unit

> For special kinds of units see also *unit of an adjunction* and *unit of a monad*. Different (but related) is *physical unit*.

A unit is a quantity $u$ such that every other quantity (of a certain type) is a multiple (in a certain sense) of $u$.

Exactly what this means depends on context. A very general definition is this:

Given sets $R$ and $M$, and a function ${\cdot}\colon R \times M \to M$, an element $u$ of $M$ is a **unit** (relative to the operation ${\cdot}$) if, given any element $x$ of $M$, there exists a unique element $a$ of $R$ such that $x = a \cdot u$.

That is, every element of $M$ is a multiple (in a unique way) of $u$, where ‘multiple’ is defined in terms of the operation ${\cdot}$.

If $R$ is a ring (or rig), then $R$ comes equipped with a multiplication map ${\cdot}\colon R \times R \to R$. So $R$ can play the role of both $R$ and $M$ above, although there are two ways to do this: on the left and on the right.

We find that $u$ is a **left unit** if and only if $u$ has a left inverse, and $u$ is a **right unit** if and only if $u$ has a right inverse. First, an element $u$ with an inverse is a unit because, given any element $x$, we have

$x = (x u^{-1}) u$

(on the left) or

$x = u (u^{-1} x)$

(on the right). Conversely, a unit must have an inverse, since there must a solution to

$1 = a u$

(on the left) or

$1 = u a$

(on the right).

In a commutative ring (or rig), a **unit** is an element of $R$ that has an inverse, period. Of course, a commutative ring $R$ is a field just when every non-zero element is a unit.

Notice that addition plays no role in the characterisation above of a unit in a ring. Accordingly, a unit in a monoid may be defined in precisely the same way.

A group is precisely a monoid in which every element is a unit.

In a rng (or, ignoring addition, in a semigroup), we cannot speak of inverses of elements. However, we can still talk about units; $u$ is a **left unit** if, for every $x$, there is an $a$ such that

$x = a u ;$

and $u$ is a **right unit** if, for every $x$, there is an $a$ such that

$x = u a .$

In a nonassociative ring (or, ignoring addition, in a magma), even if we have an identity element, an invertible element might not be a unit. So we must use the same explicit definition as in a rng (or semigroup) above.

A quasigroup is precisely a magma in which every element is a two-sided unit.

If $R$ is a ring (or rig) and $M$ an $R$-module, then a **unit** in $M$ is an element $u \in M$ such that every other $x \in M$ can be written as $x = a u$ (or $x = u a$ for a right module) for some $a \in R$. This is the same as a generator of $M$ as an $R$-module. There is no need to distinguish left and right units unless $M$ is a bimodule. Note that a (left or right) unit in $R$ *qua* ring is the same as a unit in $R$ *qua* (left or right) $R$-module.

In physics, the quantities of a given dimension generally form an $\mathbb{R}$-line, a $1$-dimensional vector space over the real numbers. Since $\mathbb{R}$ is a field, any non-zero quantity is a unit, called in this context a **unit of measurement**. This is actually a special case of a unit in a module, where $R \coloneqq \mathbb{R}$ and $M$ is the line in question.

Often (but not always) these quantities form an oriented line, so that nonzero quantities are either positive or negative. Then we usually also require a unit of measurement to be positive. In fact, for some dimensions, there is no physical meaning to a negative quantity, in which case the quantities actually form a module over the rig $\mathbb{R}_{\ge 0}$ and every nonzero element is “positive.”

For example, the kilogram is a unit of mass, because any mass may be expressed as a real multiple of the kilogram. Further, it is a positive unit; the mass of any physical object is a nonnegative quantity (so that mass quantities actually form an $\mathbb{R}_{\ge 0}$-module) and may be expressed as a nonnegative real multiple of the kilogram.

Often the term ‘unit’ (or ‘unity’) is used as a synonym for ‘identity element’, especially when this identity element is denoted $1$. For example, a ‘ring with unit’ (or ‘ring with unity’) is a ring with an identity (used by authors who say ‘ring’ for a rng). Of course, a rng with identity has a unit, since $1$ itself is a unit; conversely, a commutative rng with a unit must have an identity.

I haven't managed to find either a proof or a counterexample to the converse (in the noncommutative case): that a rng with a unit must have an identity.

Response: If $R$ is a rng with a unit $u$, then every element uniquely factors through $u$. In particular, $u$ itself does. $u = a u$, with $a$ unique. So $a$ is an identity.

Reply: Why is $a$ an identity then? This works if the rng is commutative: given any $v$, write $v$ as $b u$, and then $a v = a (b u) = b (a u) = b u = v$. But without commutativity (and associativity), this doesn't work.

It is this meaning of ‘unit’ which gives rise to the unit of an adjunction.

Revised on April 1, 2014 01:17:04
by Urs Schreiber
(185.37.147.12)