No abstract

The main aim of this survey paper is to give basic information about properties and applications ofσ-porous sets in Banach spaces(and some other infinite-dimensional spaces). This paper can be considered a partial continuation of the author's 1987 survey on porosity andσ-porosity and therefore only some results, remarks, and references (important for infinite-dimensional spaces) are repeated. However, this paper can be used without any knowledge of the previous survey. Some new results concerningσ-porosity in finite-dimensional spaces are also briefly mentioned. However, results concerning porosity (but notσ-porosity) are mentioned only exceptionally.

No abstract

We give a complete characterization of compact sets with positive reach (=proximally $C^1$ sets) in the plane and of one-dimensional sets with positive reach in ${\mathbb R}^d$. Further, we prove that if $\emptyset \neq A\subset{\mathbb R}^d$ is a set of positive reach of topological dimension $0< k \leq d$, then $A$ has its "$k$-dimensional regular part" $\emptyset \neq R \subset A$ which is a $k$-dimensional "uniform" $C^{1,1}$ manifold open in $A$ and $A\setminus R$ can be locally covered by finitely many $(k-1)$-dimensional DC surfaces. We also show that if $A \subset {\mathbb R}^d$ has positive reach, then $\partial A$ can be locally covered by finitely many semiconcave hypersurfaces

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