Today I learned about relations and Cartesian products. The Cartesian product of two sets A and B is the set consisting of all the ordered pairs (a, b) where a is an element of A and b is an element of B.
A relation of a set V is any subset of the Cartesian product of V with itself. Given an ordered pair (a, b) of a relation R, a is said to related to b by R.
Relations can have several properties: reflexive, irreflexive, symmetric, asymmetric, and transitive. A relation R on V is reflexive if for every x in V there is an element (x, x) in R; it is irreflexive if for every x in V there is no element (x, x) in R. A relation R is symmetric if for every element (a, b), there is another element (b, a); it is asymmetric if for every element (a, b) where b is not a, there is no element (b, a). And finally, a relation R is transitive if for every element (a, b) there is an element (b, c) and (a, c).
I also learned that the cardinality of a set is the number of elements in that set.
Reposted from http://invisiblegdev.blogspot.com/