One-sided limit
The limit of a function at a point from the right or left. Let
be a mapping from an ordered set X (
for example, a set lying in the real line), regarded as a topological space with the topology generated by the order relation, into a topological space Y ,
and let x _ {0} \in X .
The limit of f
with respect to any interval ( a, x _ {0} ) = \{ {x } : {x \in X, a < x < x _ {0} } \}
is called the limit of f
on the left, and is denoted by
\lim\limits _ {x \rightarrow x _ {0} - 0 } f ( x)
(it does not depend on the choice of a < x _ {0} ), and the limit with respect to the interval ( x _ {0} , b) = \{ {x } : {x \in X, x _ {0} < x < b } \} is called the limit on the right, and is denoted by
\lim\limits _ {x \rightarrow x _ {0} + 0 } f ( x)
(it does not depend on the choice of b > x _ {0} ). If the point x _ {0} is a limit point both on the left and the right for the domain of definition of the function f , then the usual limit
\lim\limits _ {x \rightarrow x _ {0} } f ( x)
with respect to a deleted neighbourhood of x _ {0} ( in this case it is also called a two-sided limit, in contrast to the one-sided limits) exists if and only if both of the left and right one-sided limits exist at x _ {0} and they are equal.
Comments
Instead of \lim\limits _ {x \rightarrow x _ {0} + 0 } ( respectively, \lim\limits _ {x \rightarrow x _ {0} - 0 } ) one also finds the notations \lim\limits _ {x \rightarrow x _ {0} + } , \lim\limits _ {x \downarrow x _ {0} } ( respectively, \lim\limits _ {x \rightarrow x _ {0} - } , \lim\limits _ {x \uparrow x _ {0} } ).
One-sided limit. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=One-sided_limit&oldid=48045