The equation above for s -particle distribution function is obtained by integration of the Liouville equation over the variables q s + 1 … … q N , p s + 1 … … p N . The problem with the above equation is that it is not closed. To solve f s , one has to know f s + 1 , which in turn demands to solve f s + 2 and all the way back to the full Liouville equation. However, one can solve f s , if f s + 1 could be modeled. One such case is the Boltzmann equation for f 1 ( q 1 , p 1 , t ) , where f 2 ( q 1 , q 2 , p 1 , p 2 , t ) is modeled based on the molecular chaos hypothesis (Stosszahlansatz). In fact, in the Boltzmann equation f 2 = f 2 ( p 1 , p 2 , t ) is the collision integral. This limiting process of obtaining Boltzmann equation from Liouville equation is known as Boltzmann–Grad limit.