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Element (mathematics)
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{{Forelements in category theoryElement (category theory)}}In mathematics, an element, or member, of a set is any one of the distinct objects that make up that set. the content below is remote from Wikipedia
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Sets
Writing A = {1, 2, 3, 4} means that the elements of the set {{mvarA}} are the numbers 1, 2, 3 and 4. Sets of elements of {{mvarA}}, for example {1, 2}, are subsets of {{mvarA}}.Sets can themselves be elements. For example, consider the set B = {1, 2, {3, 4}}. The elements of {{mvarB}} are not 1, 2, 3, and 4. Rather, there are only three elements of {{mvarB}}, 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 {{redred}}, {{greengreen}} and {{blueblue}}.Notation and terminology
File:First usage of the symbol âˆˆ.pngthumbrightFirst usage of the symbol Ïµ 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, 0126227608, 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 Ïµ , the converse relation ÏµT may be written
A ni x
meaning "A contains x", though ÏµT is used in the calculus of relations.The negation of set membership is denoted by the symbol "âˆ‰". Writing
x notin A
means that "x is not an element of A", or x in bar{A}, the complement of A.The symbol Ïµ was first used by Giuseppe Peano 1889 in his work . Here he wrote on page X:which meansThe symbol Ïµ means is. So a Ïµ b is read as a is a b; ...The symbol itself is a stylized lowercase Greek letter epsilon ("Îµ"), the first letter of the word {{wiktlanggrcá¼ÏƒÏ„Î¯}}, which means "is".The Unicode characters for these symbols are U+2208 ('element of'), U+2209 ('not an element of'), U+220B ('contains as member') and U+220C ('does not contain as member'). The equivalent LaTeX commands are "in", "notin", "ni" and "notni". Mathematica has commands "[Element]", "[NotElement]", "[ReverseElement]" and "[NotReverseElement]".{{charmapname1=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=LaTeX  Wolfram Mathematica (software)>Wolfram Mathematica}}Cardinality of setsThe 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, ... }.ExamplesUsing the sets defined above, namely A = {1, 2, 3, 4 }, B = {1, 2, {3, 4}} and C = { red, green, blue }:
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