“…In the study of submanifolds in Lorentzian-Minkowski space, the codimension two spacelike submanifolds happen to be the most interesting subjects, both from the view point of singularity theory and of the theory of relativity (cf., [10,11,13,14,15,16]). For instance, one of the important objects in the theory of relativity are the lightlike hypersurfaces because they provide good models for different types of horizons [6,20].…”