Stability of a wave and Klein-Gordon system with mixed coupling

We are interested in establishing global stability results for a system of semilinear wave and Klein-Gordon equations with mixed coupling nonlinearities, that is, we consider all of the possible quadratic nonlinear terms of the type of wave and Klein-Gordon interactions. The main difficulties are due to the absence of derivatives on the wave component in the nonlinearities. By doing a nonlinear transformation on the wave equation, we reveal a hidden null structure. Next, by using the scaling vector field on the wave component only, which is generally avoided, we are able to get good $L^2$-type estimates on the wave component. Then we distinguish high-order and low-order energy of both wave and Klein-Gordon components, which allows us to close the bootstrap argument.