Singular points of lightlike hypersurfaces of the de Sitter space
  The authors study singular points of lightlike hypersurfaces of the de Sitter space S^{n+1}_1 and the geometry of hypersurfaces and use them for construction of an invariant normalization and an invariant affine connection of lightlike hypersurfaces. 
Maks A. Akivis and Vladislav V. Goldberg