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:
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(a, + ∞ is the set of all real numbers x such that a < x.
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(- ∞ a) is the set of all real numbers x such that x < a.
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[a, + ∞ is the set of all real numbers x such that a ≤ x.
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(- ∞ , a] is the set of all real numbers x such that x ≤ a.