2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/108811It is well-known that the solution of the classical linear wave equation with compactly supported initial condition and vanishing initial velocity is also compactly supported in a set depending on time : the support of the solution at time t is causally related to that of the initially given condition. Reed and Simon have shown that for a real-valued Klein-Gordon equation with (nonlinear) right-hand side $- λu^3$, causality still holds. We show the same property for a one-dimensional Klein-Gordon problem but with transmission and with a more general repulsive nonlinear right-hand side $F$. We also prove the global existence of a solution using the repulsiveness of $F$. In the particular case $F(u) = - λu^3$, the problem is a physical model for a quantum particle submitted to self-interaction and to a potential step.23 pages, no figuresAnalysis of PDEs35A05; 35C15; 35E15; 35L70; 35L90; 42A38text