Utente:Grasso Luigi/sandbox4/Classe di equivalenza
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In matematica, quando gli elementi di un insieme hanno una nozione di equivalenza (detta relazione di equivalenza), allora si può naturalmente dividere l'insieme in classi di equivalenza. Queste classi di equivalenza sono costruite in modo che gli elementi e appartengono alla stessa classe di equivalenza se e solo se, sono equivalenti.
Formalmente, dato un insieme e una relazione di equivalenza su la classe di equivalenza di un elemento in indicata con [1]è l'insieme[2]
di elementi equivalenti ad Dalle proprietà che definiscono le relazioni di equivalenza si può dimostrare che le classi di equivalenza formano una partizione di Questa partizione (l'insieme delle classi di equivalenza) viene detto insieme quoziente o spazio quoziente di con ed è indicato con .
Quando l'insieme ha una struttura (come un'operazione di gruppo o una topologia) e la relazione di equivalenza è compatibile con questa struttura, l'insieme quoziente spesso eredita una struttura simile dal suo insieme genitore. Esempi includono gli spazi quoziente in algebra lineare, gli spazi quoziente in topologia, i gruppi quoziente, gli spazi quoziente, gli anelli quoziente, i monoidi quoziente e le categorie quoziente.
Esempi
modifica- 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 [3]
- 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 [4]
- 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.[5] 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.
Definizione e notazione
modificaAn equivalence relation on a set is a binary relation on satisfying the three properties:[6][7]
- for all (reflexivity),
- implies for all (symmetry),
- if and then for all (transitivity).
The equivalence class of an element is often denoted or and is defined as the set of elements that are related to by [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 ).[8] 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 Template:Mvar. 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 Template:Em. For example, in modular arithmetic, for every integer Template:Mvar greater than Template:Math, the [[congruence modulo m|congruence modulo Template:Mvar]] is an equivalence relation on the integers, for which two integers Template:Mvar and Template:Mvar are equivalent—in this case, one says congruent —if Template:Mvar 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 Template:Mvar by Template:Mvar.
Proprietà
modificaEvery 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.[9] 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.[10]
It follows from the properties of an equivalence relation that if and only if
In other words, if is an equivalence relation on a set and and are two elements of then these statements are equivalent:
Rappresentazione grafica
modificaAn 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.[4]
Invarianti
modificaIf 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 Template:Em or simply Template:Em 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.
Spazio quoziente in topologia
modificaIn 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.
Note
modifica- ^ (EN) 7.3: Equivalence Classes, su Mathematics LibreTexts, 20-09-2017. URL consultato il 30-08-2020.
- ^ a b (EN) Weisstein Eric W., Equivalence Class, su mathworld.wolfram.com. URL consultato il 30-08-2020.
- ^ Avelsgaard, p. 127
- ^ a b Devlin, p. 123
- ^ Maddox, pp. 77–78
- ^ Devlin, p. 122
- ^ (EN) Eric W. Weisstein, Equivalence Relation, su mathworld.wolfram.com.
- ^ Wolf, p. 178
- ^ Maddox, p. 74, Thm. 2.5.15
- ^ Avelsgaard, p. 132, Thm. 3.16
- Carol Avelsgaard, Foundations for Advanced Mathematics, Scott Foresman, 1989.
- Keith Devlin, Sets, Functions, and Logic: An Introduction to Abstract Mathematics, Chapman & Hall/ CRC Press, 2004.
- Randall B. Maddox, Mathematical Thinking and Writing, Harcourt/ Academic Press, 2002.
- Robert S. Wolf, Proof, Logic and Conjecture: A Mathematician's Toolbox, Freeman, 1998.
Bibliografia
modifica- Sundstrom, Mathematical Reasoning: Writing and Proof, Prentice-Hall, 2003.
- A Transition to Advanced Mathematics, Thomson (Brooks/Cole), 2006.
- Carol Schumacher, Chapter Zero: Fundamental Notions of Abstract Mathematics, Addison-Wesley, 1996.
- O'Leary, The Structure of Proof: With Logic and Set Theory, Prentice-Hall, 2003.
- Lay, Analysis with an introduction to proof, Prentice Hall, 2001.
- Ronald P. Morash, Bridge to Abstract Mathematics, Random House, 1987.
- An Introduction to Mathematical Thinking, Pearson Prentice-Hall, 2005.
- Foundations of Higher Mathematics, PWS-Kent.
- An Introduction to Mathematical Reasoning, MacMillan.
- Mathematical Thinking: Problem Solving and Proofs, Prentice Hall, 2000.
- Cupillari, The Nuts and Bolts of Proofs, Wadsworth.
- Bond, Introduction to Abstract Mathematics, Brooks/Cole.
- Introduction to Advanced Mathematics, Prentice Hall, 2000.
- Ash, A Primer of Abstract Mathematics, MAA.
Voci correlate
modifica- Equivalence partitioning, a method for devising test sets in software testing 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
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Collegamenti esterni
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