偏序关系 Suppose R is a relation on a set S satisfying the following three properties: [O](Reflexive)For any a e S,we have aRa. [O2](Antisymmetric)If a Rb and bRa,then a =b. [O3](Transitive)If a Rb and bRc,then aRc. Then R is called a partial order or,simply an order relation,and R is said to define a partial ordering of S.The s set S with the partial order is called a partially ordered set or,simply,an ordered set or poset.We write (S,R) when we want to specify the relation R.偏序关系