On the singularities of distance functions in Hilbert spaces

Abstract

For a given closed nonempty subset E of a Hilbert space H, the singular set \(\Sigma _E\) consists of the points in \(H\setminus E\) where the distance function \(d_E\) is not Fréchet differentiable. It is known that \(\Sigma _E\) is a weak deformation retract of the open set \(\mathcal {G}_E=\{x\in H: d_{\overline{{\text {co}}}\,E}(x)< d_E(x)\}\). This short paper sheds light on the relationship between the connected components of the three sets \(\Sigma _E\subset \mathcal {G}_E\subseteq H{\setminus } E\).