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Consider the function f(x) = -1 for x < 0 and f(x) = 1 for x ≥ 0. This function is upper semi-continuous at x0 = 0, but not lower semi-continuous.
Imagine that you are scanning a certain scenery with your eyes and record the distance to the viewed object at all times. This yields a lower semi-continuous function which in general is not upper semi-continuous (for instance if you focus on the edge of a table).
Suppose X is a topological space, x0 is a point in X and f : X → R is a real-valued function. We say that f is upper semi-continuous at x0 if for every ε > 0 there exists a neighborhood U of x0 such that f(x) < f(x0) + ε for all x in U. Equivalently, this can be expressed as
The function f is called upper semi-continuous if it is upper semi-continuous at every point of its domain. Then {x∈X : f(x) < α} is an open set for every α∈R.
We say that f is lower semi-continuous at x0 if for every ε > 0 there exists a neighborhood U of x0 such that f(x) > f(x0) - ε for all x in U. Equivalently, this can be expressed as
The function f is called lower semi-continuous if it is lower semi-continuous at every point of its domain. Then {x∈X : f(x) > α} is an open set for every α∈R.
A function is continuous at x0 if and only if it is upper and lower semi-continuous there.
If f and g are two functions which are both upper semi-continuous at x0, then so is f + g. If both functions are non-negative, then the product function fg will also be upper semi-continuous at x0. Multiplying a positive upper semi-continuous function with a negative number turns it into a lower semi-continuous function.
If C is a compact space (for instance a closed interval [a, b]) and f : C → R is upper semi-continuous, then f has a maximum on C. The analogous statement for lower semi-continuous functions and minima is also true.
Suppose fn : X → R is a lower semi-continuous function for every natural number n, and
for every x in X. Then f is lower semi-continuous. Even if all the fn are continuous, f need not be continuous.
The characteristic functionSome mathematicians use the phrase characteristic function synonymously with " indicator function". The indicator function of a subset A of a set B is the function with domain B whose value is 1 at each point in A and 0 at each point that is in B but not of an open set is lower semicontinuous. The characteristic function of a closed set is upper semicontinuous.