Utente:Grasso Luigi/sandbox4/Classe di equivalenza

La congruenza è un esempio di relazione di equivalenza. I due triangoli più a sinistra sono congruenti, mentre il terzo e il quarto triangolo non sono congruenti a nessun altro triangolo mostrato qui. Quindi, i primi due sono nella stessa classe di equivalenza, mentre il terzo e il quarto formano una propria propria classe di equivalenza.

In matematica, quando gli elementi di un insieme $S$ hanno una nozione di equivalenza (detta relazione di equivalenza), allora si può naturalmente dividere l'insieme $S$ in classi di equivalenza. Queste classi di equivalenza sono costruite in modo che gli elementi $a$ e $b$ appartengono alla stessa classe di equivalenza se e solo se, sono equivalenti.

Formalmente, dato un insieme $S$ e una relazione di equivalenza $\,\sim \,$ su $S,$ la classe di equivalenza di un elemento $a$ in $S,$ indicata con $[a],$ è l'insieme

$$\{x\in S:x\sim a\}$$

di elementi equivalenti ad $a.$ Dalle proprietà che definiscono le relazioni di equivalenza si può dimostrare che le classi di equivalenza formano una partizione di $S.$ Questa partizione (l'insieme delle classi di equivalenza) viene detto insieme quoziente o spazio quoziente di $S$ con $\,\sim \,,$ ed è indicato con $S/{\sim }$.

Quando l'insieme $S$ ha una struttura (come un'operazione di gruppo o una topologia) e la relazione di equivalenza $\,\sim \,$ 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

• 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 2 equivalence relation on the set of integers, $\mathbb {Z} ,$ 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, $[7],[9],$ and $[1]$ all represent the same element of $\mathbb {Z} /{\sim }.$
• 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 $ad=bc,$ 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.

Definizione e notazione

An equivalence relation on a set $X$ is a binary relation $\,\sim \,$ 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 $[a]$ or $[a]_{\sim },$ and is defined as the set $\{x\in X:a\sim x\}$ 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 $R$ (or the quotient set of $X$ by $R$). The surjective map $x\mapsto [x]$ from $X$ onto $X/R,$ 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 $X/R$ to Template:Mvar. 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 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 $a-b;$ this is denoted ${\textstyle a\equiv b{\pmod {m}}.}$ 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 Template:Mvar by Template:Mvar.

Proprietà

Every element $x$ of $X$ is a member of the equivalence class $[x].$ Every two equivalence classes $[x]$ and $[y]$ 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 $[x]=[y].$

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:

• $x\sim y$
• $[x]=[y]$
• $[x]\cap [y]\neq \emptyset .$

Rappresentazione grafica

Graph of an example equivalence with 7 classes

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. These graphs, called cluster graphs, are characterized as the graphs such that the connected components are cliques.

Invarianti

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 Template:Em $\,\sim \,,$ or simply Template:Em $\,\sim .$ 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 $[x]$ 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.

Spazio quoziente in topologia

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 $X,$ 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.

Note

• 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

• 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

• 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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