Increasing stability in an inverse problem for the acoustic equation
  In this work we study the inverse boundary value problem of determining the refractive index in the acoustic equation. It is known that this inverse problem is ill-posed. Nonetheless, we show that the ill-posedness decreases when we increase the frequency and the stability estimate changes from logarithmic type for low frequencies to a Lipschitz estimate for large frequencies. 
Sei Nagayasu, Gunther Uhlmann, and Jenn-Nan Wang
math.AP