Meanwhile, the following $R$ expression is not a function, since the same input, $-2$, maps to two different outputs, $5$ and $7$. It is a **non-function**. $ R=\left\{(-2, 5), (3,7), (-36,8), (-2, 7)\right\} $ A more common example of a non-function is square roots. Unless they're given as a positive or negative, we must write them in the form $\pm\sqrt x$. They can be both evaluated to a positive or negative value, therefore not satisfying the constraints of functions either.