1. R. Glauber:Phys. Rev.,131, 2766 (1963).
2. More generally, the pseudoparabolic differential equationu xy+aux+buy+cu+dut=f, wherea, b, c, d, f are given functions ofx, y, t is relevant for some physical problems. See, for instance,H. Neunzert:Zeits. Angew Math. Mech.,48, T222 (1968).
3. P. Lambropoulos:Journ. Math. Phys.,8, 2167 (1967).
4. J. L. Neuringer:Journ. Math. Phys.,10, 250 (1969).
5. M. E. Goldstein:Journ. Math. Phys.,11, 667 (1970).