Staircase Visibility and Krasnosel’skii-Type Results for Planar Compact Sets

Abstract

Some Krasnotel’skii-type results previously established for a simply connected orthogonal polygon may be extended to a nonempty compact planar set \(S\) having connected complement. In particular, if every two points of \(S\) are visible via staircase paths from a common point of \(S\), then \(S\) is starshaped via staircase paths. For \(n\) fixed, \(n \geq 1\), if every two points of \(S\) are visible via staircase \(n\)-paths from a common point of \(S\), then \(S\) is starshaped via staircase \((n+1)\)-paths. In each case, the associated staircase kernel is orthogonally convex.