In mathematics, a Boolean ring R is a ring (with identity) for which x^{2} = x for all x in R; that is, R consists only of idempotent elements.
Boolean rings are automatically commutative and of characteristic 2 (see below for proof). A Boolean ring is essentially the same thing as a Boolean algebra, with ring multiplication corresponding to conjunction or meet ∧, and ring addition to exclusive disjunction or symmetric difference (not disjunction ∨).
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There are at least four different and incompatible systems of notation for Boolean rings and algebras.
The old terminology was to use "Boolean ring" to mean a "Boolean ring possibly without an identity", and "Boolean algebra" to mean a Boolean ring with an identity. (This is the same as the old use of the terms "ring" and "algebra" in measure theory).
The simplest Boolean ring is the Twoelement Boolean algebra, with set the Boolean domain, conventionally written B = {0, 1}.
One example of a Boolean ring is the power set of any set X, 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 X, 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 (treated as a ring with these operations).
Since the join operation ∨ in a Boolean algebra is often written additively, it makes sense in this context to denote ring addition by ⊕ (which is the same as subtraction in any Boolean algebra), a symbol that is often used to denote exclusive or.
Given a Boolean ring R, for x and y in R 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 (prime ring ideal, maximal ring ideal) if and only if it is an order ideal (prime order ideal, maximal 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.
Every Boolean ring R satisfies x ⊕ x = 0 for all x in R, because we know
and since <R,⊕> is an abelian group, we can subtract x ⊕ x from both sides of this equation, which gives x ⊕ x = 0. A similar proof shows that every Boolean ring is commutative:
and this yields xy ⊕ yx = 0, which means xy = yx (using the first property above).
The property x ⊕ x = 0 shows that any Boolean ring is an associative algebra over the field F_{2} with two elements, in just one way. In particular, any finite Boolean ring has as cardinality a power of two. Not every associative algebra with one over F_{2} is a Boolean ring: consider for instance the polynomial ring F_{2}[X].
The quotient ring R/I of any Boolean ring R modulo any ideal I is again a Boolean ring. Likewise, any subring of a Boolean ring is a Boolean ring.
Every prime ideal P in a Boolean ring R is maximal: the quotient ring R/P is an integral domain and also a Boolean ring, so it is isomorphic to the field F_{2}, which shows the maximality of P. Since maximal ideals are always prime, prime ideals and maximal ideals coincide in Boolean rings.
Boolean rings are von Neumann regular rings.
Boolean rings are absolutely flat: this means that every module over them is flat.
Every finitely generated ideal of a Boolean ring is principal (indeed, (x,y)=(x+y+xy)).
In mathematics, a Boolean ring R is a ring (with identity) for which x^{2} = x for all x in R; that is, R consists only of idempotent elements.
Boolean rings are automatically commutative and of characteristic 2 (see below for proof). A Boolean ring is related to a Boolean algebra, with ring multiplication corresponding to conjunction or meet ∧, but Boolean rings need not have ring addition, nor do they need to support logical negation.
Much of the following material is about Boolean algebras and is not necessarily relevant to Boolean rings. Additionally, statements that boolean algebras and boolean rings are the same thing should be recognized as incorrect.
If * is defined as "greatest common divisor" (or "least common multiple") and x is a nonnegative integer, then x = x*x and we have a boolean ring. But we do not have any "not" operation in these examples, so they can not be a boolean algebras.
Contents 
There are at least four different and incompatible systems of notation for Boolean rings and algebras.
The old terminology was to use "Boolean ring" to mean a "Boolean ring possibly without an identity", and "Boolean algebra" to mean a Boolean ring with an identity. (This is the same as the old use of the terms "ring" and "algebra" in measure theory).
The simplest Boolean ring is the twoelement Boolean algebra, with set the Boolean domain, conventionally written B = {0, 1}.
One example of a Boolean ring is the power set of any set X, 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 X, 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 (treated as a ring with these operations).
[[File:center500pxthumbVenn diagrams for the Boolean operations of conjunction, disjunction, and complement]] Since the join operation ∨ in a Boolean algebra is often written additively, it makes sense in this context to denote ring addition by ⊕ (which is the same as subtraction in any Boolean algebra), a symbol that is often used to denote exclusive or.
Given a Boolean ring R, for x and y in R 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 (prime ring ideal, maximal ring ideal) if and only if it is an order ideal (prime order ideal, maximal 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.
Every Boolean ring R satisfies x ⊕ x = 0 for all x in R, because we know
and since <R,⊕> is an abelian group, we can subtract x ⊕ x from both sides of this equation, which gives x ⊕ x = 0. A similar proof shows that every Boolean ring is commutative:
and this yields xy ⊕ yx = 0, which means xy = yx (using the first property above).
The property x ⊕ x = 0 shows that any Boolean ring is an associative algebra over the field F_{2} with two elements, in just one way. In particular, any finite Boolean ring has as cardinality a power of two. Not every associative algebra with one over F_{2} is a Boolean ring: consider for instance the polynomial ring F_{2}[X].
The quotient ring R/I of any Boolean ring R modulo any ideal I is again a Boolean ring. Likewise, any subring of a Boolean ring is a Boolean ring.
Every prime ideal P in a Boolean ring R is maximal: the quotient ring R/P is an integral domain and also a Boolean ring, so it is isomorphic to the field F_{2}, which shows the maximality of P. Since maximal ideals are always prime, prime ideals and maximal ideals coincide in Boolean rings.
Boolean rings are von Neumann regular rings.
Boolean rings are absolutely flat: this means that every module over them is flat.
Every finitely generated ideal of a Boolean ring is principal (indeed, (x,y)=(x+y+xy)).
