Is y = ± x a function? Where x is the independent variable and Y is the dependent variable Function definition: let X be a nonempty set, y be a nonempty set, and f be a corresponding rule. If for each x in X, according to the corresponding rule F, there is a unique element X in Y corresponding to it, then the corresponding rule f is a function on X, denoted as y = f (x) Y = ± x when x takes one value, y has two values, which does not conform to the definition of function, right? What is this equation We also see a supplement: In the definition of function, for every x ∈ D, the corresponding function value y is always unique. The function defined in this way is called single valued function. If given a corresponding rule, for every x ∈ D, there is always a certain y value corresponding to it, but this y is not always unique. We call this method to determine a multi valued function. Clearly defined function should satisfy the requirement that "there is only one element X in Y corresponding to it". Now "multi valued function" comes out. What's the matter What I read is the fifth edition edited by the Department of Applied Mathematics of Tongji University

Is y = ± x a function? Where x is the independent variable and Y is the dependent variable Function definition: let X be a nonempty set, y be a nonempty set, and f be a corresponding rule. If for each x in X, according to the corresponding rule F, there is a unique element X in Y corresponding to it, then the corresponding rule f is a function on X, denoted as y = f (x) Y = ± x when x takes one value, y has two values, which does not conform to the definition of function, right? What is this equation We also see a supplement: In the definition of function, for every x ∈ D, the corresponding function value y is always unique. The function defined in this way is called single valued function. If given a corresponding rule, for every x ∈ D, there is always a certain y value corresponding to it, but this y is not always unique. We call this method to determine a multi valued function. Clearly defined function should satisfy the requirement that "there is only one element X in Y corresponding to it". Now "multi valued function" comes out. What's the matter What I read is the fifth edition edited by the Department of Applied Mathematics of Tongji University

This equation calculates two functions
Y = x and y = - x
If it's two straight lines in rectangular coordinates