# The Quasi-hyperbolicity Constant of a Metric Space

## TL;DR

This paper introduces the quasi-hyperbolicity constant, a new invariant measuring how metric spaces deviate from hyperbolicity, with exact calculations and bounds for various spaces.

## Contribution

It defines the quasi-hyperbolicity constant, establishes bounds for different classes of metric spaces, and computes exact values for specific cases like snowflakes of the Euclidean line.

## Key findings

- The constant lies in [1,2] for unbounded spaces.
- It equals 1 for Gromov hyperbolic spaces.
- For Euclidean space, the constant is rac{1}{2}.

## Abstract

We introduce the quasi-hyperbolicity constant of a metric space, a rough isometry invariant that measures how a metric space deviates from being Gromov hyperbolic. This number, for unbounded spaces, lies in the closed interval $[1,2]$. The quasi-hyperbolicity constant of an unbounded Gromov hyperbolic space is equal to one. For a CAT$(0)$-space, it is bounded from above by $\sqrt{2}$. The quasi-hyperbolicity constant of a Banach space that is at least two dimensional is bounded from below by $\sqrt{2}$, and for a non-trivial $L_p$-space it is exactly $\max\{2^{1/p},2^{1-1/p}\}$. If $0 < \alpha < 1$ then the quasi-hyperbolicity constant of the $\alpha$-snowflake of any metric space is bounded from above by $2^\alpha$. We give an exact calculation in the case of the $\alpha$-snowflake of the Euclidean real line.