Intervals
Definition of an Interval
An interval is a to set of real numbers between two given points: a and b, which are called ends of the interval.
Open Interval
An open interval, (a, b), is the set of all real numbers greater than a and smaller than b.
(a, b) = {x 
/ a < x < b}
Closed Interval
A closed interval, [a, b), is the set of all real numbers greater than or equal to a and less than or equal to b.
[a, b] = {x / a ≤ x ≤ b}
Half-Closed Intervals
(a, b] = {x / a < x ≤ b}
[a, b) = {x / a ≤ x < b}
Half-closed intervals are also called half-open intervals.
When there are a set of points formed by two or more of these intervals, the sign 
(Union) is used between them.
