1.2 does not exist. Inclusion is the relationship between sets, for example, {1} is included in the relationship between {1, 2} sets and elements, for example, 1 belongs to {1, 2} 3. The function represents the correspondence between each input value and a unique output value. The standard symbol of the output value x corresponding to the input value in the function f is f(x). The set containing all the input values of a function is called the domain of this function, and the set containing all the output values is called the range. If you define the concept of mapping first, you can simply define a function as, and the mapping defined between non-empty number sets is called a function. (that is, set A in the mapping can be regarded as the definition domain and set B as the value domain) Let A and B be two non-empty sets. If any element in set A corresponds to a unique element according to a certain correspondence F, then such correspondence (including sets A and B and the correspondence F from set A to set B) is called the mapping from set A to set B. Generally speaking, 10 is written first.