Curvatures of left invariant Randers metric on the five-dimensional   Heisenberg group
  A left invariant Z-Randers metric on the five-dimensional Heisenberg group is a left invariant Randers metric with deformation vector from the center of the Heisenberg algebra. In this note we prove that for every left invariant Z-Randers metric on the five-dimensional Heisenberg group there exist flags of strictly negative and there exist flags of strictly positive curvatures. 
A. Lengyeln\'e-T\'oth and Z. Kov\'acs
math.DG