The global uniqueness and $C^1$-regularity of geodesics in expanding   impulsive gravitational waves
  We study geodesics in the complete family of expanding impulsive gravitational waves propagating in spaces of constant curvature, that is Minkowski, de Sitter and anti-de Sitter universes. Employing the continuous form of the metric we rigorously prove existence and global uniqueness of continuously differentiable geodesics (in the sense of Filippov) and study their interaction with the impulsive wave. Thereby we justify the "$C^1$-matching procedure" used in the literature to derive their explicit form. 
Jiri Podolsky, Clemens S\"amann, Roland Steinbauer, Robert Svarc
gr-qc math-ph math.MP