# Interval Notation

Interval notation is a shorthand way to show:

• The domain of a function (i.e., where the function is defined [1],
• A range of values that make an inequality true,
• Subsets of the real number line.

## Interval Notation Reference List

The following image shows representations for inequalities, their respective interval notation, and how they appear on a graph.
The ∞ symbol is used to represent infinity; infinity is not a number, so it should never be paired with a square bracket when using interval notation.

• Positive infinity (∞) indicates there isn’t an endpoint to the right of the number (on the number line).
• Negative infinity (∞) indicates there isn’t an endpoint to the left of the number (on the number line).

Note that on a graph, a closed circle ● can be used instead of brackets [ ] and an open circle ○ can be used instead of parentheses ( ). 1

## Some General Tips

The smaller value of the interval goes first and the larger value of the interval goes second; the two values are separated by a comma. Parentheses indicate an open interval and square brackets a closed interval, although you can also have a mixture of the two, giving a half closed or half open interval. For example:

• (2, 9) = an open interval from 2 to 9 (does not include 2 and 9 as endpoints).
• [2, 9] = a closed interval from 2 to 9 (includes 2 and 9 as endpoints).
• [2, 9) = a half-closed interval from 2 to 9 (includes 2 as an endpoint, but not 9).

Not sure what closed, open and half-closed intervals are? Watch the video for an overview: