Editing Interval (mathematics) (section)

===Infinite endpoints===
In some contexts, an interval may be defined as a subset of the [[extended real number line|extended real numbers]], the set of all real numbers augmented with {{math|−∞}} and {{math|+∞}}.

In this interpretation, the notations {{closed-closed|−∞, ''b''}} , {{open-closed|−∞, ''b''}} , {{closed-closed|''a'', +∞}} , and {{closed-open|''a'', +∞}} are all meaningful and distinct. In particular, {{open-open|−∞, +∞}} denotes the set of all ordinary real numbers, while {{closed-closed|−∞, +∞}} denotes the extended reals. 

Even in the context of the ordinary reals, one may use an [[infinity (mathematics)|infinite]] endpoint to indicate that there is no bound in that direction. For example, {{open-open|0, +∞}} is the set of [[positive real numbers]], also written as <math>\mathbb{R}_+.</math> The context affects some of the above definitions and terminology. For instance, the interval {{open-open|−∞, +∞}}&nbsp;=&nbsp;<math>\R</math> is closed in the realm of ordinary reals, but not in the realm of the extended reals.