Bertrand Spaces and Projected Closed Orbits in General Relativity
  In the present work, metrics which lead to projected closed orbits are found by comparing the relativistic differential equation of orbits with the corresponding classical differential equation. Physical and geometrical properties of these peculiar spacetimes are derived and discussed. It is also shown that some of these spacetimes belong to the broader class of the Bertrand spacetimes. 
Mohsen Rahimkhanli and Nematollah Riazi
gr-qc math-ph math.MP