\begin{definition}[Definition:Continuity/Metric Subspace]
Let $M_1 = \struct {A_1, d_1}$ and $M_2 = \struct {A_2, d_2}$ be metric spaces.
Let $f: A_1 \to A_2$ be a mapping from $A_1$ to $A_2$.
Let $Y \subseteq A_1$.
By definition, $\struct {Y, d_Y}$ is a metric subspace of $A_1$.
Let $a \in Y$ be a point in $Y$.
Then $f$ is '''$\tuple {d_Y, d_2}$-continuous at $a$''' {{iff}}:
:$\forall \epsilon > 0: \exists \delta > 0: \map {d_Y} {x, a} < \delta \implies \map {d_2} {\map f x, \map f a} < \epsilon$
Similarly, $f$ is '''$\tuple {d_Y, d_2}$-continuous''' {{iff}}:
:$\forall a \in Y: f$ is $\tuple {d_Y, d_2}$-continuous at $a$
\end{definition}