example of Boolean algebras
Below is a list of examples of Boolean algebras. Note that the phrase “usual set-theoretic operations” refers to the operations of union , intersection
, and set complement
.
- 1.
Let be a set. The power set
of , or the collection
of all the subsets of , together with the operations of union, intersection, and set complement, the empty set
and , is a Boolean algebra
. This is the canonical example of a Boolean algebra.
- 2.
In , let be the collection of all finite subsets of , and the collection of all cofinite subsets of . Then is a Boolean algebra.
- 3.
More generally, any field of sets is a Boolean algebra. In particular, any sigma algebra in a set is a Boolean algebra.
- 4.
(product
of algebras
) Let and be Boolean algebras. Then is a Boolean algebra, where
(1) (2) (3) - 5.
More generally, if we have a collection of Boolean algebras , indexed by a set , then is a Boolean algebra, where the Boolean operations are defined componentwise.
- 6.
In particular, if is a Boolean algebra, then set of functions from some non-empty set to is also a Boolean algebra, since .
- 7.
(subalgebras
) Let be a Boolean algebra, any subset such that , whenever , and whenever is a Boolean algebra. It is called a Boolean subalgebra of . In particular, the homomorphic image
of a Boolean algebra homomorphism is a Boolean algebra.
- 8.
(quotient algebras
) Let be a Boolean algebra and a Boolean ideal in . View as a Boolean ring
and an ideal in . Then the quotient ring is Boolean, and hence a Boolean algebra.
- 9.
Let be a set, and be the set of all -ary relations on . Then is a Boolean algebra under the usual set-theoretic operations. The easiest way to see this is to realize that , the powerset of the -fold power of .
- 10.
The set of all clopen sets in a topological space
is a Boolean algebra.
- 11.
Let be a topological space and be the collection of all regularly open
sets in . Then has a Boolean algebraic structure
. The meet and the constant operations follow the usual set-theoretic ones: , and . However, the join and the complementation on are different. Instead, they are given by
(4) (5)