本文探讨了离散数学中的关系概念，包括从A到B的关系、函数的特殊性质、在集合A上的关系及其数量计算。还详细阐述了关系的反射性、对称性、反对称性和传递性等性质，以及如何结合和封闭关系。同时，介绍了等价关系、n-元关系、SQL操作、关系的矩阵和图表示，以及部分有序集（偏序集）和哈斯图的应用。
a relation from A to B is a subset of AxB;
function is a special relation where every elment of A is the first element of exact one ordered pair.
a relation on a set A is a subset of AxA;
so,there are 2^n^2 relations for a set with n element. such as :n=3;relation:512
properties of relations
a relation R on a set A is called reflective if (a,a) 属于R foreveryelement a 属于A
a relation R on a set A is called symmetric if (b,a)属于R whenever (a,b) 属于R，
a relation R on a set A such that for all a,b属于A,if(a,b)属于R，and (b,a)属于R,then a=b is calledantisymmetric
a relation R on a set A is called transitive if whenever (a,b)属于R,(b,c)属于R，then (a,c)属于R for all a,b,c 属于A
combining relations
RoS: where the second element o
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