Continuity

In mathematics, the notion of continuity of a function relates to the idea that the "value" of the function should not jump abruptly for any vanishingly "small" variation to its argument. Another way to think about a continuity of a function is that any "small" change in the argument of the function can only effect a correspondingly "small" change in the value of the function.

Formal definition of continuity

A function f from a topological space $(X,O_{X})$ to another topological space $(Y,O_{Y})$, usually written as $f:(X,O_{X})\rightarrow (Y,O_{Y})$, is said to be continuous at the point $x\in X$ if for every open set $U_{y}\in O_{Y}$ containing the point y=f(x), there exists an open set $U_{x}\in O_{X}$ containing x such that $f(U_{x})\subset U_{y}$. Here $f(U_{x})=\{f(x')\in Y\mid x'\in U_{x}\}$. In a variation of this definition, instead of being open sets, $U_{x}$ and $U_{y}$ can be taken to be, respectively, a neighbourhood of x and a neighbourhood of $y=f(x)$.

This definition corresponds to a generalization of the $\delta -\epsilon$ formalism which are usually taught in first year calculus courses to, among other things, define limits and continuity for functions which map the set of real numbers to itself.

Continuous function

If the function f is continuous at every point $x\in X$ then it is said to be a continuous function. An important equivalent definition, but perhaps less convenient to work with directly, is that a function $f:(X,O_{X})\rightarrow (Y,O_{Y})$ is continuous if for any open (respectively, closed) set $U\in O_{Y}$ the set $f^{-1}(U)=\{x\in X\mid f(x)\in U\}$ is an open (respectively, closed) set in $O_{x}$. In this latter definition, a continuous function is simply a function which maps open sets to open sets or, equivalently, closed sets to closed sets.