Interval
Definition
An interval is a continuous range of values, such as all of the real numbers between 
and 
inclusive. The most common uses of an interval are to specify the domain and range of a function.
Symbols
If an interval has either 
or 
in it, the values at the end are NOT included in the interval.
If an interval has either 
or ![$]$](http://latex.artofproblemsolving.com/0/7/1/07174a92e49be0bb0db1beef2bb6c245a8b212f9.png)
in it, the values at the end ARE included.
If both endpoints are not included, then the interval is open. If both endpoints are included, then the interval is closed.
Note: The symbols and are used with 
and 
by convention.
Examples
means all real numbers between 
and 
but not including or 
means all real numbers between 
and 
but not including 
means all real numbers between 
and 
including 
but not including 
means all real numbers between 
but not including 
means all real numbers greater than or equal to 
means all real numbers greater than or equal to 
