Separately continuous function

Definition at a point

Generic definition

Suppose $f$ is a function of more than one variable, where $x$ is one of the input variables to $f$ . Fix a choice $x=x_{0}$ and fix values of all the other input variables. We say that $f$ is continuous with respect to $x$ at this point in its domain if the following holds: the function that sends $x$ to $f$ evaluated at $x$ and the fixed choice of the other input variables is continuous at $x=x_{0}$ .

We say that a function $f$ of several variables is separately continuous in the variables at a point if it is separately continuous with respect to each of the variables at the point.

For a function of two variables

Suppose $f$ is a real-valued function of two variables $x,y$ , i.e., the domain of $f$ is a subset of $\mathbb {R} ^{2}$ . Suppose $(x_{0},y_{0})$ is a point in the domain of $f$ , i.e., it is the point where $x=x_{0}$ and $y=y_{0}$ (here $x_{0},y_{0}$ are actual numerical values). We define three notions:

$f$ is continuous with respect to $x$ at the point $(x_{0},y_{0})$ if the function $x\mapsto f(x,y_{0})$ (viewed as a function of one variable $x$ ) is continuous at $x=x_{0}$ .
$f$ is continuous with respect to $y$ at the point $(x_{0},y_{0})$ if the function $y\mapsto f(x_{0},y)$ (viewed as a function of one variable $y$ ) is continuous at $y=y_{0}$ .
$f$ is separately continuous at the point $(x_{0},y_{0})$ if it is continuous with respect to $x$ and continuous with respect to $y$ at the point $(x_{0},y_{0})$ .

For a function of multiple variables

Suppose $f$ is a real-valued function of variables $x_{1},x_{2},\dots ,x_{n}$ , i.e., the domain of $f$ is a subset of $\mathbb {R} ^{n}$ . Suppose $(a_{1},a_{2},\dots ,a_{n})$ is a point in the domain of $f$ , i.e., it is the point where $x_{1}=a_{1},x_{2}=a_{2},\dots ,x_{n}=a_{n}$ (here $a_{1},a_{2},\dots ,a_{n}$ are actual numerical values). We define two notions:

For each $i\in \{1,2,3,\dots ,n\}$ , we say that $f$ is continuous in $x_{i}$ at the point $(a_{1},a_{2},\dots ,a_{n})$ if the function $x_{i}\mapsto f(a_{1},a_{2},\dots ,a_{i-1},x_{i},a_{i+1},\dots ,a_{n})$ is continuous at $x_{i}=a_{i}$ .
We say that $f$ is separately continuous in terms of all the inputs $x_{1},x_{2},\dots ,x_{n}$ at a point $(a_{1},a_{2},\dots ,a_{n})$ if it is continuous with respect to $x_{i}$ at $(a_{1},a_{2},\dots ,a_{n})$ for each $i\in \{1,2,3,\dots ,n\}$ .

Definition as a function on an open domain

Generic definition

Suppose $f$ is a function of more than one variable whose domain is open (i.e., has no boundary points in it). Suppose $x$ is one of the inputs to $f$ . We say that $f$ is continuous with respect to $x$ if it is continuous with respect to $x$ at all points in its domain.

We say that $f$ is separately continuous if it is continuous with respect to each of the variables that are inputs to it.

For a function of two variables

Suppose $f$ is a real-valued function of two variables $x,y$ , i.e., the domain of $f$ is an open subset of $\mathbb {R} ^{2}$ . Then:

$f$ is continuous with respect to $x$ if $f$ is continuous with respect to $x$ at all points in its domain.
$f$ is continuous with respect to $y$ if $f$ is continuous with respect to $y$ at all points in its domain.
$f$ is separately continuous in $x,y$ if $f$ is continuous with respect to $x$ and continuous with respect to $y$ at all points in its domain.

For a function of multiple variables

Suppose $f$ is a real-valued function of variables $x_{1},x_{2},\dots ,x_{n}$ , i.e., the domain of $f$ is an open subset of $\mathbb {R} ^{n}$ :

For each $i\in \{1,2,3,\dots ,n\}$ , we say that $f$ is continuous in $x_{i}$ if $f$ is continuous in $x_{i}$ for every point in its domain.
We say that $f$ is separately continuous in terms of all the inputs $x_{1},x_{2},\dots ,x_{n}$ if it is continuous in all the inputs for every point in its domain.