where H is the Hamiltonian, and where the derivatives ∂ ∂ q ˙ ˙ i / ∂ ∂ q i and ∂ ∂ p ˙ ˙ i / ∂ ∂ p i have been evaluated using Hamilton's equations of motion. That is, viewing the motion through phase space as a 'fluid flow' of system points, the theorem that the convective derivative of the density, d ρ ρ / d t , is zero follows from the equation of continuity by noting that the 'velocity field' ( p ˙ ˙ , q ˙ ˙ ) in phase space has zero divergence (which follows from Hamilton's relations).