Suppose $f$ is a function, with domain $D$ and $G(x,y)$ is a
function of $x$ and $y$ that appears in the equation, $G(x,y) = 0$.

Definition:We say that __$f$ is an implicit function for the
equation $G(x,y) =0$__ if (and only if) for every $x \in D,
G(x,f(x))=0$.

##### Diagram IMPL Graph and Mapping
Diagram for $f$ as an implicit function for $G(x,
y)=x^2+y^2-4=0$