Hi. The formulae for the velocities of two bodies after a perfectly elastic or inelastic bodies, let's say in 2D, (e.g. billiard) can be derived from three equations: conservation of energy and conservation of momentum in two dimensions. But how do you treat collisions of three or more bodies? With each additional body the number of unknowns rises by two (velocity in x and y direction), but the number of constraints is still three. Is the problem stable with respect to shifting the bodies by arbitrarily small (or virtual) displacements such that there are only two-body collisions to consider?