Global existence without decay for quadratic Klein-Gordon equations
  The Cauchy problem for quadratic Klein-Gordon systems is considered in two spatial dimensions and higher under a suitable non-resonance condition on the masses, including the main case of equal masses. A global well-posedness and scattering result is proven for small H^s data, s >= max(1/2, (n-2)/2), using the contraction mapping technique in U^2/V^2 based spaces. The result demonstrates that global results and scattering hold in this setting without the need to impose strong decay on the initial data. 
Tobias Schottdorf
math.AP