On semiconvex sets in the plane

Abstract

The present work considers the properties of classes of generally convex sets in the plane known as $1$-semiconvex and weakly $1$-semiconvex. More specifically, the examples of open and closed weakly $1$-semiconvex but non $1$-semiconvex sets with smooth boundary in the plane are constructed. It is proved that such sets consist of minimum four connected components. In addition, the example of closed, weakly $1$-semiconvex, and non $1$-semiconvex set in the plane consisting of three connected components is constructed. It is proved that such a number of components is minimal for any closed, weakly $1$-semiconvex, and non $1$-semiconvex set in the plane.