###### Definition1.7.3Interval Notation

Interval notation tries to just say the numbers where the interval starts and stops. For example, in Figure 1.7.1, the interval starts at $$18\text{.}$$ To the right, the interval extends forever and has no end, so we use the $$\infty$$ symbol (meaning "infinity"). This particular interval is denoted:

\begin{equation*} [18,\infty) \end{equation*}

Why use “$$[$$” on one side and “$$)$$” on the other? The square bracket tells us that $$18$$ is part of the interval and the round parenthesis tells us that $$\infty$$ is not part of the interval. 1

In general there are four types of infinite intervals. Take note of the different uses of round parentheses and square brackets.

in-context