Correspondence in mathematics mainly refers to the correspondence of elements in a set, such as sets A = {1, 2} and B = {2 2,4}. If a certain mathematical rule y=2x is used (if the element in set A is called X and the element in set B is called Y), then the element in set A is1->; 2(B set), 2-> 4(B set), so that the elements in set A correspond to the elements in set B respectively. This relationship, which we call mapping, is also called that y is a function of x.