Axis of a hyperbolic-space isometry

An isometry of hyperbolic space with positive attained translation length has an invariant complete geodesic joining its two ideal fixed points. This is its axis. It translates along the axis by its translation length; in three dimensions it may additionally rotate around it. In upper half-space, $(z,t)\mapsto (kz,|k|t)$ with $|k|\ne 1$ has the vertical axis through zero and length $|\log |k||$. Every connecting path has length at least the change in $\log t$, and a vertical segment attains the bound.