The Zwanzig projection operator operates on functions in the 6 N -dimensional phase space Γ Γ = { q i , p i } of N point particles with coordinates q i and momenta p i . A special subset of these functions is an enumerable set of "slow variables" A ( Γ Γ ) = { A n ( Γ Γ ) } . Candidates for some of these variables might be the long-wavelength Fourier components ρ ρ k ( Γ Γ ) of the mass density and the long-wavelength Fourier components π π k ( Γ Γ ) of the momentum density with the wave vector k identified with n . The Zwanzig projection operator relies on these functions but does not tell how to find the slow variables of a given Hamiltonian H ( Γ Γ ) .