How Hard is it to be a Star? Convex Geometry and the Real Hierarchy

Abstract

A set is star-shaped if there is a point in the set that can see every other point in the set in the sense that the line-segment connecting the points lies within the set. We show that testing whether a non-empty compact smooth region is star-shaped is $\forall\mathbb{R}$-complete. Since the obvious definition of star-shapedness has logical form $\exists\forall$, this is a somewhat surprising result, based on Krasnosel'ski\u{\i}'s theorem from convex geometry; we study several related complexity classifications in the real hierarchy based on other results from convex geometry.