1. Oscillatory Phenomena Associated to Semilinear Wave Equations in One Spatial Dimension;Cazenave;Trans. A.M.S,1987
2. Elliptic Partial Differential Equations of Second Order;Gilbarg,1983
3. Semi-Linear Hyperbolic Problems in Bounded Domains;Haraux,1987
4. Oscillation Theory;Kreith,1973
5. Maximum Principles in Differential Equations;Protter,1984