###### 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.

*not*including \(a\text{.}\)

*including*\(a\text{.}\)

*not*including \(a\text{.}\)

*including*\(a\text{.}\)