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### Element (mathematics)

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{{For|elements in category theory|Element (category theory)}}In mathematics, an element, or member, of a set is any one of the distinct objects that make up that set.

## Sets

Writing A = {1, 2, 3, 4} means that the elements of the set {{mvar|A}} are the numbers 1, 2, 3 and 4. Sets of elements of {{mvar|A}}, for example {1, 2}, are subsets of {{mvar|A}}.Sets can themselves be elements. For example, consider the set B = {1, 2, {3, 4}}. The elements of {{mvar|B}} are not 1, 2, 3, and 4. Rather, there are only three elements of {{mvar|B}}, namely the numbers 1 and 2, and the set {3, 4}.The elements of a set can be anything. For example, C = {mathrm{color{red}red}, mathrm{color{green}green}, mathrm{color{blue}blue}}, is the set whose elements are the colors {{red|red}}, {{green|green}} and {{blue|blue}}.

## Notation and terminology

File:First usage of the symbol âˆˆ.png|thumb|right|First usage of the symbol âˆˆ in the work by Giuseppe PeanoGiuseppe PeanoThe relation "is an element of", also called set membership, is denoted by the symbol " in ". Writing
x in A
means that "x is an element of A". Equivalent expressions are "x is a member of A", "x belongs to A", "x is in A" and "x lies in A". The expressions "A includes x" and "A contains x" are also used to mean set membership, however some authors use them to mean instead "x is a subset of A".BOOK, Eric Schechter, Handbook of Analysis and Its Foundations, Academic Press, 1997, 0-12-622760-8, p. 12 Logician George Boolos strongly urged that "contains" be used for membership only and "includes" for the subset relation only.SPEECH, 24.243 Classical Set Theory (lecture), George Boolos, February 4, 1992, Massachusetts Institute of Technology, For the relation âˆˆ , the converse relation âˆˆT may be written
A ni x , meaning "A contains x".
The negation of set membership is denoted by the symbol "âˆ‰". Writing
x notin A means that "x is not an element of A".
The symbol âˆˆ was first used by Giuseppe Peano 1889 in his work . Here he wrote on page X:which meansThe symbol âˆˆ means is. So a âˆˆ b is read as a is a b; â€¦The symbol itself is a stylized lowercase Greek letter epsilon ("Ïµ"), the first letter of the word {{wikt-lang|grc|á¼ÏƒÏ„Î¯}}, which means "is".{{charmap
name1=Element of ref2char1=[Element]name2=Not an element of ref2char2=[NotElement]name3=Contains as member ref2char3=[ReverseElement]name4=Does not contain as member ref2char4=[NotReverseElement]|namedref1=LaTeXWolfram Mathematica (software)>Wolfram Mathematica}}

### Complement and converse

Every relation R : U â†’ V is subject to two involutions: complementation R â†’ bar{R} and conversion RT: V â†’ U.The relation âˆˆ has for its domain a universal set U, and has the power set P(U) for its codomain or range. The complementary relation bar{in} = mathord{notin} expresses the opposite of âˆˆ. An element x âˆˆ U may have x âˆ‰ A, in which case x âˆˆ U A, the complement of A in U.The converse relation in^textsf{T} = mathord{ni} swaps the domain and range with âˆˆ. For any A in P(U), A ni x is true when x âˆˆ A.

## Cardinality of sets

The number of elements in a particular set is a property known as cardinality; informally, this is the size of a set. In the above examples the cardinality of the set A is 4, while the cardinality of either of the sets B and C is 3. An infinite set is a set with an infinite number of elements, while a finite set is a set with a finite number of elements. The above examples are examples of finite sets. An example of an infinite set is the set of positive integers, {1, 2, 3, 4, â€¦}.

## Examples

Using the sets defined above, namely A = {1, 2, 3, 4 }, B = {1, 2, {3, 4}} and C = {red, green, blue}:
• 2 âˆˆ A
• {3,4} âˆˆ B
• 3,4 âˆ‰ B
• {3,4} is a member of B
• Yellow âˆ‰ C

{{reflist}}

## Further reading

• {{Citation|last=Halmos|first=Paul R.|author-link=Paul R. Halmos|origyear=1960|year=1974|title=Naive Set Theory|publisher=Springer-Verlag|location=NY|edition=Hardcover|series=Undergraduate Texts in Mathematics|isbn=0-387-90092-6}} - "Naive" means that it is not fully axiomatized, not that it is silly or easy (Halmos's treatment is neither).
• {{Citation|last=Jech|first=Thomas|author-link=Thomas Jech|year=2002|title=Stanford Encyclopedia of Philosophy|chapter=Set Theory|url=http://plato.stanford.edu/entries/set-theory/}}
• {{Citation|last=Suppes|first=Patrick|author-link=Patrick Suppes|origyear=1960|year=1972|title=Axiomatic Set Theory|publisher=Dover Publications, Inc.|location=NY|isbn=0-486-61630-4}} - Both the notion of set (a collection of members), membership or element-hood, the axiom of extension, the axiom of separation, and the union axiom (Suppes calls it the sum axiom) are needed for a more thorough understanding of "set element".

## External links

• {{MathWorld |title=Element |id=Element }}
{{Mathematical logic}}{{Set theory}}

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