Majorization by Hemispheres & Quadratic Isoperimetric Constants

Abstract

Let $X$ be a Banach space or more generally a complete metric space admitting a conical geodesic bicombing. We prove that every closed $L$-Lipschitz curve $\gamma :S^1\to X$ may be extended to an $L$-Lipschitz map defined on the hemisphere $f:H^2\to X$. This implies that $X$ satisfies a quadratic isoperimetric inequality (for curves) with constant $\frac{1}{2\pi }$. We discuss how this fact controls the regularity of minimal discs in Finsler manifolds when applied to the work of Alexander Lytchak and Stefan Wenger.

22 pages, 1 figure; minor correction in the proof of lemma 3.2