1. D. Bohm,Phys. Rev. 85, 166 (1952).
2. In the case where the variables are time and frequency variations of this equation has been derived by J. Ville,Cables Transm. 2A, 61 (1948); L. M. Fink, translated inProblems Inf. Transm. 2, 11 (1966); L. Mandel,Am. J. Phys. 42, 840 (1974).
3. L. Cohen and C. Lee, inAdvanced Algorithms and Architectures for Signal Processing III, Franklin T. Luk, ed.;Proc. SPIE 975, 186 (1988);
4. L. Cohen and C. Lee,Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing 3, 2451 (1990).
5. L. Cohen,IEEE Proc. 77, 941 (1989).