function
This expression does not define a function, because the output value may have two. If there are more than two output values, it is not possible to determine which number to output after entering a number. For example, the square root of 9 is 3 and -3. To define a function, you must explicitly select a square root. We can define (for the positive square root) that is, for any non-negative input value, select its non-negative square root as a function value. The function is not necessarily related to numbers, for example, to specify the current capital of each country, then, in this function. In the set theory, the function has a precise definition, here only for an informal explanation: Let A and B for the two sets, if for each element of A, B, there is a limited element of the unique element with its Corresponding, it is said that this correspondence is a function from A to B. The set A is called the definition field of the function, and the set B is called the function. ...