Horo-shrinkers in the hyperbolic space

A surface Σ in the hyperbolic space ³ is called a horo-shrinker if its mean curvature H satisfies H= N,∂z, where $(x,y,z)$ are the coordinates of ³ in the upper half-space model and N is the unit normal of Σ. In this paper we study horo-shrinkers invariant by one-parameter groups of isometries of ³ depending if these isometries are hyperbolic, parabolic or spherical. We characterize totally geodesic planes as the only horo-shrinkers invariant by a one-parameter group of hyperbolic translations. The grim reapers are defined as the horo-shrinkers invariant by a one-parameter group of parabolic translations. We describe the geometry of the grim reapers proving that they are periodic surfaces. In the last part of the paper, we give a complete classification of horo-shrinkers invariant by spherical rotations, distinguishing if the surfaces intersect or not the rotation axis.