Intrinsic Geometry of a Null Hypersurface
  We apply Cartan's method of equivalence to construct invariants of a given null hypersurface in a Lorentzian space-time. This enables us to fully classify the internal geometry of such surfaces and hence solve the local equivalence problem for null hypersurface structures in 4-dimensional Lorentzian space-times. 
Pawe{\l} Nurowski, David C. Robinson