Inverse problems for quadratic derivative nonlinear wave equations
  For semilinear wave equations on Lorentzian manifolds with quadratic derivative non-linear terms, we study the inverse problem of determining the background Lorentzian metric. Under some conditions on the nonlinear term, we show that from the source-to-solution map, one can determine the Lorentzian metric up to diffeomorphisms. 
Yiran Wang and Ting Zhou
math.AP