δ-hyperbolic space

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In mathematics, a δ-hyperbolic space is a geodesic metric space in which every geodesic triangle is δ-thin.

The δ-thin triangle condition

There are many equivalent definitions of "δ-thin". A simple definition is as follows: pick three points and draw geodesic lines between them to make a geodesic triangle. Then any point on any of the edges of the triangle is within a distance of δ from one of the other two sides.

For example, trees are 0-hyperbolic: a geodesic triangle in a tree is just a subtree, so any point on a geodesic triangle is actually on two edges. Normal Euclidean space is ∞-hyperbolic; i.e. not hyperbolic. Generally, the higher δ has to be, the less curved the space is.

The definition of δ-hyperbolic space is generally credited to Eliyahu Rips. There is also a definition of δ-hyperbolicity due to Mikhail Gromov. A geodesic metric space is said to be a Gromov δ-hyperbolic space if, for all p, x, y and z in X,

(x, z)_{p} \geq \min \big\{ (x, y)_{p}, (y, z)_{p} \big\} - \delta,

where (xy)p denotes the Gromov product of x and y at p. X is said to be simply Gromov hyperbolic if it is Gromov δ-hyperbolic for some δ ≥ 0.

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