A function is written as the function’s name followed by parentheses containing the input to the function. The most common way to denote a function is , where the expression is read as “f of x.” Typically, a function appears in text like this:

In this notation, represents the function, and is the input. The output is the value that the function produces when it receives the input and is defined by the expression to the right of the equals sign.

To see an introduction to the concept of a function see this page. To see a reference of the different functions in math see this page.