1. Shore GM 2002 Superluminal Light InTime and Matter: an International Colloquium on the Science of Time, edited by I. Bigi and M. Faessler (Venice), gr-qc/0302116
2. Gravitational Lenses
3. Hawking SW 1972 The Event Horizon InLes Houches Lectures 1972, Vol 72 Black Holes, edited by C. De Witt, (Gordon and Breach, New York)
4. The Large Scale Structure of Space-Time
5. Brillouin L 1960Wave Propagation and Group Velocity(London: Academic Press)