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Difference between revisions of "Identity"

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There are at least two possible meanings in mathematics for the word identity.
 
There are at least two possible meanings in mathematics for the word identity.
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== Equations ==
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An '''identity''' is "a general relationship which always holds, usually over some choice of variables."  For example, <math>(x+1)^2=x^2+2x+1</math> is an identity, since it holds regardless of choice of variable. We therefore sometimes write <math>(x+1)^2\equiv x^2+2x+1</math>.
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== Abstract Algebra ==
 
== Abstract Algebra ==
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This usage of the word identity is common in [[abstract algebra]].
 
This usage of the word identity is common in [[abstract algebra]].
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== Equations ==
 
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An alternative meaning for the word identity is "a general relationship which always holds, usually over some choice of variables."  For example, <math>(x+1)^2=x^2+2x+1</math> is an identity, since it holds regardless of choice of variable. We therefore sometimes write <math>(x+1)^2\equiv x^2+2x+1</math>.
 

Revision as of 05:11, 18 July 2006

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There are at least two possible meanings in mathematics for the word identity.


Equations

An identity is "a general relationship which always holds, usually over some choice of variables." For example, $(x+1)^2=x^2+2x+1$ is an identity, since it holds regardless of choice of variable. We therefore sometimes write $(x+1)^2\equiv x^2+2x+1$.


Abstract Algebra

Given a binary operation G on a set S, $G: S \times S \to S$, an identity for G is an element $e\in S$ such that for all $a \in S$, $G(e, a) = G(a, e) = a$. For example, in the real numbers, if we take G to be the operation of multiplication $G(a, b) = a\cdot b$, the number 1 will be the identity for G. If instead we took G to be addition ($G(a, b) = a + b$), 0 would be the identity.

Identities in this sense are unique. Imagine we had two identities, $e$ and $e'$, for some operation $G$. Then $e = G(e, e') = e'$, so $e = e'$, and so e and e' are in fact equal.

This usage of the word identity is common in abstract algebra.

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