An D4767: Unsigned real number $C \in [0, \infty)$ is a

**minimal Lipschitz constant**for $f$ with respect to $M_X$ and $M_Y$ if and only if \begin{equation} C = \inf \left\{ L \in [0, \infty) \mid \forall \, x, y \in X : d_Y ( f(x), f(y)) \leq C d_X(x, y) \right\} \end{equation}