Definition:Real Function/Definition by Equation

Definition
It is often convenient to refer to an equation or formula as though it were a function.

What is meant is that the equation defines the function, i.e. it specifies the rule by which we obtain the value of $y$ from a given $x$.

Square Function
For example, let $x, y \in \R$.

We may express this as $y = x^2$, and use this equation to define this function.

This may be conceived as:
 * For each $x \in \R$, the number $y \in \R$ assigned to it is that which we get by squaring $x$.

Also see

 * Definition:Real-Valued Function