Intervals which extend indefinitely in both the directions are known as unbounded intervals. These are written with the aid of symbols +∞ and - ∞ . The various types of intervals, if "a" happens to be a real number, are:
(a, + ∞ is the set of all real numbers x such that a < x. (- ∞ a) is the set of all real numbers x such that x < a. [a, + ∞ is the set of all real numbers x such that a ≤ x. (- ∞ , a] is the set of all real numbers x such that x ≤ a.
(a, + ∞ is the set of all real numbers x such that a < x.
(- ∞ a) is the set of all real numbers x such that x < a.
[a, + ∞ is the set of all real numbers x such that a ≤ x.
(- ∞ , a] is the set of all real numbers x such that x ≤ a.