Just about the only time you don’t need to use parentheses is when the function is an exponential function—for example, if h(x) = 3x, then you can just write h(x2 + 6) = 3x2 + 6. You don’t need parentheses since you’re already writing the x2 + 6 as a superscript. (link)
In other words, the inverse of the inverse is the original function (link)
So, this leads us to the vertical line test: if you have some graph and you want to know whether it’s the graph of a function, see whether any vertical line intersects the graph more than once. If so, it’s not the graph of a function; but if no vertical line intersects the graph more than once, you are indeed dealing with the graph of a function. (link)
So we have found that the domain is the set (−8, 13] except for the number 2. This set could be written as (−8, 13]{2}. Here the backslash means “not including.” (link)
An interval of the form (a, b) is called open (link)
An interval such as [a, b] is called closed. (link)
a function must assign a unique output for each valid input (link)