Topological size of the set of universal and ultrahomogeneous retractions on the Urysohn space

Abstract

In this paper, we investigate the set $\mathcal{U}(\mathbb{U})$ of universal and ultrahomogeneous $1$-Lipschitz retractions acting on the Urysohn space as the subspace of the space $\mathcal{R}(\mathbb{U})$ of all $1-$Lipschitz retractions defined on the Urysohn space. Especially, we study Borel complexity and density $\mathcal{U}(\mathbb{U})$ in $\mathcal{R}(\mathbb{U}).$ In order to do that, we introduce a new extension property $(U{R}^{\ast })$ that is equivalent to the universality and ultrahomogeneity of a retraction, and a new pointwise retract topology.