Semi-continuity
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In mathematical analysis, semi-continuity (or semicontinuity) is a property of real-valued functions that is weaker than continuity. A real-valued function f is upper semi-continuous at a point x0 if, roughly speaking, the function values for arguments near x0 are either close to f(x0) or less than f(x0). If "less than" is replaced by "greater than", the function is called lower semi-continuous at x0.
[edit] Examples
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.
The floor function 
, which returns the greatest integer less than or equal to a given x, is everywhere upper semi-continuous. Similarly, the ceiling function 
is lower semi-continuous.
[edit] Formal definition
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
- lim sup x → x0f(x) ≤ f(x0)
(see limit superior and limit inferior for the definition of lim sup). 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
- lim inf x → x0 f(x) ≥ f(x0)
(see limit superior and limit inferior for the definition of lim inf). 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.
[edit] Properties
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
- f(x) := sup {fn(x) : n in N} < ∞
for every x in X. Then f is lower semi-continuous. Even if all the fn are continuous, f need not be continuous.
The indicator function of any open set is lower semicontinuous. The indicator function of a closed set is upper semicontinuous.
