PartialOrder

A partial order <= on a set X is a MathematicalRelation satisfying the following conditions: for every a,b,c in X, 

    a <= a                              (reflexive property)
    If a <= b, b <= a, then a = b       (antisymmetric property)
    If a <= b, b <= c, then a <= c      (transitive property)

A set with a partial order on it is called a partial ordered set, or poset.  A poset where any two elements have both a greatest lower bound and a least upper bound forms an algebraic structure called a LatticE.

Given any a,b in X, we define the OpenInterval (a,b) = {x in X : a < x < b}.  Arbitrary unions of OpenInterval''s'' define a topology on the poset, called the OrderTopology.  Most familiar topologies are built up from objects of this sort, e.g. the RealNumbers.