A function is a correspondence relationship in mathematics that corresponds from a set of non empty numbers A to a set of real numbers B. Simply put, as A changes with B, A is a function of B. To be precise, let X be a non empty set, Y be a non empty set of numbers, and f be a correspondence rule. If for each x in X, according to the correspondence rule f, there exists a unique element y in Y that corresponds to it, then the correspondence rule f is a function on X, denoted as y=f (x), and X is the domain of the function f (x). The set {y | y=f (x), x ∈ R} is its domain (the domain is a subset of Y), x is the independent variable, y is the dependent variable, and traditionally y is also a function of x. The correspondence rule and domain are two elements of a function.