Boolean Algebra
Boolean algebra Definition Basic theorems Boolean operations on sets
Definition
Properties of both set operations and logic operations are used in an algebraic structure called a Boolean algebra.
A Boolean algebra is a set A on which are defined; two binary operations, + (called OR),  · (called AND), a unary operation, denoted ' (called NOT) and two distinct elements 0 and 1, such that, for all elements a, b and c of the set A, the following axioms hold:
associative law:               a + (b + c) = (a + b) + c                   a · (b · c) = (a · b) ·
commutative law:             a + b = b + a                                     a · b = b · a
distributive law:                a + (b · c) = (a + b) · (a + c)              a · (b + c) = (a · b) + (a · c)
identity law:                     a + 0 = a                                            a · 1 = a
complement law:              a + a' = 1                                           a · a' = 0
From these axioms derived are following identities:
idempotent law:                a + a = a                                            a · a = a
boundedness law:             a + 1 = 1                                            a · 0 = 0
absorption law:                 a + (a · b) = a                                     a · (a + b) = a
involution law:                  ( a' )' = a
De Morgan's laws:            (a + b)' = a' · b                                  (a · b)' = a' + b
0 and 1 complements:       0' = 1                                                   1' = 0
Boolean Operations on Sets
Using noticed coincidence between truth tables and Venn's diagrams we can prove any of the axioms or theorems of Boolean algebra. Thus, below is shown the proof of the distributive law:
· ( B + C ) = A  · B + A  · C
 A B C B + C A  · ( B + C ) 1 0 0 0 0 0 2 1 0 0 0 0 3 0 1 0 1 0 4 0 0 1 1 0 5 1 1 0 1 1 6 1 0 1 1 1 7 0 1 1 1 0 8 1 1 1 1 1
 A B C A  · B A  · C A  · B + A  · C 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 1 1 0 1 0 1 1 0 1 0 1 1 0 1 1 0 0 0 1 1 1 1 1 1
 Notice that required Boolean (binary) operations are performed on the three sets so, in the above tables there are 23 = 8 combinations. In both tables, at the resultant columns, obtained are ones (1) for the same combinations of input values of the sets, A, B and C, which correspond with the three colored areas in the Venn's diagram. Using rules of  Boolean algebra each of the eight areas of the Venn's diagram can be represented as a product involving given sets,  A, B and C:
 1 A' · B' · C' 3 A' · B · C' 5 A · B · C' 7 A' · B · C 2 A · B' · C' 4 A' · B' · C 6 A · B' · C 8 A · B · C
In general, the following rules must always be followed when evaluating a Boolean expression:
1. First, perform all complementations of single terms.
2. Then perform all operations within parentheses.
3. Perform an AND operation before an OR operation unless parentheses indicate otherwise.   Beginning Algebra Contents 