Let a and b be two nonempty sets. If there is a unique element B corresponding to any element A in the set A according to a certain correspondence F, then such correspondence is called the mapping from the set A to the set B, which is marked as F: A → B. Among them, B is called the image of A under the mapping F, which is marked as B = F (a); A is called the original image of B on the mapping F, and the set of images with many elements in the set A is denoted as f(A). In mathematics and related fields, mapping or projection is usually equivalent to a function. Based on this, partial mapping is equivalent to partial function, and complete mapping is equivalent to complete function. In many specific mathematical fields, this term is used to describe functions with specific properties associated with this field, such as continuous functions in topology, linear transformations in linear algebra, and so on. If the two sets in the function definition are extended from non-empty sets to sets of arbitrary elements (not limited to numbers), we can get the concept of mapping: mapping mathematically describes a special corresponding relationship between the elements of the two sets.