1. Sturmian theorems for hyperbolic equations;Kreith, Kurt;Proc. Amer. Math. Soc.,1969
2. Oscillation theorems for characteristic initial value problems for linear hyperbolic equations;Pagan, Gordon;Atti. Accad. Naz. Lincei Rend. Cl. Sci. Fis. Mat. Natur. (8),1973
3. Mathematics in Science and Engineering, Vol. 48;Swanson, C. A.,1968
4. A dichotomy of PDE Sturmian theory;Swanson, C. A.;SIAM Rev.,1978