logic (After the logician
George Boole)
1. Commonly, and especially in computer science and digital electronics, this term is used to mean
two-valued logic.
2. This is in stark contrast with the definition used by pure mathematicians who in the 1960s introduced "Boolean-valued
models" into logic precisely because a "Boolean-valued model" is an interpretation of a
theory that allows more than two possible truth values!
Strangely, a Boolean algebra (in the mathematical sense) is not strictly an
algebra, but is in fact a
lattice. A Boolean algebra is sometimes defined as a "complemented
distributive lattice".
Boole's work which inspired the mathematical definition concerned
algebras of
sets, involving the operations of intersection, union and complement on sets. Such algebras obey the following identities where the operators ^, V, - and constants 1 and 0 can be thought of either as set intersection, union, complement, universal, empty; or as two-valued logic AND, OR, NOT, TRUE, FALSE; or any other conforming system.
a ^ b = b ^ a a V b = b V a (commutative laws) (a ^ b) ^ c = a ^ (b ^ c) (a V b) V c = a V (b V c) (associative laws) a ^ (b V c) = (a ^ b) V (a ^ c) a V (b ^ c) = (a V b) ^ (a V c) (distributive laws) a ^ a = a a V a = a (idempotence laws) --a = a -(a ^ b) = (-a) V (-b) -(a V b) = (-a) ^ (-b) (de Morgan's laws) a ^ -a = 0 a V -a = 1 a ^ 1 = a a V 0 = a a ^ 0 = 0 a V 1 = 1 -1 = 0 -0 = 1
There are several common alternative notations for the "-" or
logical complement operator.
If a and b are elements of a Boolean algebra, we define a partial ordering, though it is not necessarily a linear ordering since some Boolean algebras contain incomparable values.
Note that these laws only refer explicitly to the two distinguished constants 1 and 0 (sometimes written as
LaTeX \top and \bot), and in
two-valued logic there are no others, but according to the more general mathematical definition, in some systems variables a, b and c may take on other values as well.
(1997-02-27)