Global existence of quasilinear, nonrelativistic wave equations   satisfying the null condition
  We prove global existence of solutions to multiple speed, Dirichlet-wave equations with quadratic nonlinearities satisfying the null condition in the exterior of compact obstacles. This extends the result of our previous paper by allowing general higher order terms. In the currect setting, these terms are much more difficult to handle than for the free wave equation, and we do so using an analog of a pointwise estimate due to Kubota and Yokoyama. 
Jason Metcalfe, Makoto Nakamura, and Christopher D. Sogge