Signorini problem

In abstract algebra, a partially ordered ring is a ring (A, +, · ), together with a compatible partial order, i.e. a partial order $\leq$ on the underlying set A that is compatible with the ring operations in the sense that it satisfies:

$x\leq y$ implies $x+z\leq y+z$

and

$0\leq x$ and $0\leq y$ imply that $0\leq x\cdot y$

for all $x,y,z\in A$. Various extensions of this definition exist that constrain the ring, the partial order, or both. For example, an Archimedean partially ordered ring is a partially ordered ring $(A,\leq )$ where $A$'s partially ordered additive group is Archimedean.

An ordered ring, also called a totally ordered ring, is a partially ordered ring $(A,\leq )$ where $\leq$ is additionally a total order.

An l-ring, or lattice-ordered ring, is a partially ordered ring $(A,\leq )$ where $\leq$ is additionally a lattice order.

Properties

The additive group of a partially ordered ring is always a partially ordered group.

The set of non-negative elements of a partially ordered ring (the set of elements x for which $0\leq x$, also called the positive cone of the ring) is closed under addition and multiplication, i.e., if P is the set of non-negative elements of a partially ordered ring, then $P+P\subseteq P$, and $P\cdot P\subseteq P$. Furthermore, $P\cap (-P)=\{0\}$.

The mapping of the compatible partial order on a ring A to the set of its non-negative elements is one-to-one; that is, the compatible partial order uniquely determines the set of non-negative elements, and a set of elements uniquely determines the compatible partial order if one exists.

If S is a subset of a ring A, and:

1. $0\in S$
2. $S\cap (-S)=\{0\}$
3. $S+S\subseteq S$
4. $S\cdot S\subseteq S$

then the relation $\leq$ where $x\leq y$ iff $y-x\in S$ defines a compatible partial order on A (ie. $(A,\leq )$ is a partially ordered ring).

In any l-ring, the absolute value $|x|$ of an element x can be defined to be $x\vee (-x)$, where $x\vee y$ denotes the maximal element. For any x and y,

$|x\cdot y|\leq |x|\cdot |y|$

holds.

f-rings

An f-ring, or Pierce–Birkhoff ring, is a lattice-ordered ring $(A,\leq )$ in which $x\wedge y=0$ and $0\leq z$ imply that $zx\wedge y=xz\wedge y=0$ for all $x,y,z\in A$. They were first introduced by Garrett Birkhoff and Richard S. Pierce in 1956, in a paper titled "Lattice-ordered rings", in an attempt to restrict the class of l-rings so as to eliminate a number of pathological examples. For example, Birkhoff and Pierce demonstrated an l-ring with 1 in which 1 is negative, even though being a square. The additional hypothesis required of f-rings eliminates this possibility.

Example

Let X be a Hausdorff space, and ${\mathcal {C}}(X)$ be the space of all continuous, real-valued functions on X. ${\mathcal {C}}(X)$ is an Archimedean f-ring with 1 under the following point-wise operations:

$[f+g](x)=f(x)+g(x)$

$[fg](x)=f(x)\cdot g(x)$

$[f\wedge g](x)=f(x)\wedge g(x).$

From an algebraic point of view the rings ${\mathcal {C}}(X)$ are fairly rigid. For example localisations, residue rings or limits of rings of the form ${\mathcal {C}}(X)$ are not of this form in general. A much more flexible class of f-rings containing all rings of continuous functions and resembling many of the properties of these rings, is the class of real closed rings.

Properties

A direct product of f-rings is an f-ring, an l-subring of an f-ring is an f-ring, and an l-homomorphic image of an f-ring is an f-ring.

$|xy|=|x||y|$ in an f-ring.

The category Arf consists of the Archimedean f-rings with 1 and the l-homomorphisms that preserve the identity.

Every ordered ring is an f-ring, so every subdirect union of ordered rings is also an f-ring. Assuming the axiom of choice, a theorem of Birkhoff shows the converse, and that an l-ring is an f-ring if and only if it is l-isomorphic to a subdirect union of ordered rings. Some mathematicians take this to be the definition of an f-ring.

Formally verified results for commutative ordered rings

IsarMathLib, a library for the Isabelle theorem prover, has formal verifications of a few fundamental results on commutative ordered rings. The results are proved in the ring1 context.

Suppose $(A,\leq )$ is a commutative ordered ring, and $x,y,z\in A$. Then:

	by
The additive group of A is an ordered group	OrdRing_ZF_1_L4
$x\leq y$ iff $x-y\leq 0$	OrdRing_ZF_1_L7
$x\leq y$ and $0\leq z$ imply $xz\leq yz$ and $zx\leq zy$	OrdRing_ZF_1_L9
$0\leq 1$	ordring_one_is_nonneg
$|xy|=|x||y|$	OrdRing_ZF_2_L5
$|x+y|\leq |x|+|y|$	ord_ring_triangle_ineq
x is either in the positive set, equal to 0, or in minus the positive set.	OrdRing_ZF_3_L2
The set of positive elements of $(A,\leq )$ is closed under multiplication iff A has no zero divisors.	OrdRing_ZF_3_L3
If A is non-trivial ($0\neq 1$), then it is infinite.	ord_ring_infinite

Further reading

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• Gillman, Leonard; Jerison, Meyer Rings of continuous functions. Reprint of the 1960 edition. Graduate Texts in Mathematics, No. 43. Springer-Verlag, New York-Heidelberg, 1976. xiii+300 pp

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