Absence of critical points of solutions to the Helmholtz equation in 3D

In this talk I will discuss a method to prove the absence of critical points for the Helmholtz equation in 3D. The key element of the approach is the use of multiple frequencies in a fixed range, and the proof is based on the spectral analysis of the associated problem. This question is strictly connected with the Rado-Kneser-Choquet theorem, whose direct extension to the Helmholtz equation or to three dimensions is not possible.