There are many reasons why the average of the time derivative might vanish, ⟨ ⁠ dG / dt ⁠ ⟩ τ = 0. One often-cited reason applies to stably-bound systems, that is to say systems that hang together forever and whose parameters are finite. In that case, velocities and coordinates of the particles of the system have upper and lower limits so that G, is bounded between two extremes, G min and G max, and the average goes to zero in the limit of infinite τ :