Global solutions of coupled Klein-Gordon equations with different   velocities in four space-time dimensions
  In this article one will discuss the system of coupled nonlinear Klein-Gordon equations with different velocities and different masses. The nonlinearity considered is a general quadratic nonlinearity without any restriction. The method is a classical boot-strap argument combined with a serious of techniques including conformal energy estimate, global sobolev's lemma and Hardy type inequalities. 
Yue Ma
math.AP