A **function** is a relationship that maps **one input** (the independent variable) to **one output** (the dependent variable). The reverse, however, is not true. Different inputs could produce the same output, and the expression would still be a function. Consider for instance the following function: $ S=\left\{(-2, 16), (13,14), (15,-7), (17, 16)\right\} $ Two different inputs, $-2$ and $17$ map to the same output, $16$. While it does not satisfy the properties of a *one-to-one* function, it is still a function. Functions can be written $y=f(x)$, $f(x)=2x+11$, $f(x)$ or $y=11x+2$.