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In mathematics, a unit in a ring R is an element u such that there is v in R with
- uv = vu = 1R.
That is, u is an invertible element of the multiplicative monoid of R. The units of R form a group U(R) under multiplication, the group of units of R.
The orbits of U(R) acting on R by multiplication are called sets of associates; in other words there is an equivalence relation on R called associatedness such that
- r ~ s
means that there is a unit u with r = us. For example in the ring Z of integers n and −n are associates.
Any root of unity is a unit. In algebraic number theory Dirichlet's unit theorem shows the existence of many units in most rings of algebraic integers. For example, we have
- (√5 + 2)(√5 − 2) = 1.
In fact that is the source for the unit terminology — which shouldn't be confused with the 'unit' of unital rings.
One can check that U is a functor from the category of rings, to the category of groups: a ring homomorphism must map units to units. It has a left adjoint, the integral group ring construction.
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