Imagine some kind of system, where you have at t=0 N single atoms (a gas). Now in a later instant dt there is a certain probability that 2 atoms will have collided and formed a 2-atom unit. Similarly dt after this event there is a certain probability that this 2-atom unit has either collided with another atom to form a 3-atom unit or a probability that it has decayed back to 2 single atoms. From this should it be possible to find the probability at time t, that an n-atom unit has been formed. Do you guys know of any theoretical work that describes a dynamic system like this?