Definition 2. 20: Let r be an equivalence relation on a set a. The set of all element that are related to an element a of a is called the equivalence class of a. The equivalence class of a with respect to R is denoted by lairs When only one relation is under consideration, we will delete the subscript r and write a for this equivalence class. Example: equivalence classes of congruence modulo 2 are 0 and 1 0}={…-4,-2,0,2,4,}=[2=4]=-2=|-4}= ={,-3,-1,1,3,}=|3=-1l-3}= the partition of ZIL/R=o1Definition 2.20: Let R be an equivalence relation on a set A. The set of all element that are related to an element a of A is called the equivalence class of a. The equivalence class of a with respect to R is denoted by [a]R, When only one relation is under consideration, we will delete the subscript R and write [a] for this equivalence class. Example：equivalence classes of congruence modulo 2 are [0] and [1]。 [0]={…,-4,-2,0,2,4,…}=[2]=[4]=[-2]=[-4]=… [1]={…,-3,-1,1,3,…}=[3]=[-1]=[-3]=… the partition of Z =Z/R={[0],[1]}