Let $f: \mathbb R^n \to \mathbb R^m$ be a Lipschitz map. We define the *local Lipschitz constant* $Lf$ of $f$ at $x \in \mathbb R^n$ by

$$Lf(x) := \lim_{r \to 0_+} \text{Lip}(f, B_r (x)),$$

where $\text{Lip}(f, U) := \sup_{y,z  \in U} \frac{f(y) - f(z)}{y- z}$ denotes the Lipschitz constant of $f$ on the set $U$.

Define the *stretch set* $S$ of $f$ by 

$$S := \{x \in \mathbb R^n \, | \, Lu(x) = \text{Lip}(f, \mathbb R^n)\}.$$

 Roughly, the stretch set is the set on which $f$ achieves its maximal Lipschitz constant. 

**Question:** Is it true that $\text{Lip}(f, \mathbb R^n) = \max(\text{Lip} (f, S), \text{Lip}(f, \mathbb R^n \setminus S))$?

**Remark:** The stretch set plays a crucial role in Thurston's "best Lipschitz maps" approach to Teichmuller theory, see [here](https://people.math.harvard.edu/~ctm/home/text/others/thurston/stretch/stretch.pdf) for a reference.