Gromov hyperbolic John is quasihyperbolic John I

In this paper, we introduce a concept of quasihyperbolic John spaces and provide a necessary and sufficient condition for a space to be quasihyperbolic John. Using this criteria, we exhibit a simple proof to show that a John space with a Gromov hyperbolic quasihyperbolization is quasihyperbolic John, quantitatively. This answers affirmatively to an open question proposed by Heinonen (Rev.~Math.~Iber, 1989), which was studied by Gehring et al. (Math.~Scand, 1989). As a tool, we study its connection between several geometric conditions.