The division induced by relation R is to divide the original set into several parts according to the conditions described by relation (actually equivalent relation).

Each part is an equivalence class, and then different equivalence classes, * * * together form the original set.

In short, division is to recombine the elements in the original set (the total number of elements is unchanged, there is no addition or deletion), and the elements in the group satisfy the equivalence relation r,

Elements between groups are not equivalent and cannot be put together.