Quasi-continuous function

In mathematics, the notion of a quasi-continuous function is similar to, but weaker than, the notion of a continuous function. All continuous functions are quasi-continuous but the converse is not true in general.

Definition

Let $X$ be a topological space. A real-valued function $f:X\to \mathbb{ℝ}$ is quasi-continuous at a point $x\in X$ if for any $ϵ>0$ and any open neighborhood $U$ of $x$ there is a non-empty open set $G\subset U$ such that

$|f(x)-f(y)|<ϵ\mathrm{\forall }y\in G$

Note that in the above definition, it is not necessary that $x\in G$.

Properties

• If $f:X\to \mathbb{ℝ}$ is continuous then $f$ is quasi-continuous
• If $f:X\to \mathbb{ℝ}$ is continuous and $g:X\to \mathbb{ℝ}$ is quasi-continuous, then $f+g$ is quasi-continuous.

Example

Consider the function $f:\mathbb{ℝ}\to \mathbb{ℝ}$ defined by $f(x)=0$ whenever $x\le 0$ and $f(x)=1$ whenever $x>0$. Clearly f is continuous everywhere except at x=0, thus quasi-continuous everywhere except (at most) at x=0. At x=0, take any open neighborhood U of x. Then there exists an open set $G\subset U$ such that $y<0\mathrm{\forall }y\in G$. Clearly this yields $|f(0)-f(y)|=0\mathrm{\forall }y\in G$ thus f is quasi-continuous.

In contrast, the function $g:\mathbb{ℝ}\to \mathbb{ℝ}$ defined by $g(x)=0$ whenever $x$ is a rational number and $g(x)=1$ whenever $x$ is an irrational number is nowhere quasi-continuous, since every nonempty open set $G$ contains some $y_1,y_2$ with $|g(y_1)-g(y_2)|=1$.

See also

• Cliquish function