Higher-order preconnections in synthetic differential geometry of jet bundles. Nishimura, Hirokazu (2004) Beiträge zur Algebra und Geometrie
Hyperconvexity of ℝ-trees W. Kirk (1998) Fundamenta Mathematicae It is shown that for a metric space (M,d) the following are equivalent: (i) M is a complete ℝ-tree; (ii) M is hyperconvex and has unique metric segments.