The length, width, and inradius of space curves

Abstract

The width $w$ of a curve $\gamma$ in Euclidean space $R^n$ is the infimum of the distances between all pairs of parallel hyperplanes which bound $\gamma$, while its inradius $r$ is the supremum of the radii of all spheres which are contained in the convex hull of $\gamma$ and are disjoint from $\gamma$. We use a mixture of topological and integral geometric techniques, including an application of Borsuk Ulam theorem due to Wienholtz and Crofton's formulas, to obtain lower bounds on the length of $\gamma$ subject to constraints on $r$ and $w$. The special case of closed curves is also considered in each category. Our estimates confirm some conjectures of Zalgaller up to $99\%$ of their stated value, while we also disprove one of them.