The limit as x decreases in value approaching a (x approaches a "from the right" or "from above") can be denoted:
- or or or
The limit as x increases in value approaching a (x approaches a "from the left" or "from below") can be denoted:
- or or or
In probability theory it is common to use the short notation:
- for the left limit and for the right limit.
The two one-sided limits exist and are equal if the limit of f(x) as x approaches a exists. In some cases in which the limit
does not exist, the two one-sided limits nonetheless exist. Consequently, the limit as x approaches a is sometimes called a "two-sided limit".
In some cases one of the two one-sided limits exists and the other does not, and in some cases neither exists.
The right-sided limit can be rigorously defined as
and the left-sided limit can be rigorously defined as
where I represents some interval that is within the domain of f.
One example of a function with different one-sided limits is the following:
Relation to topological definition of limit
The one-sided limit to a point p corresponds to the general definition of limit, with the domain of the function restricted to one side, by either allowing that the function domain is a subset of the topological space, or by considering a one-sided subspace, including p. Alternatively, one may consider the domain with a half-open interval topology.