Rotation inside convex Kakeya sets

TL;DR

This paper investigates the continuous motion and selection of rotated copies of a convex body within a convex set, revealing dimension-dependent possibilities and limitations, especially for line segments and in four dimensions.

Contribution

It provides new results on the continuous motion and selection of rotated bodies inside convex sets, with dimension-specific answers and counterexamples.

Findings

In 2D, the stronger continuous selection question always has an affirmative answer.

In 3D, the answer is negative for general bodies but positive for line segments.

In 4D, the first question has a negative answer for general bodies.

Abstract

Let $K$ be a convex body (a compact convex set) in $\mathbb{R}^d$, that contains a copy of another body $S$ in every possible orientation. Is it always possible to continuously move any one copy of $S$ into another, inside $K$? As a stronger question, is it always possible to continuously select, for each orientation, one copy of $S$ in that orientation? These questions were asked by Croft. We show that, in two dimensions, the stronger question always has an affirmative answer. We also show that in three dimensions the answer is negative, even for the case when $S$ is a line segment -- but that in any dimension the first question has a positive answer when $S$ is a line segment. And we prove that, surprisingly, the answer to the first question is negative in dimension four for general $S$.