
 Amazon.com: Singularity Theory and Gravitational Lensing: Books: Arlie O. Petters,Harold Levine,Joachim Wambsganss (Site not responding. Last check: 20071106) 
  This monograph, unique in the literature, is the first to develop a mathematical theory of gravitational lensing. 
  Among the lensing topics discussed are multiple quasars, giant luminous arcs, Einstein rings, the detection of dark matter and planets with lensing, time delays and the age of the universe (Hubble's constant), microlensing of stars and quasars. 
  Mathematical topics discussed: Morse theory, Whitney singularity theory, Thom catastrophe theory, Mather stability theory, Arnold singularity theory, and the Euler characteristic via projectivized rotation numbers. 
 www.amazon.com /exec/obidos/tg/detail//0817636684?v=glance (621 words) 
