It is well-known that quasi-isometries between R-trees induce power
quasi-symmetric homeomorphisms between their ultrametric end spaces. This paper
investigates power quasi-symmetric homeomorphisms between bounded, complete,
uniformly perfect, ultrametric spaces (i.e., those ultrametric spaces arising
up to similarity as the end spaces of bushy trees). A bounded distortion
property is found that characterizes power quasi-symmetric homeomorphisms
between such ultrametric spaces that are also pseudo-doubling. Moreover,
examples are given showing the extent to which the power quasi-symmetry of
homeomorphisms is not captured by the quasiconformal and bi-H\"older conditions
for this class of ultrametric spaces.Comment: 20 pages, 1 figure. To appear in Ann. Acad. Sci. Fenn. Mat