash ,\backslash sim\backslash ,,$ according to which $x\_1\; \backslash sim\; x\_2$ if and only if $f\backslash left(x\_1\backslash right)\; =\; f\backslash left(x\_2\backslash right).$ The equivalence class of $x$ is the set of all elements in $X$ which get mapped to $f(x),$ that is, the class $;\; href="/html/ALL/s/.html"\; ;"title="">$inverse image
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

of $f(x).$ This equivalence relation is known as the kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine learnin ...

of $f.$
More generally, a function may map equivalent arguments (under an equivalence relation $\backslash sim\_X$ on $X$) to equivalent values (under an equivalence relation $\backslash sim\_Y$ on $Y$). Such a function is a morphism
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in mod ...

of sets equipped with an equivalence relation.
Quotient space in topology

Intopology
In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric object that are preserved under Continuous function, continuous Deformation theory, deformations, such ...

, a quotient space is a topological space
In mathematics, a topological space is, roughly speaking, a Geometry, geometrical space in which Closeness (mathematics), closeness is defined but cannot necessarily be measured by a numeric Distance (mathematics), distance. More specifically, a to ...

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 group (mathematics), groups, ring (mathematics), rings, field (mathematics), fields, module (mathe ...

, congruence relation
In abstract algebra, a congruence relation (or simply congruence) is an equivalence relation on an algebraic structure (such as a group (mathematics), group, ring (mathematics), ring, or vector space) that is compatible with the structure in the ...

s 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
Linear algebra is the branch of mathematics concerning linear equations such as:
:a_1x_1+\cdots +a_nx_n=b,
linear maps such as:
:(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n,
and their representations in vector spaces and through matrix (mat ...

, a quotient space is a vector space formed by taking a quotient group
A quotient group or factor group is a mathematical group (mathematics), 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 "factor ...

, where the quotient homomorphism is a linear map
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

. By extension, in abstract algebra, the term quotient space may be used for quotient modules, quotient ring
In ring theory, a branch of abstract algebra, a quotient ring, also known as factor ring, difference ring or residue class ring, is a construction quite similar to the quotient group in group theory and to the quotient space (linear algebra), quo ...

s, quotient group
A quotient group or factor group is a mathematical group (mathematics), 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 "factor ...

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
In mathematics, a group action on a space (mathematics), space is a group homomorphism of a given group (mathematics), group into the group of transformation (geometry), transformations of the space. Similarly, a group action on a mathematical ...

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 coset
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...

s 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 insoftware 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
In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in m ...

, the quotient space of Lie group
In mathematics, a Lie group (pronounced ) is a group (mathematics), group that is also a differentiable manifold. A manifold is a space that locally resembles Euclidean space, whereas groups define the abstract concept of a binary operation a ...

s
*
*
*
*
Notes

References

* * * *Further reading

* * * * * * * * * * * * * *External links

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