Oleksandr V. Maslyuchenko, Ziemowit M. Wójcicki
We introduce the generalized notion of semicontinuity of a function defined on a topological space and derive the useful classification of the so-called Lipschitz derivatives of functions defined on a metric space. Secondly, we investigate some connections of the Lipschitz derivatives defined on normed spaces to the Fréchet derivative and relations between little, big and local Lipschitz derivatives (denoted by $\lip f$, $\Lip f$ and $\LLip f$ respectively) in terms of Baire limit functions. In particular, we prove that $\lip f$ is $\mathcal{F}_σ$-lower, $\Lip f$ is $\mathcal{F}_σ$-upper, $\LLip f$ is upper semicontinuous. Moreover, for a function $f$ defined on an open or convex subset of a normed space, the upper Baire limit function of functions $\lip f$ and $\Lip f$ are equal to $\LLip f$.
Oleksandr V. Maslyuchenko, Ziemowit M. Wójcicki
Using a modification of a generalized Takagi-van der Waerden function on a metric space we prove that for any closed subset of a metric space without isolated points there exists a continuous function such that its big and local Lipschitz derivatives are equal to infinity exactly on this set. Moreover, if given space is hermetic (for example, if it is normed) then the little Lipschitz derivative has the same property.
Ziemowit M. Wójcicki
We study the local Lipschitz one subsets of a finite dimensional space, that is, sets for which there exists a continuous function whose local Lipschitz derivative is the characteristic function of said set. We give a characterization of a local Lipschitz one set on the real line in terms of a certain measure-theoretic density condition, which we call quasi-density. We show that any local Lipschitz one set needs to be quasi-dense, but the converse does not hold. Finally, we show that any regular closed subset of a normed space is a local Lipschitz one set, but there exist local Lipschitz one sets that are not regular closed.