M.H.S. Amin, E.V. Bezuglyi, A.S. Kijko, and A.N. Omelyanchouk, Wigner distribution function formalism for superconductors and collisionless dynamics of the superconducting order parameter: Сверхпроводимость и мезоскопические структуры, Low Temp. Phys. 30, (2004) [Fiz. Nizk. Temp. 30, 874-880, (2004)] DOI: https://doi.org/10.1063/1.1789939.