A key insight applied by Boltzmann was to determine the collision term resulting solely from two-body collisions between particles that are assumed to be uncorrelated prior to the collision. This assumption was referred to by Boltzmann as the " Stosszahlansatz " and is also known as the " molecular chaos assumption". Under this assumption the collision term can be written as a momentum-space integral over the product of one-particle distribution functions: ( ∂ ∂ f ∂ ∂ t ) coll = ∬ ∬ g I ( g , Ω Ω ) [ f ( r , p ′ A , t ) f ( r , p ′ B , t ) − − f ( r , p A , t ) f ( r , p B , t ) ] d Ω Ω d 3 p B , where p A and p B are the momenta of any two particles (labeled as A and B for convenience) before a collision, p′ A and p′ B are the momenta after the collision, g = | p B − − p A | = | p ′ B − − p ′ A | is the magnitude of the relative momenta (see relative velocity for more on this concept), and I (g, Ω) is the differential cross section of the collision, in which the relative momenta of the colliding particles turns through an angle θ into the element of the solid angle d Ω, due to the collision.