The 4-point condition and subsets of CAT(κ) spaces
It is a longstanding open question to characterize those metric spaces that admit a distance-preserving embedding into a CAT(κ) space. On the other hand, among all geodesic metric spaces, CAT(κ) spaces can be characterized by various simple conditions. Gromov's Cycl4(κ) condition, which is equivalent to the so-called CAT(κ) 4-point condition, is one of them. In this talk, we study the difference between the class of metric spaces that admit a distance-preserving embedding into a CAT(κ) space and the class of metric spaces that satisfy the Cycl4(κ) condition. We show that an analogue of Reshetnyak's majorization theorem holds for all metric spaces that satisfy the Cycl4(κ) condition. We also show that a metric space that contains at most five points admits a distance-preserving embedding into a CAT(0) space if and only if it satisfies the Cycl4(0) condition.