On Boundaries of $\varepsilon$-neighbourhoods of Planar Sets, Part I: Singularities.

We study geometric and topological properties of singularities on the boundaries of $\varepsilon$-neighbourhoods $E_\varepsilon = \{x \in \mathbb{R}^2 : \textrm{dist}(x, E) \leq \varepsilon \}$ of planar sets $E \subset \mathbb{R}^2$. We develop a novel technique for analysing the boundary and obtain, for a compact set $E$ and $\varepsilon > 0$, a classification of singularities (i.e. non-smooth points) on $\partial E_\varepsilon$ into eight categories. We also show that the set of singularities is either countable or the disjoint union of a countable set and a closed, totally disconnected, nowhere dense set.