Order (ring theory)

"Maximal order" redirects here. For the maximal order of an arithmetic function, see Extremal orders of an arithmetic function.

In mathematics, an order in the sense of ring theory is a subring ${\displaystyle {\mathcal {O}}}$ of a ring ${\displaystyle A}$, such that

1. ${\displaystyle A}$ is a finite-dimensional algebra over the field ${\displaystyle \mathbb {Q} }$ of rational numbers
2. ${\displaystyle {\mathcal {O}}}$ spans ${\displaystyle A}$ over ${\displaystyle \mathbb {Q} }$, and
3. ${\displaystyle {\mathcal {O}}}$ is a ${\displaystyle \mathbb {Z} }$-lattice in ${\displaystyle A}$.

The last two conditions can be stated in less formal terms: Additively, ${\displaystyle {\mathcal {O}}}$ is a free abelian group generated by a basis for ${\displaystyle A}$ over ${\displaystyle \mathbb {Q} }$.

More generally for ${\displaystyle R}$ an integral domain with fraction field ${\displaystyle K}$, an ${\displaystyle R}$-order in a finite-dimensional ${\displaystyle K}$-algebra ${\displaystyle A}$ is a subring ${\displaystyle {\mathcal {O}}}$ of ${\displaystyle A}$ which is a full ${\displaystyle R}$-lattice; i.e. is a finite ${\displaystyle R}$-module with the property that ${\displaystyle {\mathcal {O}}\otimes _{R}K=A}$.^[1]

When ${\displaystyle A}$ is not a commutative ring, the idea of order is still important, but the phenomena are different. For example, the Hurwitz quaternions form a maximal order in the quaternions with rational co-ordinates; they are not the quaternions with integer coordinates in the most obvious sense. Maximal orders exist in general, but need not be unique: there is in general no largest order, but a number of maximal orders. An important class of examples is that of integral group rings.

Examples

Some examples of orders are:^[2]

• If ${\displaystyle A}$ is the matrix ring ${\displaystyle M_{n}(K)}$ over ${\displaystyle K}$, then the matrix ring ${\displaystyle M_{n}(R)}$ over ${\displaystyle R}$ is an ${\displaystyle R}$-order in ${\displaystyle A}$
• If ${\displaystyle R}$ is an integral domain and ${\displaystyle L}$ a finite separable extension of ${\displaystyle K}$, then the integral closure ${\displaystyle S}$ of ${\displaystyle R}$ in ${\displaystyle L}$ is an ${\displaystyle R}$-order in ${\displaystyle L}$.
• If ${\displaystyle a}$ in ${\displaystyle A}$ is an integral element over ${\displaystyle R}$, then the polynomial ring ${\displaystyle R[a]}$ is an ${\displaystyle R}$-order in the algebra ${\displaystyle K[a]}$
• If ${\displaystyle A}$ is the group ring ${\displaystyle K[G]}$ of a finite group ${\displaystyle G}$, then ${\displaystyle R[G]}$ is an ${\displaystyle R}$-order on ${\displaystyle K[G]}$

A fundamental property of ${\displaystyle R}$-orders is that every element of an ${\displaystyle R}$-order is integral over ${\displaystyle R}$.^[3]

If the integral closure ${\displaystyle S}$ of ${\displaystyle R}$ in ${\displaystyle A}$ is an ${\displaystyle R}$-order then the integrality of every element of every ${\displaystyle R}$-order shows that ${\displaystyle S}$ must be the unique maximal ${\displaystyle R}$-order in ${\displaystyle A}$. However ${\displaystyle S}$ need not always be an ${\displaystyle R}$-order: indeed ${\displaystyle S}$ need not even be a ring, and even if ${\displaystyle S}$ is a ring (for example, when ${\displaystyle A}$ is commutative) then ${\displaystyle S}$ need not be an ${\displaystyle R}$-lattice.^[3]

Algebraic number theory

The leading example is the case where ${\displaystyle A}$ is a number field ${\displaystyle K}$ and ${\displaystyle {\mathcal {O}}}$ is its ring of integers. In algebraic number theory there are examples for any ${\displaystyle K}$ other than the rational field of proper subrings of the ring of integers that are also orders. For example, in the field extension ${\displaystyle A=\mathbb {Q} (i)}$ of Gaussian rationals over ${\displaystyle \mathbb {Q} }$, the integral closure of ${\displaystyle \mathbb {Z} }$ is the ring of Gaussian integers ${\displaystyle \mathbb {Z} [i]}$ and so this is the unique maximal ${\displaystyle \mathbb {Z} }$-order: all other orders in ${\displaystyle A}$ are contained in it. For example, we can take the subring of complex numbers of the form ${\displaystyle a+2bi}$, with ${\displaystyle a}$ and ${\displaystyle b}$ integers.^[4]

The maximal order question can be examined at a local field level. This technique is applied in algebraic number theory and modular representation theory.

See also

• Hurwitz quaternion order – An example of ring order