A note on the minimal displacement function

Abstract

Let (X,d) be a metric space and Iso(X,d) the associated isometry group. We study the subadditivity of the minimal displacement function $f : Iso(X, d) \to R$ for different metric spaces. When (X,d) is ultrametric, we prove that the minimal displacement function is subadditive. We show, by a simple algebraic argument, that subadditivity does not hold for the direct isometry group of the hyperbolic plane. The same argument can be used for other metric spaces.