1. Picone-type theorems for semidiscrete hyperbolic equations;Kreith,1983
2. A class of comparison theorems for nonlinear hyperbolic initial value problems;Kreith,1980
3. Oscillation theorems for characteristic initial value problems for linear hyperbolic equations;Pagan;Atti Accad. naz. Lincei RC,1973
4. An oscillation theorem for characteristic initial value problems in linear hyperbolic equations;Pagan,1977