Equivalence class


In mathematics, when the elements of some set have a notion of equivalence (formalized as an equivalence relation), then one may naturally split the set into equivalence classes. These equivalence classes are constructed so that elements and belong to the same equivalence class if, and only if, they are equivalent.

Congruence is an example of an equivalence relation. The leftmost two triangles are congruent, while the third and fourth triangles are not congruent to any other triangle shown here. Thus, the first two triangles are in the same equivalence class, while the third and fourth triangles are each in their own equivalence class.

Formally, given a set and an equivalence relation on the equivalence class of an element in is denoted or, equivalently, to emphasize its equivalence relation The definition of equivalence relations implies that the equivalence classes form a partition of meaning, that every element of the set belongs to exactly one equivalence class. The set of the equivalence classes is sometimes called the quotient set or the quotient space of by and is denoted by

When the set has some structure (such as a group operation or a topology) and the equivalence relation 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 groups, homogeneous spaces, quotient rings, quotient monoids, and quotient categories.

Definition and notation edit

An equivalence relation on a set   is a binary relation   on   satisfying the three properties:[1]

  •   for all   (reflexivity),
  •   implies   for all   (symmetry),
  • if   and   then   for all   (transitivity).

The equivalence class of an element   is defined as[2]


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   with respect to an equivalence relation   is denoted as   and is called   modulo   (or the quotient set of   by  ).[3] The surjective map   from   onto   which maps each element to its equivalence class, is called the canonical surjection, 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   to X. Since its composition with the canonical surjection is the identity of   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 canonical representatives. For example, in modular arithmetic, for every integer m greater than 1, the congruence modulo m is an equivalence relation on the integers, for which two integers a and b are equivalent—in this case, one says congruent—if m divides   this is denoted   Each class contains a unique non-negative integer smaller than   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   and produces the remainder of the Euclidean division of a by m.

Properties edit

Every element   of   is a member of the equivalence class   Every two equivalence classes   and   are either equal or disjoint. Therefore, the set of all equivalence classes of   forms a partition of  : every element of   belongs to one and only one equivalence class.[4] Conversely, every partition of   comes from an equivalence relation in this way, according to which   if and only if   and   belong to the same set of the partition.[5]

It follows from the properties in the previous section that if   is an equivalence relation on a set   and   and   are two elements of   the following statements are equivalent:


Examples edit

  • Let   be the set of all rectangles in a plane, and   the equivalence relation "has the same area as", then for each positive real number   there will be an equivalence class of all the rectangles that have area  [6]
  • Consider the modulo 2 equivalence relation on the set of integers,   such that   if and only if their difference   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   all represent the same element of  [2]
  • Let   be the set of ordered pairs of integers   with non-zero   and define an equivalence relation   on   such that   if and only if   then the equivalence class of the pair   can be identified with the rational number   and this equivalence relation and its equivalence classes can be used to give a formal definition of the set of rational numbers.[7] The same construction can be generalized to the field of fractions of any integral domain.
  • If   consists of all the lines in, say, the Euclidean plane, and   means that   and   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.

Graphical representation edit

Graph of an example equivalence with 7 classes

An undirected graph may be associated to any symmetric relation on a set   where the vertices are the elements of   and two vertices   and   are joined if and only if   Among these graphs are the graphs of equivalence relations. These graphs, called cluster graphs, are characterized as the graphs such that the connected components are cliques.[2]

Invariants edit

If   is an equivalence relation on   and   is a property of elements of   such that whenever     is true if   is true, then the property   is said to be an invariant of   or well-defined under the relation  

A frequent particular case occurs when   is a function from   to another set  ; if   whenever   then   is said to be class invariant under   or simply invariant under   This occurs, for example, in the character theory of finite groups. Some authors use "compatible with  " or just "respects  " instead of "invariant under  ".

Any function   is class invariant under   according to which   if and only if   The equivalence class of   is the set of all elements in   which get mapped to   that is, the class   is the inverse image of   This equivalence relation is known as the kernel of  

More generally, a function may map equivalent arguments (under an equivalence relation   on  ) to equivalent values (under an equivalence relation   on  ). Such a function is a morphism of sets equipped with an equivalence relation.

Quotient space in topology edit

In topology, a quotient space 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, 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 is a vector space formed by taking a quotient group, 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 groups, 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   either to an equivalence relation that induces some structure on the set of equivalence classes from a structure of the same kind on   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 edit

Notes edit

  1. ^ Devlin 2004, p. 122.
  2. ^ a b c Devlin 2004, p. 123.
  3. ^ Wolf 1998, p. 178
  4. ^ Maddox 2002, p. 74, Thm. 2.5.15
  5. ^ Avelsgaard 1989, p. 132, Thm. 3.16
  6. ^ Avelsgaard 1989, p. 127
  7. ^ Maddox 2002, pp. 77–78

References edit

  • Avelsgaard, Carol (1989), Foundations for Advanced Mathematics, Scott Foresman, ISBN 0-673-38152-8
  • Devlin, Keith (2004), Sets, Functions, and Logic: An Introduction to Abstract Mathematics (3rd ed.), Chapman & Hall/ CRC Press, ISBN 978-1-58488-449-1
  • Maddox, Randall B. (2002), Mathematical Thinking and Writing, Harcourt/ Academic Press, ISBN 0-12-464976-9
  • Wolf, Robert S. (1998), Proof, Logic and Conjecture: A Mathematician's Toolbox, Freeman, ISBN 978-0-7167-3050-7

Further reading edit

  • Sundstrom (2003), Mathematical Reasoning: Writing and Proof, Prentice-Hall
  • Smith; Eggen; St.Andre (2006), A Transition to Advanced Mathematics (6th ed.), Thomson (Brooks/Cole)
  • Schumacher, Carol (1996), Chapter Zero: Fundamental Notions of Abstract Mathematics, Addison-Wesley, ISBN 0-201-82653-4
  • O'Leary (2003), The Structure of Proof: With Logic and Set Theory, Prentice-Hall
  • Lay (2001), Analysis with an introduction to proof, Prentice Hall
  • Morash, Ronald P. (1987), Bridge to Abstract Mathematics, Random House, ISBN 0-394-35429-X
  • Gilbert; Vanstone (2005), An Introduction to Mathematical Thinking, Pearson Prentice-Hall
  • Fletcher; Patty, Foundations of Higher Mathematics, PWS-Kent
  • Iglewicz; Stoyle, An Introduction to Mathematical Reasoning, MacMillan
  • D'Angelo; West (2000), Mathematical Thinking: Problem Solving and Proofs, Prentice Hall
  • Cupillari, The Nuts and Bolts of Proofs, Wadsworth
  • Bond, Introduction to Abstract Mathematics, Brooks/Cole
  • Barnier; Feldman (2000), Introduction to Advanced Mathematics, Prentice Hall
  • Ash, A Primer of Abstract Mathematics, MAA

External links edit

  •   Media related to Equivalence classes at Wikimedia Commons