Definition:Continuity/Metric Subspace

Definition
Let $M_1 = \left({A_1, d_1}\right)$ and $M_2 = \left({A_2, d_2}\right)$ 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, $\left({Y, d_Y}\right)$ is a metric subspace of $A_1$.

Let $a \in Y$ be a point in $Y$.

Then $f$ is $\left({d_Y, d_2}\right)$-continuous at $a$ iff:
 * $\forall \epsilon > 0: \exists \delta > 0: d_Y \left({x, a}\right) < \delta \implies d_2 \left({f \left({x}\right), f \left({a}\right)}\right) < \epsilon$

Similarly, $f$ is $\left({d_Y, d_2}\right)$-continuous iff:
 * $\forall a \in Y: f$ is $\left({d_Y, d_2}\right)$-continuous at $a$

Also see

 * Restriction of Non-Continuous Mapping on Metric Space to Subspace may be Continuous