Function is one kind of correspondence in math, from the non-empty number set A to the real number set B. Simply put, a changes with B, and a is Function of B. To be precise, let X be one non-empty Collection, Y a non-empty set of numbers, and f a correspondence rule, if for every X in x, according to the correspondence rule f, there exists only one elemental y and a correspondence in y, denote f as one Function on X, denote y=f (x), denote X as the domain of Function f (x), Collection {y|y=f (x), x∈R} as its range (the range is Y subsets), x is called the independent variable. y is called the dependent variable, and it's customary to say y is an x Function. The law of correspondence and the domain of definition are two elements of Function.