where f ( p → → , x → → , t ) = f 0 ( p → → ) + δ δ f ( p → → , x → → , t ) is the density of quasiparticles (here we ignore spin) with momentum p → → and position x → → at time t , and E ( p → → , x → → , t ) = E 0 ( p → → ) + δ δ E ( p → → , x → → , t ) is the energy of a quasiparticle of momentum p → → ( f 0 and E 0 denote equilibrium distribution and energy in the equilibrium distribution). The semiclassical limit assumes that f fluctuates with angular frequency ω ω and wavelength λ λ = 2 π π / k , which are much lower than E F / ℏ ℏ and much longer than ℏ ℏ / p F respectively, where E F and p F are the Fermi energy and momentum respectively, around which f is nontrivial. To first order in fluctuation from equilibrium, the equation becomes