In "mathematics, an **element**, or **member**, of a "set is any one of the distinct "objects that make up that set.

Writing means that the elements of the set A are the numbers 1, 2, 3 and 4. Sets of elements of A, for example , are "subsets of A.

Sets can themselves be elements. For example, consider the set . The elements of B are *not* 1, 2, 3, and 4. Rather, there are only three elements of B, namely the numbers 1 and 2, and the set .

The elements of a set can be anything. For example, , is the set whose elements are the colors red, green and blue.

The "relation "is an element of", also called **set membership**, is denoted by the symbol "". Writing

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*".^{[1]} Logician "George Boolos strongly urged that "contains" be used for membership only and "includes" for the subset relation only.^{[2]}

Another possible notation for the same relation is

meaning "*A* contains *x*", though it is used less often.

The "negation of set membership is denoted by the symbol "∉". Writing

means that "*x* is not an element of *A*".

The symbol ϵ was first used by Giuseppe Peano 1889 in his work *"Arithmetices principia, nova methodo exposita*. Here he wrote on page X:

Signum ϵ significat est. Ita a ϵ b legitur a est quoddam b; ...

which means

The symbol ϵ means

is. So a ϵ b is read as ais ab; ...

The symbol itself is a stylized lowercase Greek letter "epsilon ("ε"), the first letter of the word ἐστί, 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 "\not\ni". "Mathematica has commands "\[Element]", "\[NotElement]", "\[ReverseElement]" and "\[NotReverseElement]".

Character | ∈ | ∉ | ∋ | ∌ | ||||
---|---|---|---|---|---|---|---|---|

Unicode name |
ELEMENT OF | NOT AN ELEMENT OF | CONTAINS AS MEMBER | DOES NOT CONTAIN AS MEMBER | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

"Unicode | 8712 | U+2208 | 8713 | U+2209 | 8715 | U+220B | 8716 | U+220C |

"UTF-8 | 226 136 136 | E2 88 88 | 226 136 137 | E2 88 89 | 226 136 139 | E2 88 8B | 226 136 140 | E2 88 8C |

"Numeric character reference | ∈ | ∈ | ∉ | ∉ | ∋ | ∋ | ∌ | ∌ |

"Named character reference | ∈ | ∉ | ∋ | |||||

"LaTeX | \in | \notin | \ni | \not\ni | ||||

"Wolfram Mathematica | \[Element] | \[NotElement] | \[ReverseElement] | \[NotReverseElement] |

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, ... }.

Using the sets defined above, namely *A* = {1, 2, 3, 4 }, *B* = {1, 2, {3, 4}} and *C* = { red, green, blue }:

- 2 ∈
*A* - {3,4} ∈
*B* - 3,4 ∉
*B* - {3,4} is a member of
*B* - Yellow ∉
*C* - The cardinality of
*D*= {2, 4, 8, 10, 12} is finite and equal to 5. - The cardinality of
*P*= {2, 3, 5, 7, 11, 13, ...} (the "prime numbers) is infinite (proved by "Euclid).

**^**"Eric Schechter (1997).*Handbook of Analysis and Its Foundations*. "Academic Press. "ISBN "0-12-622760-8. p. 12**^**"George Boolos (February 4, 1992).*24.243 Classical Set Theory (lecture)*(Speech). "Massachusetts Institute of Technology.

- "Halmos, Paul R. (1974) [1960],
*Naive Set Theory*, "Undergraduate Texts in Mathematics (Hardcover ed.), NY: Springer-Verlag, "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). - "Jech, Thomas (2002), "Set Theory",
*Stanford Encyclopedia of Philosophy* - "Suppes, Patrick (1972) [1960],
*Axiomatic Set Theory*, NY: Dover Publications, Inc., "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".