ABSTRACT
The particles move with constant velocity unless they collide, which is natural because the matter is so cold that its pressure and viscosity are negligible and no forces are included. The motion of free particles gives rise to the intersection of trajectories and the formation of regions of infinite density, that is, at collision the colliding particles stick and form a new massive particle. The initial discontinuity emits either a contact discontinuity or a delta-shock. To obtain the global solution, the authors therefore have to deal with the interactions of delta-shocks themselves, delta-shock and contact discontinuity, and contact discontinuities themselves, and study how they match together.
