# Geometry of sets of non-differentiability

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Olga Maleva

created by dimarino on 18 Sep 2018

**Abstract.**

There are many subsets $N$ of $\mathbb{R}^n$ for which one can find a real-valued Lipschitz function $f$ defined on the whole of $\mathbb{R}^n$ but non-differentiable at every point of $N$. Of course, by the Rademacher theorem any such set $N$ is Lebesgue null, however, due to a celebrated result of Preiss from 1990 not every Lebesgue null subset of $\mathbb{R}^n$ gives rise to such a
Lipschitz function $f$. In this talk, I explain that a sufficient condition on a set $N$ for such $f$ to exist is to be locally unrectifiable with respect to curves in a cone of directions. In particular, every $1$-purely unrectifiable set $U$ possesses a Lipschitz function non-differentiable on $U$ in the strongest possible sense. I also give an example of a universal differentiability set unrectifiable with respect to a fixed cone of directions, showing that one cannot relax the conditions.
This is joint work with D. Preiss.