Boolean ring


In mathematics, a Boolean ring is a ring for which for all in, that is, a ring that consists of only idempotent elements. An example is the ring of integers modulo 2.
Every Boolean ring gives rise to a Boolean algebra, with ring multiplication corresponding to conjunction or meet, and ring addition to exclusive disjunction or symmetric difference. Conversely, every Boolean algebra gives rise to a Boolean ring. Boolean rings are named after the founder of Boolean algebra, George Boole.

Notation

There are at least four different and incompatible systems of notation for Boolean rings and algebras:
  • In commutative algebra the standard notation is to use for the ring sum of and, and use for their product.
  • In logic, a common notation is to use for the meet and use for the join, given in terms of ring notation by.
  • In set theory and logic it is also common to use for the meet, and for the join. This use of is different from the use in ring theory.
  • A rare convention is to use for the product and for the ring sum, in an effort to avoid the ambiguity of.
Historically, the term "Boolean ring" has been used to mean a "Boolean ring possibly without an identity", and "Boolean algebra" has been used to mean a Boolean ring with an identity. The existence of the identity is necessary to consider the ring as an algebra over the field of two elements: otherwise there cannot be a ring homomorphism of the field of two elements into the Boolean ring.

Examples

One example of a Boolean ring is the power set of any set, where the addition in the ring is symmetric difference, and the multiplication is intersection. As another example, we can also consider the set of all finite or cofinite subsets of, again with symmetric difference and intersection as operations. More generally with these operations any field of sets is a Boolean ring. By Stone's representation theorem every Boolean ring is isomorphic to a field of sets.

Relation to Boolean algebras

Since the join operation in a Boolean algebra is often written additively, it makes sense in this context to denote ring addition by, a symbol that is often used to denote exclusive or.
Given a Boolean ring, for and in we can define
These operations then satisfy all of the axioms for meets, joins, and complements in a Boolean algebra. Thus every Boolean ring becomes a Boolean algebra. Similarly, every Boolean algebra becomes a Boolean ring thus:
If a Boolean ring is translated into a Boolean algebra in this way, and then the Boolean algebra is translated into a ring, the result is the original ring. The analogous result holds beginning with a Boolean algebra.
A map between two Boolean rings is a ring homomorphism if and only if it is a homomorphism of the corresponding Boolean algebras. Furthermore, a subset of a Boolean ring is a ring ideal if and only if it is an order ideal of the Boolean algebra. The quotient ring of a Boolean ring modulo a ring ideal corresponds to the factor algebra of the corresponding Boolean algebra modulo the corresponding order ideal.

Properties of Boolean rings

Every Boolean ring satisfies for all in, because we know
and since is an abelian group, we can subtract from both sides of this equation, which gives. A similar proof shows that every Boolean ring is commutative:
The property shows that any Boolean ring is an associative algebra over the field with two elements, in precisely one way. In particular, any finite Boolean ring has as cardinality a power of two. Not every unital associative algebra over is a Boolean ring: consider for instance the polynomial ring.
The quotient ring of any Boolean ring modulo any ideal is again a Boolean ring. Likewise, any subring of a Boolean ring is a Boolean ring.
Any localization of a Boolean ring by a set is a Boolean ring, since every element in the localization is idempotent.
The maximal ring of quotients of a Boolean ring is a Boolean ring, since every partial endomorphism is idempotent.
Every prime ideal in a Boolean ring is maximal: the quotient ring is an integral domain and also a Boolean ring, so it is isomorphic to the field, which shows the maximality of. Since maximal ideals are always prime, prime ideals and maximal ideals coincide in Boolean rings.
Every finitely generated ideal of a Boolean ring is principal (indeed,. Furthermore, as all elements are idempotents, Boolean rings are commutative von Neumann regular rings and hence absolutely flat, which means that every module over them is flat.

Unification

Unification in Boolean rings is decidable, that is, algorithms exist to solve arbitrary equations over Boolean rings. Both unification and matching in finitely generated free Boolean rings are NP-complete, and both are NP-hard in finitely presented Boolean rings.
Unification in Boolean rings is unitary if all the uninterpreted function symbols are nullary and finitary otherwise.