Relativistic astrometry and Synge's world function
  Almost all of the studies devoted to relativistic astrometry are based on the integration of the null geodesic differential equations. However, the gravitational deflection of light rays can be calculated by a different method, based on the determination of the bifunction giving half the squared geodesic distance between two arbitrary points-events, the so-called Synge's world function. We give a brief review of the main results obtained by this method. 
P. Teyssandier (1), C. Le Poncin-Lafitte (2) ((1) Observatoire de   Paris, (2) Technische Universitat Dresden)