1. W.-T. Hsiang and M. K. Kwong, A comparison theorem for the first nodal line of the solutions of quasilinear hyperbolic equations with nonincreasing initial values, Proc. Roy. Soc. Edinburgh, in press.
2. W.-T. Hsiang and M. K. Kwong, Oscillation of second order hyperbolic equations with non-integrable coefficients, Proc. Roy. Soc. Edinburgh, in press.
3. K. Kreith, A class of comparison theorems for nonlinear hyperbolic initial value problems, Proc. Roy. Soc. Edinburgh, in press.
4. Oscillation theorems for characteristic initial value problems for linear hyperbolic equations;Pagan;Rend. Accad. Lincei,1973
5. An oscillation theorem for characteristic initial value problems in linear hyperbolic equations;Pagan,1977