In mathematics, an identity element, or neutral element, is a special type of element of a setwith respect to a binary operation on that set, which leaves any element of the set unchanged when combined with it. This concept is used in algebraic structures such as groups and rings. The term identity element is often shortened to identity, when there is no possibility of confusion, but the identity implicitly depends on the binary operation it is associated with.
Definitions
Let be a set equipped with a binary operation ∗. Then an element of is called a left identity if for all in , and a right identity if for all in . If is both a left identity and a right identity, then it is called a two-sided identity, or simply an identity. An identity with respect to addition is called an additive identity and an identity with respect to multiplication is called a multiplicative identity. These need not be ordinary addition and multiplication—as the underlying operation could be rather arbitrary. The distinction is used most often for sets that support both binary operations, such as rings, integral domains, and fields. The multiplicative identity is often called unity in the latter context. This should not be confused with a unit in ring theory, which is any element having a multiplicative inverse. By its own definition, unity itself is necessarily a unit.
Examples
Properties
As the last example shows, it is possible for to have several left identities. In fact, every element can be a left identity. In a similar manner, there can be several right identities. But if there is both a right identity and a left identity, then they must be equal, resulting in a single two-sided identity. To see this, note that if is a left identity and is a right identity, then. In particular, there can never be more than one two-sided identity: if there were two, say and, then would have to be equal to both and. It is also quite possible for to have no identity element, such as the case of even integers under the multiplication operation. Another common example is the cross product of vectors, where the absence of an identity element is related to the fact that the direction of any nonzero cross product is always orthogonal to any element multiplied. That is, it is not possible to obtain a non-zero vector in the same direction as the original. Yet another example of group without identity element involves the additive semigroup of positivenatural numbers.
